Master's thesis

The peri-urban landscape interface’s contribution to flood management of the metropolitan area, the case of Phnom Penh City

Waseda Univeristy, Faculty of Creative Science and Engineering Department of Arhitecture Regenerative Design of the Built Environment, Yaguchi Lab Srey Sokuncharia 5220AG06

Acknowledgement This research would not be possible if not for the Monbukagakusho (MEXT) Scholarship so I want to thank the Japanese government for providing me with financial support throughout my studies. It is because of this scholarship that I am able to fulfill my wish of continuing to higher education and immerse in the study of a field that I love.

I want to express my deepest gratitude to my academic supervisor professor Tetsuya Yaguchi for taking me under his wing and providing invaluable advice and feedback that made this thesis possible. Without his tireless guidance to steer me in the right direction, I would not have been so passionate and immersed in my work. The relaxed and communal atmosphere that he cultivated in the lab amongst his pupils also greatly helped me during my research. On that note, I want to extend my thank to a particular lab-mate, Joy, for the time she spent on encouragement and idea-exchange with me.

Last but not least, my heartfelt love and gratitude to my parents and little brother who are endlessly understanding when it comes to my pursuit for higher education. I am neither the most filial of daughters nor the kindest sister but I hope you are proud of me and how far I’ve come.

Acknowledgement

a

Table of Contents Acknowledgement �������������������������������������������������������������������������������������a List of Figures ������������������������������������������������������������������������������������������� e List of Tables �������������������������������������������������������������������������������������������� g Introduction ���������������������������������������������������������������������������������������������1 1.1

Overview

2

1.2

Existing studies

2

1.3

Problem statement

3

1.4

Research goals and flow

3

Research Background : The Peri-urban Area �����������������������������������������5 2.1

A brief introduction to the peri-urban area

6

2.1.1​​Peri-urban delineation

6

2.1.2 Hydrological dynamic between the peri-urban area and the urban area

7

2.2

The peri-urban area of Phnom Penh

7

2.2.1​​Phnom Penh’s peri-urban delineation

8

2.2.2 Peri-urbanization trend of Phnom Penh

9

I. Historic trend

10

II. Contemporary Trend

11

2.2.3 The need for sustainable peri-urbanization practice

13

Research Background: Flooding in Phnom Penh ���������������������������������14 3.1

Flooding issues in Phnom Penh

15

3.2

Limitation of flood management in Phnom Penh

16

3.2.1 Insufficient grey infrastructure

16

3.2.2 Reliance on the peri-urban area

16

3.2.3 Limited flood protection projects

17

3.2.4 Lack of policies and regulations

18

3.3

Peri-urbanization effect on flooding in Phnom Penh

18

3.3.1 Peri-urban flooding

18

3.3.2 Flood risk in urban core

18

3.4

Existing local adaptations to flooding in the peri-urban community

19

3.4.1 Coping strategies

19

3.4.2 Adaptations

19

Study Area: Boeung Cheung Ek (BCE) ���������������������������������������������������21 4.1 Boeung Cheung Ek as Phnom Penh’s flood management system

b

22

4.1.1 From the urban core to Boeung Cheung Ek

23

4.1.2 From Bassac River to Boeung Cheung Ek

23

Table of content

4.3

Official demarcation of Boeung Cheung Ek

24

4.4

Development plan for Boeung Cheung Ek

24

4.4.1 Flexibilities in development

25

Methodology �������������������������������������������������������������������������������������������26 5.1

Research questions and objectives

27

5.2

Research design

27

5.2.1 Analysing the peri-urban area’s contribution to flood management

27

5.2.2 Determining the suitable peri-urbanization pattern

28

Peri-urban contribution to flood management �����������������������������������29 6.1

Flood susceptibility mapping method

30

6.2

Weighted sum model

31

6.3

Flood conditioning factors

31

6.3.1 Factors before BCE development

32

I. Spatial processing

32

II. Hydrological processing

32

III. LULC and landscape coverage criteria

33

6.3.2 Factors after BCE development

33

6.3.3 Rainfall

34

6.4

Results

34

6.4.1 Increased flood risk

34

6.4.2 Changes in urban hydrology

35

6.5

Discussion

36

6.5.1 Cause of increased flood risk

36

6.5.2 Landscape coverage as main flood protection factor

39

6.5.3 Conclusion

40

Suitable peri-urbanization pattern �������������������������������������������������������41 7.1

Green infrastructure

42

I. ‘Attached’ green infrastructure type

43

II. ‘Detached’ green infrastructure type

44

III. ‘End-of-pipe’ green infrastructure type

45

7.1.1 Green infrastructure movement in Phnom Penh

46

7.2

Scenario planning

46

7.3

Focal issue & scenario driver

47

7.3.1 Landscape coverage guidelines

47

7.4

I. Cambodia’s guideline to landscape coverage

47

II. Impervious cover model (ICM)

49

Scenarios

50

7.4.1 Existing regulation scenario

50

Table of content

c

7.4.2 Mild landscape coverage scenario

51

7.4.3 Moderate landscape coverage scenario

52

7.4.4 Intense landscape coverage scenario

53

7.5

Testing and results

57

7.5.1 Overall flood risk of each scenario

57

7.5.2 Flood risk of each scenario in ING City

57

7.6

Discussion

58

7.6.1 Suitable peri-urbanization pattern

58

7.6.2 Expected challenges

62

7.6.3 Vision

63

Chapter 8 �������������������������������������������������������������������������������������������������66 Summary ���������������������������������������������������������������������������������������������������66 8.1

Summary of findings

67

8.1.1 Peri-urban’s contribution to flood management

67

8.1.2 Suitable peri-urbanization pattern

67

8.2

Further studies

69

Reference �������������������������������������������������������������������������������������������������71 Appendix ���������������������������������������������������������������������������������������������������76

d

Table of content

List of Figures Fig 1.1 Location of Phnom Penh in relation to the major water bodies in Cambodia ����������������������������������� 2 Fig 1.2 Research flow �������������������������������������������������������������������������������������������������������������������������������������4 Fig 2.1 Administrative map and boundary changes of Phnom Penh ������������������������������������������������������������� 8 Fig 2.2 Phnom Penh’s peri-urban area defined by EO4SD ���������������������������������������������������������������������������9 Fig 2.3 Delineation of Phnom Penh’s peri-urban area in this study ��������������������������������������������������������������� 9 Fig 2.4 The colonial period (1890 - 1937) ���������������������������������������������������������������������������������������������������10 Fig 2.5 The independence period (1943 - 1958) ����������������������������������������������������������������������������������������� 11 Fig 2.6 The peace and rehabilitation period (1943 - 1958) ��������������������������������������������������������������������������� 11 Fig 2.7 Current mega construction projects in Phnom Penh �����������������������������������������������������������������������12 Fig 2.8 Loss of wetlands (2003 - 2018) �������������������������������������������������������������������������������������������������������12 Fig 3.1 Maximum flood extent of Phnom Penh �������������������������������������������������������������������������������������������15 Fig 3.2 Land reclamation and water evacuation �������������������������������������������������������������������������������������������16 Fig 3.3 Regulation ponds and flow of water �����������������������������������������������������������������������������������������������17 Fig 4.1 Existing wetland system and reclaimed land �����������������������������������������������������������������������������������22 Fig 4.2 Boeung Cheung Ek’s existing flood management ���������������������������������������������������������������������������23 Fig 4.3 Change in demarcation of Boeung Cheung Ek (2) �������������������������������������������������������������������������24 Fig 4.4 ING City’s conceptual land use masterplan �������������������������������������������������������������������������������������25 Fig 6.1 Schema of Phnom Penh’s flood susceptibility mapping, before and after BCE development ����������� 31 Fig 6.2a Flood risk map of Phnom Penh before BCE’s development ����������������������������������������������������������� 35 Fig 6.2b Flood risk map of Phnom Penh after BCE’s development ������������������������������������������������������������� 36 Fig 6.3a Flood risk and LULC maps of BCE after development ������������������������������������������������������������������� 37 Fig 6.3b Flood risk map of BCE after development �������������������������������������������������������������������������������������37 Fig 7.1a Evolution of urban drainage management �������������������������������������������������������������������������������������42 Fig 7.1b Urban drainage practices classification, their focuses, and specificities ����������������������������������������� 42 Fig 7.2a Green roofs ������������������������������������������������������������������������������������������������������������������������������������43 Fig 7.2b Living wall with infiltration trench �����������������������������������������������������������������������������������������������������44 Fig 7.2c Rain barrel ���������������������������������������������������������������������������������������������������������������������������������������44 Fig 7.3a Enhanced tree pit ���������������������������������������������������������������������������������������������������������������������������44 Fig 7.3b Permeable pavement ���������������������������������������������������������������������������������������������������������������������44 Fig 7.3c Rain garden �������������������������������������������������������������������������������������������������������������������������������������44 Fig 7.3d Dry swale and wet swale �����������������������������������������������������������������������������������������������������������������45 Fig 7.3d Dry swale and wet swale �����������������������������������������������������������������������������������������������������������������45 Fig 7.4b Constructed wetland �����������������������������������������������������������������������������������������������������������������������45 Fig 7.4c Infiltration trench �����������������������������������������������������������������������������������������������������������������������������46 Fig 7.5 Relationship between impermeable surface and runoff ������������������������������������������������������������������� 47 Fig 7.6 Schema of scenario planning to determine a suitable peri-urbanization pattern ����������������������������� 48 Fig 7.7 Visuals of the existing regulation scenario (baseline for comparison) ������������������������������������������� 55 Fig 7.8 Visuals of the mild scenario �������������������������������������������������������������������������������������������������������������55 Fig 7.9 Visuals of the moderate scenario �����������������������������������������������������������������������������������������������������56 Fig 7.10 Visuals of the intense scenario �������������������������������������������������������������������������������������������������������56 Table of content

e

Fig 7.11a Phnom Penh’ s flood risk map (‘existing regulation’ BCE’s development scenario) ������������������� 59 Fig 7.11b Phnom Penh’ s flood risk map (‘mild’ BCE’s development scenario) ����������������������������������������� 59 Fig 7.11c Phnom Penh’ s flood risk map (‘moderate’ BCE’s development scenario) ��������������������������������� 60 Fig 7.11d Phnom Penh’ s flood risk map (’intense’ BCE’s development scenario) ������������������������������������� 60 Fig 7.12a ING City’s flood risk map (existing regulation scenario) ������������������������������������������������������������� 61 Fig 7.12c ING City’s flood risk map (moderate scenario) �����������������������������������������������������������������������������61 Fig 7.12b ING City’s flood risk map (mild scenario) �������������������������������������������������������������������������������������61 Fig 7.12d ING City’s flood risk map (intense scenario) �������������������������������������������������������������������������������61 Fig 7.13a Public/green space and water resources �������������������������������������������������������������������������������������64 Fig 7.13b Mixed-used / mixed commercial ���������������������������������������������������������������������������������������������������64 Fig 7.13c Low rise residential �����������������������������������������������������������������������������������������������������������������������65 Fig 8.1 Conceptual framework of study on the peri-urban contribution to flood management ��������������������� 68 Table 8.1 Proposed landscape coverage ratio for all three scenarios ����������������������������������������������������������� 69 A-Fig. 4.1 ING City’s 2017 conceptual land use masterplan �������������������������������������������������������������������������77 A-Fig 6.2a Elevation before BCE’s development �����������������������������������������������������������������������������������������78 A-Fig 6.4b LULC before BCE development �������������������������������������������������������������������������������������������������78 A-Fig 6.3a Catchment and natural stream network before BCE’s development ����������������������������������������� 78 A-Fig 6.2b Elevation after BCE’s development ���������������������������������������������������������������������������������������������78 A-Fig 6.4c LULC after BCE development �����������������������������������������������������������������������������������������������������78 A-Fig 6.3b Catchment and natural stream network after BCE’s development ��������������������������������������������� 78 A-Fig 6.5 (a - i) Spatial and hydrological flood inducing factors, before BCE’s development ����������������������� 83 A-Fig 6.6 (a - i) Spatial and hydrological flood inducing factors, after BCE’s development ������������������������� 83 A-Fig 6.7 Rainfall flood inducing factor of Phnom Penh (2011 - 2020) ��������������������������������������������������������� 83

f

List of Figures

List of Tables Table 2.1 Phnom Penh density by Khan �������������������������������������������������������������������������������������������������������8 Table 4.1 Company and project involved in ING City �����������������������������������������������������������������������������������25 Table 6.1 Description of data used and derived �������������������������������������������������������������������������������������������30 Table 6.2 Flood susceptibility ranges and rating �������������������������������������������������������������������������������������������32 Table 6.3a Flood risk area in all of Phnom Penh, before and after BCE development ������������������������������� 38 Table 6.3b Flood risk area in the urban core area, before and after BCE development ����������������������������� 38 Table 6.3c Flood risk area in the peri-urban area, before and after BCE development ����������������������������� 38 Table 6.3d Flood risk area in the study area, before and after BCE development ������������������������������������� 38 Table 7.1 Guideline on land use type, FAR, BCR, and landscape coverage in urban area of Cambodia ����� 49 Table 7.2 Impervious and pervious cover index of ICM ���������������������������������������������������������������������������������49 Table 7.3 Landscape coverage index and flood risk level of existing regulation scenario ������������������������� 51 Table 7.4 Landscape coverage index and flood risk level of the mild scenario ������������������������������������������� 52 Table 7.5 Landscape coverage index and flood risk level of the moderate scenario ����������������������������������� 53 Table 7.6 Landscape coverage index and flood risk level of intense scenario ��������������������������������������������� 54 Table 7.7a Flood risk area in all of Phnom Penh in every scenario ������������������������������������������������������������� 58 Table 7.7b Flood risk area in the study area in every scenario ������������������������������������������������������������������� 58 A-Table 6.1 LULC classification system for use with remote sensor data ����������������������������������������������������� 79 A-Table 6.2 Cohen’s Kappa (Accuracy assessment of Phnom Penh’s LULC detection) ����������������������������� 79

List of Tables

g

Chapter 1 Introduction

1.1

Overview

In recent years, rapid urbanization has generated more surface runoff due to the increase in impervious cover, making urban flooding an even bigger issue (DeFries, & Eshleman, 2004) (Bhaduri, Harbor, Engel, & Grove, 2000), and developing nations are the most impacted by this problem (Miguez, Rezende, & Veról, 2015). Cambodia is one such country and its capital city, Phnom Penh, is greatly affected by urban flooding especially because the city’s urban development is closely related to its water bodies (Vann, 2004).

Phnom Penh is a flood-prone city in the Cambodian floodplain, located at the point of confluence of four major rivers: Tonlé Sap River, Mekong River, and the Bassac Rivers. Its development from a rural landscape to an urban one has always been enabled by flood control in the form of systematic canal drainage, dike construction, land reclamation, and construction on raised earth (Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004). Even now, the city is still expanding at an accelerated rate into the surrounding wetlands and lakes in the peri-urban area and this trend is expected to continue (GGGI, 2016) (Percival & Waley, 2012). Existing studies indicate that Phnom Penh city is inherently weak against flood hazards as it relies heavily on grey infrastructure and the natural flow of water to evacuate stormwater out of its interior. It also depends solely on natural water networks in the peri-urban area such as wetland, marshes, and lakes for water retention and treatment (Pierdet, 2008). However, as the peri-urban arena is being urbanized, this poses a problem in terms of flood management in the city.

1.2

Existing studies

Many research in the developed world demonstrated that the peri-urban has a close relationship with the urban area and its natural assets, as well as the urbanization pattern, can greatly influence the city. Unsustainable peri-urbanization practice and unregulated urban sprawl can drastically alter not only the ecosystem service but also the socio-economic structure and cultural dynamic of the city as well (Ravetz, Fertner, & Nielsen, 2013) (UNESCO, 2014). Furthermore, recent reports on Phnom Penh’s peri-urban areas show that more than 65% of the surveyed peri-urban area experiences flooding for an extended period that greatly exceeds the urban area and it currently retains most of the stormwater from the urban core as well as the surrounding area (Flower & Fortnam, 2015). Therefore, this implies that flooding in the peri-urban area reduces flooding in the urbanized area, and, for this reason, there should be further studies into how to sustainably develop the peri-urban area so that it can be transformed into a powerful tool for flood management.

Fig 1.1 Location of Phnom Penh in relation to the major water bodies in Cambodia

Previous research on how to sustainably achieve urban resilience against flood hazards are also mostly focused on developed nations and even though cities in developing countries, such as Cambodia, are more vulnerable to flooding, little has been written about them. In Cambodia in particular, existing studies on flooding and stormwater management are heavily centred around the insufficient drainage system (Pierdet, 2008) and, in the case of the peri-urban context, water treatment (Irvine, Sovann, Suthipong, Kok, & Chea,

Ch1 Introduction

2

2015) (Sovann, Irvine, Suthipong, Kok, & Chea, 2015). Meanwhile, research on the peri-urban area of Phnom Penh are still mainly focused on agriculture and urban risks such as water insecurity and water sanitation (Flower & Fortnam, 2015). Although there are indications by some studies (STT, 2019) (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019) of the importance of the peri-urban’s assets in terms of flood control, there is no concrete evident to back up this claim. There is a gap in knowledge in how the landscape interface and the spatial structure of the peri-urban parameter can positively contribute to the sustainable development of cities. So, it is important to investigate the role that peri-urban areas can play in helping the cities achieve urban resilience against flood events.

1.3

Problem statement

The target area is the Boeung Cheung Ek wetlands area (herein referred to as BCE), located in the south of Phnom Penh. It is officially demarcated at 107ha but actually spans up to 2859ha, consisting of several lakes, rivers, lagoons, and flood plains. This wetland is a green and blue space that provides climate cooling function, spawning ground for fishes, and supports aquatic agricultural farming. In addition to its environmental and agricultural role, the Boeung Cheung Ek also serves as an important stormwater retention and wastewater treatment facility, handling up to 70% of the rainwater and wastewater from Phnom Penh city (STT, 2019) (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, (2019).

The lifestyle here used to be agricultural, and the land-use pattern remained predominantly rural (Beckwith, 2020) (STT, 2019) despite its existence within the city’s boundary. However, as the city develops, Boeung Cheung Ek is slowly reclaimed in the usual urban expansion practice of Phnom Penh and the formal urbanization of this area will result in a satellite city called ING City. Loss of this peri-urban wetland area from unsustainable development is expected to result in a devasting flood within Phnom Penh city as well as downstream areas (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, (2019). Therefore, it is essential that the flood management capacity of this peri-urban area be properly studied and to ensure that it is sustainably developed.

1.4

Research goals and flow

As shown in the previous sections, it is clear that there is limited research on the peri-urban arena so the knowledge on how peri-urban area can help manage flooding is lacking as well. Additionally, the effect of large-scale developments in this area can affect the city, especially its hydrological dynamic, as a whole is yet to be understood and proven. There is also a lack of formal study on how stormwater management techniques can be introduced into peri-urbanization practices to achieve sustainability and resilience against urban flooding. So, this study will contribute to a better comprehension of how the changes in peri-urban areas and their natural ecosystem services can influence the whole city, especially in terms of flooding.The results can serve as a platform for further discussion on how peri-urbanization practice effects the urban hydrology and how to utilize the peri-urban’s natural asset for flood control effectively. Furthermore, it can be used to create a framework to integrate the peri-urban area into the urban fabric, as a tool for stormwater retention, to achieve urban resilience against flood hazards for cities in low-lying regions. It could aids the sustainable development of cities in developing countries as they have similar urban development trends to Phnom Penh.

The assumption of this research is the following: Sustainable development of the peri-urban area and maintenance of its existing landscape asset can contribute to flood management.

The hypothesis is as follow: Peri-urbanization and flooding are interconnected and unavoidable; however a sustainable development practice that balances the urban built-up area while maintaining the natural landscape interface can contribute to flood management of the whole metropolitan area.

3

Ch1 Introduction

This thesis is composed of eight parts and its structure is shown in Fig 1.2.

Introduction 1

An introduction that presents the necessity of this research by giving an overview, explaining the existing studies and problem statement, and the research goals

Peri-urban area 2

Information on the peri-urban area (its general definition as well as specific delineation in Phnom Penh) and a study on Phnom Penh’s peri-urbanization trend, pointing to the need for sustainable peri-urban development practice in order to ensure continuous adequate flood management system.

Flooding issues 3

Illustration of the general flooding problems of Phnom Penh and how new peri-urban development projects influence flooding within the city.

Literature review

Study area 4

Background research on the site (BCE): Emphasize its importance to Phnom Penh in terms of flood control, therefore the need to study this area, and present the intended development plan for BCE.

Methodology 5

Demonstration of research questions and research objectives as well as the research design: (a) how to study peri-urban’s contribution to flooding (b) how to determine the best development pattern for peri-urban area

Analyze peri-urban area’s contribution to flood management 6

Study how BCE contributes to flood control of Phnom Penh by generating flood risk maps of the city before and after the development of BCE.

Suitable peri-urbanization pattern 7

Determining the landscape coverage pattern and stormwater management techniques that are suitable for flood management while also capable of meeting the high pressure for development in the peri-urban area

Tools: ArcGIS Pro 2.8.3

Tools: Scenario planning ArcGIS Pro 2.8.3

Summary 8

- Summary of findings - Further studies Fig 1.2 Research flow

Ch1 Introduction

4

Chapter 2 Research Background : The Peri-urban Area

2.1

A brief introduction to the peri-urban area

2.1.1​​ Peri-urban delineation As cities develop, they continue to expand past their boundaries, spilling into the nearby countryside and transforming the adjacent landscape into a mixture of urban-rural ones. In the case of developed nations, particularly European ones, this kind of mixed-use zone of urban usage intermingling with a rural morphology is considered as a ‘peri-urban area’ as long as it is under the influence of and/or in the jurisdiction of an urban area (Caruso, 2001) In developing countries, newly urbanized territories at the urban edge with a transitional landscape of the city (infrastructure and services) mixed with rural attributes (natural and/or agricultural landscape) are referred to as ‘peri-urban interface’ (Adell, 1999) (McGregor, Simon, & Thompson., 2006).

Furthermore, in most developing nations, it is a place with organic or unregulated urbanization that is shifting from a rural lifestyle toward an urban one which usually leads to urban sprawl and usually facing social and environmental problems (Hudalah & Firman, 2011) (Ravetz et al., 2013).

In North America, urban expansion into the urban fringes has created Edge Cities that are an agglomeration of shops, businesses, and entertainment facilities mixed with the semi-rural communities or residential suburbs can be considered as a form of peri-urban area (Ravetz et al., 2013) (Garreau, 1991). Meanwhile, in developing Asian nations like Indonesia, the peri-urban area grew from an urban core into its peripheral surrounding, including new towns (Hudalah & Firman, 2011). Similar phenomenon has also been observed in East Asia, particularly in China where housings and industrial development encroach upon the farmland territories at the outskirt of the cities (Pengjun, Bin, & Woltjer, 2009).

Besides its physical attributes, the peri-urban area is also characterized by social features, such as population density, demographics, or cultural dynamics, and economic activities, which could be derived from its market factors or usage in relation to the surrounding parameter. (Iaquinta & Drescher, 2000) This area usually has a relatively low density by the urban standard, with some studies determining the density threshold of between 40 per km2 and settlements that contain less than 20,000 people (Ravetz et al., 2013) (Pior, Ravetz, & Tosics, 2011). In addition, the peri-urban interface is often associated with poor spatial and urban governance issues which include scattered settlements and fragmented communities of gated communities and unplanned development as well as the lack of urban infrastructures and services (Hudalah & Firman, 2011) (Woltjer, 2014). Migrants and the urban poor can be found aggregating within this zone and they are frequently faced with threats of gentrification, segregation, and inequality (Iaquinta & Drescher, 2000) (Woltjer, 2014).

However, in spite of these issues, the peri-urban area is a valuable space with great potential for urban growth, mostly due to its unique mosaic-like land use pattern and availability of vacant land (Woltjer, 2014) (Bertrand, 2007). Additionally, the pressure to reclaim and develop wetlands and other natural habitats surrounding the cities have always been high as they are considered to be an inhospitable and insalubrious environment (Miller & Klemens, 2005). For instance, many cities in Asia have seen large-scale peri-urbanization thanks to the influx of foreign direct investments (Woltjer, 2014). In the developing nations of Southeast Asia such as Jakarta, Indonesia, Hanoi and Ho Chi Minh, Vietnam, and Phnom Penh, Cambodia, the peri-urban interface is appropriated for privatized urban growth (Leaf, 2002). In the case of Phnom Penh in particular, peri-urbanization is driven by foreign investors as well as local tycoons spread into the city edge in the form of gigantic satellites cities that dominate the urban landscape (Woltjer, 2014) (Percival & Waley, 2012).

In conclusion, the peri-urban area should not be seen simply as a transitional zone between the city and the countryside but as an entity of its own, with its own sets of problems, yet still interdependent and interwoven with the larger urban fabric (Ravet et al., 2013). It can therefore be defined as a dynamic hybrid area within the urban confinement, with potential for urban expansion and under high pressure to develop, that is still somewhat rural in nature and lifestyle but is undergoing a transformation toward a more urban one (Bertrand, 2007) (Taylor & Hurley, 2016).

Ch2 The peri-urban area

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2.1.2 Hydrological dynamic between the peri-urban area and the urban area It is an undeniable fact that the peri-urban interface originates from the urban area due to the ever-expanding nature of the cities. Factors such as changes in demographics and social dynamics, economic growth and job patterns, environmental dynamics, and constraints, as well as the built environment and urban infrastructures not only drive peri-urbanization but also shape it (Ravet et al., 2013). Additionally, the boundaries of this zone are flexible and temporal, evolving in tandem with its associated city as urbanization continues to sprawl into the surrounding hinterlands (UNESCO, 2014). Therefore, it is safe to conclude that the peri-urban interface is vastly influenced by the urban area and it will always be shaped by and defined in relation to the urban area.

However, that is not to say that the peri-urban and urban relationship is a one-sided affair. Urban development ignites alterations in the peri-urban and rural arena and the local responses to urbanization can, in turn, affect the city’s environment as well. This mutual relation can create an inter-urban or regional agglomeration effect where the peri-urban area is linked with the urban core not only via its proximity but also through other means such as legal and institutional issues, land-use changes, socio-economic restructuring, and cultural dynamics (Ravet et al., 2013).

Furthermore, it has also been noted that the urban territories and their peri-urban hinterlands are closely intertwined in terms of natural resources and ecosystem services. Changes within the peri-urban areas not only impact themselves but the surrounding urban and rural areas as well, tampering with regional food production, energy supplies, water supplies, natural habitats, and ecosystem functions (UNESCO, 2014). Many problems associated with hydrology have also been observed in the peri-urban catchment and overall urban watershed after peri-urbanization occur, (Braud, Flectcher, & Andrieu, 2013) especially that of increased flood risk which results in greater challenges for flood hazard mitigation (Rose & Peters, 2001).

Take the case of the southern U.S. metropolitan areas such as Washington DC-Baltimore, MD and Atlanta, GA for example. The continuous reclamation of wetlands and tree covers in the peri-urban areas of these cities not only generates a greater amount of urban runoff but also doubling and quadrupling the peak flows within the urban catchment as well as causing a more frequent occurrence of extreme flow events (Driscoll, Clinton, Jefferson, Manda, & McMillan, 2010). Meanwhile, in developing countries like Sri Lanka, the peri-urbanization of the Gampaha district, situated in the northeast of Colombo, the capital city, brought about numerous negative environmental impacts, especially urban flooding. Unplanned and uncontrolled filling of low-lying lands in the Gampaha district caused areas within the city’s parameter to be waterlogged and further exacerbated flooding problems in the urban territory during the rainy seasons (Dangalle & Närman, 2005).

As more and more of the peri-urban’s semi-natural and natural landscape surfaces are converted into impervious urban land use, its hydrological cycle and water balance are severely transformed, putting itself and its associated urban area at greater risk of flooding (Shuster, Bonta, Thurston, Warnemuende, & Smith, 2005) (Fletcher, Andrieu, & Hameul, 2013). It is estimated that peri-urbanization occurs at four times the rate of urbanization and it is expected that this trend will continue (Pior et al., 2011). Therefore, it is imperative to maximize water infiltration and retention in the peri-urban landscape interface as it has the potential for mitigating urban flood risks (Kalantari, Ferreira, Walsh, Ferreira, & Destouni, 2017).

In brief, it seems that in both the developed North and developing South, sustainable development of the peri-urban area is crucial since it is proven that modification of its landscape interface and spatial infrastructure have a big impact on the urban area’s environment and ecology.

2.2

The peri-urban area of Phnom Penh

Phnom Penh is a low-lying city with flat terrain that is a part of the Mekong Floodplain & the point of confluence of four rivers: Tonlé Sap River, Upper Mekong River, Lower Mekong River, and the Bassac River. It began as a village along the bank of the Mekong River in the early 15th century and only experience rigorous 7

Ch2 The peri-urban area

urban expansion in the early 1990s. Phnom Penh developed from a city with 4 districts (khans), protected from flood within the inner dike, to a city with 10 districts in 2010, extending well beyond the outer dike. Now, Phnom Penh has 12 districts, sprawling even further west into the floodplain. (Fig 2.1) (Igout & Dubuisson, 1993) (JICA, 2016) (World Bank Group, 2017) (NCDD, n.d.)

Due to the addition of new administrations, its population increased 1.5 times from 1998 to 2008 and the area of the built-up urban zone and permeable rural territory also grew exponentially (Table 2.1) (JICA, 2016) (NIS, 2013).

2.2.1​​Phnom Penh’s peri-urban delineation As the peri-urban territory is indistinct and irregular in general, seen as an urban-rural continuum that spreads from the city core into the rural-urban fringe, peri-urban delineation varies depending on spatio-temporal, anthropogenic, and institutional context. (Simon, 2008) The same can also be said about the outline of the peri-urban interface in Phnom Penh as well since its boundary varies according to reports and discussions, especially since Cambodia, in general, is still predominantly rural and Phnom Penh’s boundary itself continuously expanded over time.

A report by Earth Observation for Sustainable Development (EO4SD) used the 2010 boundary of the city to delineate the peri-urban interface of Phnom Penh. Despite the lack of definition, EO4SD outlined the peri-urban zone as the exterior area surrounding the city which was part of the Kandal province in 2010 (Fig 2.2) (EO4SD, 2019).

Fig 2.1 Administrative map and boundary changes of Phnom Penh Source: (EO4SD, 2019) & (Phnom Penh City Hall) Table 2.1 Phnom Penh density by Khan

Source: Phnom Penh City Hall

The Ministry of Land Management, Urban Planning and Construction (MLMUPC) vaguely defined the peri-urban area as the outskirt area at the fringe of Phnom Penh. It is a vulnerable place, still rural in its morphology and severely lacking in urban amenities, where rural-urban migrants usually settled down in informal and poor urban communities. However, because of the city’s fast pace economic development, these areas at the city edge are under high pressure to urbanize (General Department of Housing, 2016). Meanwhile, a report by World Vision placed the urban poor communities in the outer Khans such as Dang Kao, Meanchey, Por Senchey, Russey Keo, and Sen Sok. In these districts, there exist numerous informal setCh2 The peri-urban area

8

tlements that have from 10 households to 400, settling on wetlands, canals, and riverbanks. (World Vision, 2015).

Papers by the World Bank Group also consistently located the peri-urban area within wetlands and floodplain in the outer Khans of Meanchey, Dang Kao Por Senchey, Sen Sok, Russey Keo, Prek Pnov, Chroy Changvar, and Chbar Ampov (World Bank Group, 2017) (Menzies, Ketya, & Adler, 2008).

A few academic papers written on the peri-urban area of Phnom Penh refer to the exterior Khans as the peri-urban zone (Percival, & Waley, 2012) while others consider the wetland system of Boeung Cheung Ek as the peri-urban interface (Sar, Chervier, Lim, Cristy, & Warrender, 2010) (Ro, Sovann, Bun, Yim, Bun, Yim, & Irvine, 2020).

Despite varying delineation, one certain thing that can be said about the peri-urban area of Phnom Penh is that its morphology is largely rural; its land use is mostly natural and agrarian, usually waterlogged, and under high pressure to develop. Therefore, in this paper, the peri-urban interface is defined as any natural and semi-natural (agricultural) area beyond the outer dike. Natural landscape between the outer dike and the city core can be considered as inner peri-urban landscape. The delineation of Phnom Penh’s peri-urban area is depicted in Fig 2.3.

Fig 2.2 Phnom Penh’s peri-urban area defined by EO4SD Source: (EO4SD, 2019)

Fig 2.3 Delineation of Phnom Penh’s peri-urban area in this study

2.2.2 Peri-urbanization trend of Phnom Penh Its location within the flat alluvial floodplain on the bank of the Mekong River, provides many benefits, including economic, transportation, and agrarian. However, as the city began to expand beyond the raised earth along the river’s embankment, it became clear that Phnom Penh will be faced with many risks, especially flooding. So, to support urban expansion while also protecting itself from the flood, the city has been developing from a rural landscape to an urban one through systemic flood control practices such as canal draining, dike construction, and land reclamation of the floodplain ever since its conception in the 15th centu-

9

Ch2 The peri-urban area

ry. (Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004) (Englund & Ryttar, 2008).

Therefore, it can be concluded that the development of Phnom Penh is akin to that of the peri-urbanization process as the city developed from an agricultural landscape to an urban one.

I. Historic trend The historical urbanization of Phnom Penh can be sorted into 6 periods as the following (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004) (Englund & Ryttar, 2008): a. Precolonial period Phnom Penh was established on the raised earth along the western bank of the Mekong River during the precolonial era. The area near the riverbank was filled in with soil extracted from further within the floodplain, creating pockets of swamps, lakes, and ponds further inland. Additionally, canals and drainage ditches were dug, and earthen walls were built around the town to form the hydraulic system for the city, proving water for the people while protecting them from daily rainy season flood as well as episodic floodplain events.

It was during this era that the plain outside the city came to have more canals and wetlands, the biggest of them was Boeung Decho, due to earth extraction to elevate the riverbanks. The method of infilling areas liable to flooding also came into practice (Igout & Dubuisson, 1993) (Englund & Ryttar, 2008). b. Colonial period (1863 - 1953) (Fig 2.4) Under the French Protectorate, Phnom Penh saw a tremendous change from a poor, rural landscape to a more modern, urban one as the city was equipped with the appropriate urban services and beautified with parks and public spaces. Road networks were created, the paved road themselves were built on raised earth, to avoid damage from flooding while also serving as protective dikes for the city. However, Phnom Penh’s development was enabled largely by land reclamation as many of the interior canals and marshes that were created pre-French occupation were drained and filled to create land for construction. Moreover, the raised earth created by the roads caused water to fill the hollows, increasing flood risk and, therefore, had to be filled, consequently providing new land for development and encouraging urban expansion.

It was during this period that Phnom Penh developed a systematic policy of draining and filling in wetlands and canals to urbanize the city (Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004).

Fig 2.4 The colonial period (1890 - 1937) Source: (Ministere de la culture, Departement des affaires internationales, 1997)

c. Independence period (Fig 2.5) The independence era is the heyday of urban development in Phnom Penh’s history. The city’s built-up area expanded north toward the Tonlé Sap lake and south along the Bassac river as economic development Ch2 The peri-urban area

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and industrialization spurred on urbanization. An airport, a national stadium, new markets, and other modern amenities, as well as factories and powerplants, were also introduced into the city’s boundary. The city’s hinterland was developed into residential areas while the riverfront was cleared of floating villages and replaced with large-scale development like the Bassac National Theater and housing development.

Despite its achievement, Phnom Penh also sprawled from the center and encroAched further upon the wetlands and lakes as its built-up area doubled. The riverfront improvement project protrudeD into the river, reclaiming dozens of hectares of the river at the junction of the Tonlé Sap and the Bassac River(Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004). d. Khmer Rouge period Not much can be said about this period as the people were forced out of Phnom Penh and the city was abandoned, causing severe destruction to its tree covers, water reservoirs, and stormwater protection system. Its hydraulic infrastructure fell into disarray as the city’s floodgates were left open to be filled with alluvial sediments (Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004). e. Vietnamese occupation period

Fig 2.5 The independence period (1943 - 1958) Source: (Ministere de la culture, Departement des affaires internationales, 1997)

The city was repopulated after the war, but it seemed to revert to its previous peasant way of life.

However, efforts were put into re-establishing urban services such as renovating the road networks and sewage systems. New housings were also built to accommodate migrants but were insufficient so pockets of slum settlements also popped up throughout the city’s urban landscape (Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004) (Englund & Ryttar, 2008). f.

Peace and rehabilitation period

(1989 – 1995) (Fig 2.6) The city began to reform in 1991 after the signage of the Paris Peace Agreement, extensive repair works taking place on government buildings, and restoring the green spaces of the city. The city center, where most businesses were situated, was also revitalized and any remaining lakes and wetlands within the core area were reclaimed as the city densified, with the exception of Boeung Kak which served as floodwater retention and wastewater treatment (Ministere de la culture, Departement des affaires internationales, 1997) (Englund & Ryttar, 2008).

Fig 2.6 The peace and rehabilitation period (1943 - 1958) Source: (Ministere de la culture, Departement des affaires internationales, 1997)

II. Contemporary Trend Ever since the early 2000s, tremendous changes were made in the natural landscape as the city’s development was largely driven by private investors who purchased public spaces, especially green spaces in the city center and wetlands at the edge, transforming them into ambitious projects sites. (Fig 2.7) Development at the urban core mostly came in the form of skyscrapers while peri-urbanization in the form of large-scale satellite cities dominated the urban fringe. These satellite cities will take up to nearly 8,000 hectares, or 12% of the city’s area, and its construction remains largely unregulated due to the lack of comprehension construc11

Ch2 The peri-urban area

tion law. Furthermore, these megaprojects were financed and implemented mostly by private foreign investors, looking to garner profit, alongside master plan development and land use planning funded by the French government (Brod, 2014) (GGGI, 2019) (Lim, 2017) (Paling, 2012) (Percival & Waley, 2012). Planning policy no longer has considerations for stormwater management and water retention, instead favoring aesthetic and return on investments. (JICA, 2015)

Fig 2.7 Current mega construction projects in Phnom Penh Source: (JICA, 2016b), (Lim, 2017), (STT, 2019), (STT, 2021)

Fig 2.8 Loss of wetlands (2003 - 2018) (Source: Sahmakum Teang Tnaut

Boeung Kak lake and its surrounding hinterlands, one of the remaining natural reservoirs in Phnom Penh’s urban core that used to host organic peri-urban community, was completely reclaimed by 2013 and the construction of the Phnom Penh City Center urban district began soon after (JICA, 2016) (GGGI, 2019) (Lim, 2017) (Paling, 2012) (Percival & Waley, 2012) (STT, 2019).

Peri-urban areas in the outskirt saw a similar fate as more and more major wetlands and their nearby agrarian landscapes were reclaimed and subsequently developed into housing projects and mega satellite cities. To the north, Boeung Reach Sey lake and a majority of the Boeung Kbal Damrey lake were reclaimed to create the Grand Phnom Penh International City (Lim, 2017) (Percival & Waley, 2012) (STT, 2019).

In the northwest region of Phnom Penh peri-urban interface, Boeung Pong Peay lake was filled in to construct the Camko City in 2005. Boeung Tamok, also located in the northwest peri-urban region, the largest natural lake that is not only the city’s rainwater reservoir and flood protection but also home to a diverse ecosystem of fishes and birds, began urbanization in 2016 (GGGI, 2019) (STT, 2019). To the south, Boeung Trabek lake shrunk as residents began to consistently occupy the lake. Meanwhile, the Boeung Cheung Ek is currently being reclaimed by various mega urbanization projects. More agricultural lands in the southern peri-urban area were also converted to built-up urbanized areas as well (JICA, 2016) (Lim, 2017) (STT, 2019) (Sokun, 2019). On the western side of the Mekong riverbank, construction works also encroached upon major wetlands such as Boeung Kham

Ch2 The peri-urban area

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Pong by Orkide Villa satellite city and the OCIC satellite city in the northeast. An island of alluvial swamps was commandeered to create the Diamond Island city on the Bassac River while, in the south-east, Boeung Snor and Boeung Chhouk were filled in to make way for gated communities and others (JICA, 2016) (STT, 2019).

The existing settlements within these peri-urban hinterlands were also relocated to the peri-urban area in the further away Khans and the people subsequently began locally and organically urbanizing those paddy fields as well as the canals and swamps. Additionally, factories and special economic zones were also settled in these outskirt areas of the city as it continued to expand into the rural landscape and, with it, came the unplanned low-income residential development to cope with the influx of population (Paling, 2012) (Percival & Waley, 2012) (STT, 2012).

The peri-urban area continues to grow exponentially since the turn of the century (Percival & Waley, 2012) and it was reported that, between 2003 and 2015, up to 50% of Phnom Penh’s lakes, wetlands, and marshes were reclaimed for the purpose of urbanization (JICA, 2015). By 2019, all 26 lakes within Phnom Penh’s jurisdiction have been impacted by housing development and satellite city constructions, with 16 of them completely filled in. There is also a (projected) loss of 41.3km2 of wetlands throughout the city to construction projects. That is 61% of the lakes and 41% of wetlands lost to the urbanization of the peri-urban areas, further straining the fragile urban ecology (Fig 2.8) (STT, 2019).

2.2.3 The need for sustainable peri-urbanization practice As peri-urbanization intensifies in Phnom Penh, the peri-urban territories have been put under the spotlight, the topic ranging from institutional and governance perspective (General Department of Housing, 2016) (Paling, 2012) (Percival & Waley, 2012) (STT, 2012) to urban poor and human rights perspective (Manzies, Ketya, & Adler, 2008) (STT, 2020) (World Vision, 2015) (Ville de Paris & Municipality of Phnom Penh, 2009) (Ville de Paris & Municipality of Phnom Penh, 2019) to economic value perspective (Ro, Sovann, Bun, Yim, Bun, Yim, & Irvin, 2020) (Sar et al., 2010). Great emphasis has also been put on the lakes and marshlands in peri-urban interfaces of the city for its invaluable flood protection service, especially that of Boeung Cheung Ek and Boeung Tamok (Menzies et al., 2008) (GGGI, 2019) (STT, 2019) (STT, 2020) (STT, 2021) (Ville de Paris & Municipality of Phnom Penh, 2009) (Ville de Paris & Municipality of Phnom Penh2019) (World Bank Group, 2017). However, despite the need to sustainably develop the peri-urban territory, the city continues to follow the trend of reclamation and construction on raised earth. Moreover, flood protection efforts in these areas are still focused on robust grey infrastructure as extensive studies and planning are undertaken by JICA on these peri-urban areas focus mainly on the drainage system and wastewater treatment (JICA, 2015) (JICA, 2016).

In the study area, Boeung Cheung Ek, in particular, urbanization began in 2013, in the form of filling in the wetlands system. The Municipality of Phnom Penh, in cooperation with Ville de Paris, published a report in 2019 on the development of the southern peri-urban area of Phnom Penh, focusing a majority of the writing on Boeung Cheung Ek. It also made some suggestions to ‘green’ the satellite city by proposing for the project to employ small-scale green infrastructure in its building design and including street trees within the development parcels (Ville de Paris & Municipality of Phnom Penh, 2019). Sahmakum Teang Tnaut (2019) (2020) (2021) also wrote about the wetlands system, emphasizing its capacity for flood control and water purification while highlighting the social injustice and human rights issue that the urbanization of this peri-urban area inflicts upon its existing residents. The organization also called for reclamation to halt and suggested that alternative options for wetlands preservation be integrated into existing development plans as their natural flood reduction and wastewater treatment outweigh the private benefit of the private development. Nonetheless,, the suggestions and calls for urbanization to cease are disregarded as intense wetland reclamation work to set the foundation for conversion of permeable wetlands to impermeable urban built-up is still ongoing.

Therefore, as the pressure to develop this peri-urban area is particularly high, it is crucial to present a comprehensive sustainable design strategy for the built-up area so that the reserved wetlands and lakes within it can continue to function effectively as flood management for the city.

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Chapter 3 Research Background: Flooding in Phnom Penh

3.1

Flooding issues in Phnom Penh

Phnom Penh has always had flooding issues and it has been reported that flooding has become more serious with disastrous flood events increasing in severity and occurring in 5 years intervals (Flower & Fortnam, 2015) (Igout & Dubuisson, 1993) (GGGI, 2019) (Vann, 2004).

Due to its location in the Mekong Floodplain at the intersection of four rivers, Phnom Penh’s topography has always been defined by interdependence between its rivers and populations and it has the inherent disadvantage against flooding. Newer developments spread from the raised riverbanks, heading further west into the swampy low-lying area filled with wetlands, Fig 3.1 Maximum flood extent of Phnom Penh lakes, and canals where flooding (Source: Mekong River Comission) has always been difficult to control.

Heavy rain from the tropical monsoon also causes localized flash floods, putting a majority of the city at risk.

Therefore, Phnom Penh is under threat of urban flash flooding from the rainy season and episodic largescale floodplain inundation events from the Tonlé Sap and Mekong rivers (Flower & Fortnam, 2015) (Igout & Dubuisson, 1993) (Ministere de la culture, Departement des affaires internationales, 1997) (Vann, 2004).

Phnom Penh’s landscape is relatively flat; a survey by JICA has shown that 30% of the city is lower than 8 meters, 45% is lower than 9 meters, and 60% is 10m lower than the river’s elevation. Since the rain can sometimes raise the water level by more than 10 meters, this results in the inundation of many areas within the urban parameter (Vann, 2004) (JICA, 2015). However, it is noted that the city does not suffer from alluvial inundation as much as other regions in Cambodia, instead frequently experiencing floods caused by excessive runoff generated by storm events. Stormwater runoff from the northern part of the country inundates the northern peri-urban area of Phnom Penh. Meanwhile, in the south of the city, floodwater can dominate from 15 to 2 0km of the peri-urban landscape from the Basaac river (Fig 3.1) (GGGI, 2019) (Vann, 2004).

Furthermore, due to the rapid urbanization taking over public green spaces and filling in the wetlands system that has always acted as natural flood control and wastewater management tool, flood events in Phnom Penh have worsened as the urban built-up area accelerates the flow velocity. The reclaimed lands are also found to be flooded during the rainy season due to inadequate drainage systems as well. The uncontrolled developments in the city worsen the already insufficient stormwater management infrastructure, increasing Phnom Penh’s vulnerability to flooding even more (GGGI, 2019) (Lim, 2017).

Additionally, it was reported that 66% of the surveyed peri-urban areas suffer from flooding with a flood duration of over 3 months per year in a third of the peri-urban peripheries. Meanwhile, only 27% of the urban area experiences inundation, 80% of it lasting less than a day (Flower & Fortnam, 2015). This less severe flooding situation is due to the extensive work done to improve the drainage pipe and channel within the urban core as well as the rehabilitation and installation of pump stations to evacuate water from the core to the city edge (JICA, 2015).

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Ch3 Flooding in Phnom Penh

All in all, Phnom Penh is a flood-prone city and inundation problems are exacerbated by urbanization of the peri-urban landscape as well as the insufficient drainage system. Loss of the natural landscape in the peri-urban area of Phnom Penh is immense and it has worsened flood events throughout the city. Evidence all point to the need to preserve the networks of canals and marshlands in the peri-urban area (GGGI, 2019) (World Bank Group, 2017) or to present alternative options for integrating stormwater management techniques into the development projects of the peri-urban interface (STT, 2020). As the country continues to rapidly develop, so too must the peri-urban region. Therefore there must be another urbanization method that can effectively balance urban development and natural landscape so that the city can maintain its resilience against flooding.

3.2

Limitation of flood management in Phnom Penh

3.2.1 Insufficient grey infrastructure Due to its vulnerabilities against flooding, Phnom Penh has continuously worked on its flood management infrastructure.

Hydraulic in-filling

Water evacuation during dry season

Water evacuation during rainy season

In the pre-colonial and colonial eras, flood control came in the form of land reclamations of vulnerable areas, dike constructions, and canal diggings. Stormwater evacuation out of the urban area depended solely on the gravitational flow of water.

From the independence period onward, pump stations were introduced to facilitate the transportation of water from the city core to the wetlands system in the urban peripheries. The city largely relied on grey infrastructure and the natural flow of water to protect itself from flood events (Fig 3.2) (Igout & Dubuisson, 1993) (JICA, 2016) (Ministere de la culture, Departement des affaires internationales, 1997) (Pierdet, 2008) (Vann, 2004).

It must also be noted that the stormwater and wastewater in of Phnom Penh are not separated.

The water from the rain and households flow into the same underground pipes and open canals, ultimately draining into wetlands in the peri-urban area. Furthermore, most of the drainage facilities are not fully functional as they are old, constructed in the 1960s, and severely mismanaged during the Khmer Rouge regime.

Most of the pipes and canals are also under dimensioned, unable to keep up with the ever-expanding city and fast-growing population (JICA, 2016) (GGGI, 2019) (Englund & Ryttar, 2008) (Pierdet, 2008) (Vann, 2004).

Fig 3.2 Land reclamation and water evacuation

3.2.2 Reliance on the peri-urban area Besides grey infrastructure, Phnom Penh has a network of intricate canals and some regulation ponds within the city center and peri-urban area (Phnom Penh City Hall, 2011) (Phnom Penh City Hall & Ambassade de France au Cambodge, 2007) The independence era also laid the groundwork for the Olympics Stadium regulation ponds in the urban core, located in the Khan 7 Makara. These ponds originated from existing low-lying ponds borrow pit, retaining runoff from upstream before discharging it downstream. However, these ponds were completely reclaimed by 2015, worsening the flooding issue in their immediate vicinity (Ministere de la culture, Departement des affaires internationales, 1997) (JICA, 2016). Another regulation pond that has been in use since

Ch3 Flooding in Phnom Penh

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the historic period is the Boeung Trabek, located in Khan Meanchey, directly above Boeung Cheung Ek. It is a natural lake and canal system that has served as the city’s water retention facility since 1960. Nonetheless, it is gradually being filled in over the past years as its peri-urban community organically encroaches on the lake (JICA, 2016) (Vann, 2004).

In the northern peri-urban area, there is the Boeung Tamok natural regulation pond. Historically, it served as a retention basin for the city, trapping water that flows down from the northern part of the country from entering the city center. It has always been effectively reducing flooding in the city and surrounding areas from excessive overland flow. In addition, after the Prek Phnov Bridge and the Tomnub Kobsrov dam were built, this natural reservoir is even more crucial to the city as more water from Phnom Penh is being pumped into the lake (STT, 2019) (STT, 2019). In the southern part, there is the study area, Boeung Cheung Ek, which was designated as another natural regulation pond of the city after its integration into Phnom Penh’s parameter.

It has been shown to be an extremely important tool for the flood protection of Phnom Penh, claims attribting up to 70% of the city’s runoff and wastewater are stored here. It can also retain a large amount of water over an extended period of time (Englund & Ryttar, 2008) (STT, 2019) (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019). However, these two wetland systems in the peri-urban periphery are being threatened by the pressure of urbanization and the risk of being completely reclaimed. (See Fig 3.3 for the locations of and flow of water into the regulation ponds)

3.2.3 Limited flood protection projects Phnom Penh has also been receiving supports from many donors for flood protection projects in recent years, namely from the ADB and JICA. Between 2000 and 2003, the ADB granted loans for a project that rehabilitated the canal drainage of Boeung Trabek and its pumping station, improving its function. Dredging work also took place in this regulation pond but ceased by 2011 due to massive opposition campaigns from the local peri-urban communities and negative media backlash. Meanwhile, JICA gave assistance to improve flood mitigation in the form of revitalizing the existing drainage system and creating new robust grey inFig 3.3 Regulation ponds and flow of water frastructures. The materials in their Source: Phnom Penh City Hall (2016) projects include drainage channels, pumping stations, underground reservoirs, and the creation of regulation ponds (GGGI, 2019) (JICA, 2016a) (JICA, 2016b). A master plan for drainage and sewerage system was also prepared by the Department of Public Works and Transport of Phnom Penh in cooperation with JICA to comprehensively ameliorate the city’s drainage and sewage system (Englund & Ryttar, 2008) (GGGI, 2019).

These projects contribute greatly to the flood protection of Phnom Penh, mainly in the urban core. The first and second phases of JICA’s assistance work successfully lessen flood damage in the southeast, southwest, and northeast of Phnom Penh’s inner Khans. However, projects and studies mainly focused on the wastewater treatment facility for the densely populated parts of Phnom Penh. The peri-urban zone of Phnom Penh is

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Ch3 Flooding in Phnom Penh

still exempted from the benefits of this robust grey infrastructure network as it has always been treated as an arena for floodwater retention and wastewater treatment. It continues to be at great risk of harsh flood events that keep on increasing in frequency (GGGI, 2019) (JICA, 2016a) (JICA, 2016b) (Flower & Fortnam, 2015).

The part within the inner dike, the city core, is mostly protected against inundation and experiences less severe flood damages. JICA is still assisting the parts of the city center that are still suffering from flooding, expecting to rehabilitate and/or construct new pumping stations, rehabilitate the existing drainage channels as well as improve the drainage pipe network of those areas. It also seems that the outer dike of the city offers some protection against flood events since drainage issues in the area between the outer dike and inner dike are less prominent. Even so, the increase in inundation frequency brought new hydraulic problems to the area and there are now requests for drainage improvement at the eastern side of Pochentong Airport, Chroy Changvar area, and Chbar Ampov area (GGGI, 2019) (JICA, 2016a) (JICA, 2016b).

3.2.4 Lack of policies and regulations Another big issue is that Phnom Penh lacks comprehensive policies and a regulatory framework in terms of flood management. Neither Phnom Penh nor the Ministry of Land Management, Urban Planning and Construction have a standard for drainage facilities for large-scale developments. Sub-Decree No.86 on Construction Permit provides that the developers are responsible for the stormwater drainage of their project.

Sub-Decree No.43 on Urbanization of the Capital City, Towns, and Urban Areas demands that there be an opportunity for onsite rainwater capture or infiltration to curtail flooding. However there are no elaborations on it. Due to this gap in law and regulations, developers of satellite cities took liberty to design drainage systems, none of them are unified. Moreover, these projects are usually not connected to Phnom Penh’s main stormwater system, putting them at risk of flooding or tampering with the city’s flood protection system (GGGI, 2019) (JICA, 2016a) (Percival & Waley, 2012).

3.3

Peri-urbanization effect on flooding in Phnom Penh

It is undeniable that peri-urbanization increases the risk of flooding and, although there is a lack of comprehensive data and mapping of the effect of peri-urbanization on the flooding situation of Phnom Penh, the same can be said of the city as reports come to the same conclusion (Doyle, 2012) (Flower & Fortnam, 2015).

3.3.1 Peri-urban flooding The increased risk of flooding due to urban development can be explicitly observed in the peri-urban area of Phnom Penh. In the peri-urban community, Chamrouen, located in Khan Chbar Ampov, at the southeast of Phnom Penh, flooding hazards shift from river flooding to development-induced inundation. This shift in the source of flooding happens due to the reclamation of the wetlands of the Chamrouen area to cope with rapid urban development. The land-use change from permeable, semi-natural surface to urban built-up increased surface runoff and flooding. In another area, Vealsbov, in the same Khan, the community also blamed rapid urbanization for the rise in flood frequency and its prolonged duration (Flower & Fortnam, 2015).

3.3.2 Flood risk in urban core Meanwhile, the loss of inner peri-urban wetlands is linked to increased inundation in Phnom Penh as well.

A flood assessment report of Boeung Kak, an inner peri-urban interface that was completely reclaimed by 2013, showed that even though it was a closed lake and marshlands system, it was critical in reducing storm runoff in the neighbouring low-lying areas. Furthermore, the report showed that urbanization of the Boeung Kak peri-urban interface will generate large volumes of runoff that have the potential to impact on property and cause a hazard to life downstream (Benham & Caddis, 2008).

A study by JICA also reported the increase of inundation experienced in the city center caused by the loss

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of the Olympics stadium regulation ponds. The simulation showed that more neighbourhoods around the regulation ponds suffer from flooding and there was an overall increase in inundation depth (JICA, 2016b).

Although lacking in data, there is a clear implication that rapid peri-urbanization and unregulated land reclamation in the area not only affect its immediate vicinity but increase flood risk in the city as a whole as well. Furthermore, this lack of discussion of peri-urbanization on flood management capacity of Phnom Penh means that a study needs to be undertaken to further understand the synergy between the peri-urban interface and flood protection of the city.

3.4

Existing local adaptations to flooding in the peri-urban community

Historically, Phnom Penh was blessed with the natural hydrological infrastructure to not only manage flooding issues but also thrive alongside the seasonal flooding. A network of lakes, canals, and wetlands that spread across the urban and dominated the peri-urban interfac. These hydrologic systems absorbed the annual floods then fed the cultivation of rice and vegetables as well as encouraging aquaculture. People have also learned to live alongside these flood events, especially those living in the peri-urban interface (Benham & Caddis, 2008) (Doyle, 2012) (Flower & Fortnam).

Nonetheless, flooding is very disruptive to the peri-urban communities as they not only affect the living environment and transportation but also the livelihood since inundation can destroy agricultural areas in which the people depend on. The community living in these high-risk areas has proven to be very adaptive to flood events. However, since the populations living in the peri-urban zone are predominantly the urban poor, their coping and adaptions strategies remain severely limited (Doyle, 2012) (Flower & Fortnam).

3.4.1 Coping strategies To cope with the flooding, the population band together to construct floating walkways in between residences and buildings. Transportation methods sometimes switched to a water-based one, people using boats to commute when roads are submerged underwater. On the household level, people would build protective fences around their homes or employ sandbags on the road in an effort to keep floodwater out of the living area. A drastic response to flooding is relocation; their small wooden houses allow for movement to higher locations. Temporary housings on higher grounds are also provided to those affected by the flooding (Benham & Caddis, 2008).

3.4.2 Adaptations In terms of adaptations, one of the main methods employed by the urban poor in the peri-urban interface is constructing their housings in the vernacular style where residences are built on stilts. Wooden houses are also strengthened with concrete foundations and constructed with platforms in the living areas, with structures on the rooftop to escape flood levels. Utilities such as water pipes also reach the rooftop structure, (Benham & Caddis, 2008) and this allows the people to live with the flooding events that occur in the area.

On the community level, adaptations are more focused on engineering solutions that follow the pattern of Phnom Penh’s general peri-urbanization trend. Such solutions include dike constructions, which can protect the village from river flash floods, and building roads on raised earth. In certain areas, raised walkways are built in place of the temporary wooden ones (Benham & Caddis, 2008).

Coping methods and adaptations to flooding in the peri-urban area are widespread but unsustainable, mainly due to its organic nature and lack of funding. The people living in the peri-urban vicinity are adapted to live with the flooding and has devised small scale strategies that aid them in living alongside flood events.

However, current urbanization tactics employed by the local peri-urban communities, mostly organic and unregulated, proved to be inconsequential in the long run (Benham & Caddis, 2008).

As these urban villages are faced with high pressure of urbanization, stronger resilience against flooding 19

Ch3 Flooding in Phnom Penh

must be achieved. However, there is also the need to maintain the existing ecosystem service of the natural landscape as its wetland system is undeniably invaluable to the city. Therefore, the built environment in the peri-urban zone must develop sustainably, being able to support a more urban lifestyle and needs while also preserving the ecological function of its landscape interface, especially that of flood management as the city depends greatly on it.

As the great Cambodian architect, Vann Molyvann, once said, Cambodia, in general, is a society of halfearth and half-water, so urbanization should not occur upon reclaimed land but through the incorporation of water into its design (STT, 2020). Phnom Penh’s development plan must respect the natural environment, paying special recognition and respect to the bodies of water that are dotted across the urban landscape as water reservoirs. These water bodies are an integral part of the city’s ecosystem and can be incorporated to achieve an attractive urban aesthetic (Vann, 2004).

No truer words have been spoken because, as the peri-urban landscape of Phnom Penh which has always served as stormwater retention facilities is turned into an arena for privatized development, peri-urbanization needs to balance urban built-up while maintaining its original flood management capacity. A sustainable way for people to live alongside the rising water must be achieved.

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Chapter 4 Study Area: Boeung Cheung Ek (BCE)

Based on existing studies done on the background of peri-urbanization and flooding issues in Phnom Penh, the most suitable study area is the Boeung Cheung Ek area (BCE). Not only is it under high pressure to develop into a satellite city (ING City) but it is also linked to flood management in the city as well.

Boeung Cheung Ek is a system of lakes, wetlands, canals, and lagoons that is located in the southern peri-urban interface of Phnom Penh, spanning parts of Khan Meanchey and Khan Dang Kao and bordering the Ta Khmao city of Kandal Province. The total wetlands system originally measured between 2,800 hectares and 2,850 hectares in the rainy season, but there is no official declaration on this number. Plus, it provides many social and environmental benefits for Phnom Penh and its inhabitants (STT, 2019) (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019).

Due to its size, Boeung Cheung Ek has halted many major large-scale developments in the southFig 4.1 Existing wetland system and reclaimed land ern peri-urban zone of the city for a long time. There Source: (STT , 2020) are two major dike-roads built around the wetland area, connecting the city to other regions in the country. Additionally, there has been some small-scale urbanization around the lake by the local communities and pockets of villages growing along the dike roads, most of which are unplanned and very rural in morphology. The local community also took to practicing agriculture and aquaculture in the vicinity of Boeung Cheung Ek, which boosts the wetland’s ability to purify sewage water before discharging it downstream (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019).

Nonetheless, ever since the early 2000s, Phnom Penh city continued to expand due to economic prosperity, an increase in population, as well as an influx of foreign investment, and the southern peri-urban interface came under immense pressure to be urbanized. For this reason, the Boeung Cheung Ek wetlands system also turned into an prized real estate for privatized development, and, as of 2020, nearly a third of the wetlands have been reclaimed. Only 1,000 hectares of the peri-urban interface of Boeung Cheung Ek is expected to remain (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019). (See Fig 4.1 for existing wetland system, water bodies, and currently reclaimed land).

4.1 Boeung Cheung Ek as Phnom Penh’s flood management system Boeung Cheung Ek has always been critical in Phnom Penh’s flood mitigation even though it’s situated downstream. Many articles have claimed that, in addition to serving as the city’s natural wastewater treatment, it handles up to 70% of the city’s stormwater. Due to its enormous size, this wetland system can retain a large quantity of water, storing up to 2.5 to 3 million cubic meters of water and raising the water level of the lake by 2 meters. During the rainy season, especially in August, September, and October, when the Mekong River’s water level reaches 7 meters to 9 meters, this wetlands system is responsible for absorbing runoff from the urban core (Khan 7 Makara, Khan Daun Penh, Khan Toul Kork, and Khan Chamkar Mon) as well as the two southern outer Khans (Khan Dang Kao and Khan Meanchey). Its water retention capacity also means that Cheung Ek is responsible for preventing flooding from occurring downstream (STT, 2019) (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019).

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4.1.1 From the urban core to Boeung Cheung Ek This peri-urban interface has always been mostly treated as a water retention and wastewater treatment basin since it is the end stream of all the runoff and wastewater from the city center. During the rainy season, especially in August, September, and October, when the Mekong River’s water level reaches 7 meters to 9 meters, 3 inner Khans, Khan Daun Penh, Khan 7 Makara, and Khan Chamkar Mon, drain their runoff and wastewater into Boeung Trabek, an inner peri-urban pocket at the edge of the urban core that sits directly on top of Boeung Cheung Ek. Water from the south of Toul Kork as well as the two southern outer Khans (Khan Dang Kao and Khan Meanchey), eventually flows into this peri-urban area via the city’s network of canals and drainages (Fig 3.3 and Fig 4.2) (STT, 2020) (Van, 2004) (Ville de Paris & Municipality of Phnom Penh, 2019).

Due to the elevated levees in the form of the road network around the wetland, water usually needed to be evacuated into Boeung Cheung Ek via the pumping stations on the northern edge of the wetland system.

Furthermore, there is also a floodgate on the Cheung Ek canal, located in the south-west side, regulating water flowing into this area (Ville de Paris & Municipality of Phnom Penh, 2019).

4.1.2 From Bassac River to Boeung Cheung Ek Boeung Cheung Ek’s water retention capacity also means that it is responsible for preventing flooding from occurring downstream. It can store water for a long period of time before releasing it downstream. This vast wetland system only has one exit, Steung Chrov, located at the south-west, that evacuates water into the Prek Thnaot stream before finally discharging into the Bassac River, a tributary of the Mekong River.

However, during the rainy season, water levels in the Bassac River rise above the water level of the Prek Thnaot stream and excess water flows via this stream into Boeung Cheung Ek. So, on top of inflow from Phnom Penh, this peri-urban landscape also receives ‘fresh water pulse’ from the Bassac River. When the wetland reaches its capacity, the floodgate at Stueng Chrov comes into play, preventing more water from the Bassac River from entering. However, it must be noted that this floodgate is not very effective, freely allowing water to travel up and down the Prek Thnaot stream to and from Boeung Cheung Ek. It is during this time that the Fig 4.2 Boeung Cheung Ek’s existing flood management wetland interface prevents Phnom Penh from exSource: (Ville de Paris & Municipality of Phnom Penh, 2019) periencing alluvial flooding (Fig 4.2) (Irvine et al., 2015) (Sovann et al., 2015) (STT, 2020) (Van, 2004) (Ville de Paris & Municipality of Phnom Penh, 2019).

In short, nearly all of the stormwater and wastewater from Phnom Penh end up in Boeung Cheung Ek. This transportation of water depends on the gravitational flow of water through various canals and drainage networks as well as pumping stations within the city. Water draining in and out of this peri-urban area is also regulated by two floodgates. During the rainy season, this wetland interface also receives water from the Bassac River as the river’s water levels rise, preventing Phnom Penh from being overtaken by riverine flooding.

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4.3

Official demarcation of Boeung Cheung Ek

Boeung Cheung Ek was first legally demarcated on September 3rd, 2008 in Sub-Decree No.124. It was declared to be at the size of 520 hectares and designated as state public property (RCG, 2008). After this initial delineation, Boeung Cheung Ek’s boundary has been revised multiple times (STT, 2020).

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February 2012: Sub-Decree No.26 ‘Amendment of Sub-Decree No. 124 2008’ removed 17 hectares from the lake surface. Although not stated to whom the land was leased to, the area was later occupied by the Chip Mong group (RCG, 2012).

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May 2017: Sub-Decree No.70 allocated 47.07 hectares of Boeung Cheung Ek to the local community as compensation. (RCG, 2017)

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January 2018: Sub-Decree No.5 ‘Amendment of the Cheung Ek Lake and Canals in Khan Meanchey and Khan Dangkao of Phnom Penh and Ta Khmao Town of Kandal Province’, an additional 30 hectares was given to ING holdings company in exchange for 10 hectares of land that the company provided to the Ministry of Interior. (STT, 2020)

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February 2018: Sub-Decree No.12 was issued, 37 hectares were given to Orkidé villa and another 10 hectares were granted to an unknown individual. (RCG, 2018a)

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December 2018: the Sub-Decree No.168 ‘Amendment of the Cheung Ek Lake and Canals in Khan Meanchey and Khan Dangkao of Phnom Penh and Ta Khmao Town of Kandal Province’ allocated nearly 20 hectares to Phnom Penh City Hall for construction of wastewater treatment facilities in cooperation with JICA. (RCG, 2018b)

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September 2019: Sub-Decree No.142 subdivided the leased land in Sub-Decree No.5 ‘Amendment of the Cheung Ek Lake and Canals in Khan Meanchey and Khan Dangkao of Phnom Penh and Ta Khmao Town of Kandal Province’ to the government and six individuals. (RCG, 2018c)

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October 2019: the Sub-Decree No.159 ‘Amendment of the Cheung Ek Lake and Canals in Khan Meanchey and Khan Dangkao of Phnom Penh and Ta Khmao Town of Kandal Province’ provisioned an additional 190 hectares to two individuals for an unknown purpose. (RCG, 2018d)

From these divisions of the Boeung Cheung Ek, only about 180 (RCG, 2018d) to 107 hectares (STT, 2020) of the once vast and biodiverse lake and wetlands system will remain completely intact. (Fig 4.3)

Fig 4.3 Change in demarcation of Boeung Cheung Ek (2) Source: (STT, 2020)

4.4

Development plan for Boeung Cheung Ek

For the last decade, there has been some formal development in the shape of gated communities, factories, and storage facilities in the southern region’s peri-urban interface of Phnom Penh. Large scale plans to urbanize the area only officially began in 2010 when ING holdings proposed a master plan for the development of the southern peri-urban area into ING city (Fig 4.4). As of 2020, there are 14 private companies Ch4 Study area

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involved in the ING city and other development projects (Table. 4.1) (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019).

Land reclamation first began in 2013 with the creation of the Hun Sen Boulevard that cuts across the length of the area. In-filling is an ongoing process in the wetland system as companies begin to execute their developments. Urbanization proposals have yet to discuss flood hazards or present a comprehensive plan for flood protection measures in detail as well (STT, 2020) (Ville de Paris & Municipality of Phnom Penh, 2019).

Table 4.1 Company and project involved in ING City

Source: (STT, 2020) & ING Holdings

However, since a majority of this wetland interface still remains and nearly all constructions are at its beginning, there is still hope yet for this peri-urban area to be developed sustainably. The local NGO, Sahmakum Teang Tnaut, has always been vocal about a balanced model of peri-urbanization that balances social justice, environmental protection, and urban developments STT, 2020).

In short, it is evident that Boeung Cheung Ek plays a crucial role in the flood protection of Phnom Penh since it is not only the end point of nearly all of the city’s runoff but also the barrier that contains the Basaac River’s alluvial flood from encroaching further into the city. However, the current urbanization method being implemented in the area is the usual unsustainable peri-urbanization trend of land reclamation. The official demarcation also points toward the reduction of the vast peri-urban interface from 2,800-2,850 hectares to a mere river of 108-107 hectares in size. The continuation of this peri-urbanFig 4.4 ING City’s conceptual land use masterplan ization trend will pose a huge flood management (ING Holdings, 2021)downstream. problem in the city and runoff generated from the developed area willSource: also impact the areas

4.4.1 Flexibilities in development ING City is still in its primary phase and still very flexible to changes. This flexibility is due to the fact that the actual development of the Boeung Cheung Ek area will not be directly undertaken by ING Holdings. Instead, ING Holdings is the owner of the area who will be parceling it and leave the actual urban development in the hands of national and international investors (ING Holdings, 2021). The conceptual zoning masterplan (Fig 4.4) merely acts as a guide for the urbanization of each land plots. (ING Holdings, 2017) It is of note that there are several discrepancies between the conceptual zoning masterplan in the brochure that was published in 2017 (A-Fig 4.1) and the current conceptual zoning masterplan displayed in the company’s official website, which is recognized by the Municipality of Phnom Penh (2019) and STT (2020). There are changes in land use plan as well as consolidations of stormwater management facilities, water reservoirs, and electrical substations between the current conceptual zoning masterplan (Fig 4.4) with the one that was first proposed in 2017 (A-Fig 4.2). The most notable change are the vast reduction in the areas dedicated to green spaces and the omission of industrial land use.

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Chapter 5 Methodology

5.1

Research questions and objectives

It is undeniable that the peri-urban and urban area have a very close relationship in terms of hydrology as development in the urban and peri-urban areas creates hydrological changes within the peri-urban catchment and overall urban watershed (Rose & Peters, 2001) (Driscoll et al., 2010). However, there is a lack of discussion about how the development in the peri-urban arena affects the flood protection capacity of the city as a whole. In Phnom Penh especially, discussion on peri-urbanization and its influence on the city in terms of flooding is superficial at best, only lightly discussing the issues of land reclamation when it comes to spatial issues. So, this thesis will investigate the extent in which the peri-urban area contributes to flood management in the metropolitan area by analysing how the development of Boeung Cheung Ek modifies Phnom Penh’s spatial character, beyond the in-filling of wetland systems, and how that affects the city’s current flood management capacity.

Recognizing the inadequate evidence of sustainability in the design schemes and regulations proposed and implemented for the peri-urban development of Phnom Penh, this research will strive to provide a suitable urbanization pattern for this area. This will begin with the exploration of how the peri-urban area can develop and in what way. Then, existing stormwater management techniques will be examined and analyse.

The discussion of how these practices can be effectively applied into the peri-urban area will be demonstrated and this will contribute in terms of understanding how the hydrological asset of the peri-urban area that aids with flood control can be preserved so that flooding issues in Phnom Penh will not worsen too greatly.

In short, this thesis is an evident-based study that seeks to understand the altered spatial character of Phnom Penh after the development of Boeung Cheung Ek and how that change affects the city’s current flood management capacity. It will also present a plausible peri-urbanization pattern that can withstand the high pressure to develop and still contribute to flood control of the whole city. The key questions that guide this research are: 1. How will the flooding situation in Phnom Penh change after the development of Boeung Cheung Ek? a. Will flood risk in the city increase or decrease, and by how much? b. What causes the change — what spatial and/or hydrological characters will be altered and what impacts do they have on the flood issue?

2. How to sustainably develop the peri-urban area while also preserving its flood management capacity? a. What are the stormwater management techniques that can maintain the existing peri-urban area’s flood control capacity? b. In what way can these stormwater management practices be introduced into the development?

5.2

Research design

This research is empirical and data driven in nature, relying in on remote sensing (RS) and geographic information system (GIS) techniques. Due to lack of digital spatial data from the Royal Government of Cambodia, most data are acquired from open sources such as Open Development Cambodia (ODC), United States Geological Survey (USGS) data portal, and Center for Hydrometeorology & Remote Sensing (CHRS) data portal.

To answer the research questions and comprehensively analyse peri-urbanization and its relationship to flooding in the city, this research will be divided into two parts which will be explained in the following parts.

5.2.1 Analysing the peri-urban area’s contribution to flood management In terms of flood-related research in the urban and peri-urban areas, many existing studies have approached it with simulation method to assess flooding, determine flood risk, or study flood mitigation.

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Existing studies on flood protection in Phnom Penh also follow this trend. The JICA study team used a simulation model to analyse the flood situation in and around the Olympics Stadium regulation ponds, a cluster of small wet ponds in the urban core, to analyse inundation in two scenarios. The two options were ‘with regulation ponds’ and ‘without regulation ponds’ (JICA, 2016b). Additionally, a flood assessment of the loss of the inner peri-urban interface Boeung Kak, within the urban core, used hydrological modelling in different development scenarios to study the rate and volume of runoff entering the drainage system in the Boeung Kak catchment (Benham & Caddis, 2008). Both analyses by the JICA team and the flood assessment of the Boeung Kak also only considered rainfall events since the city experiences floods caused by excessive runoff generated by storm events more than alluvial inundation and the likelihood of river inundation is much lower than in other regions.

Following precedents set by existing studies, this research will determine how the urbanization in the peri-urban area can affect the whole city by simulating the flood susceptibility maps of Phnom Penh, before and after the development of Boeung Cheung Ek (study area).

Detailed explanation of the flood mapping method and its processing are explained in Chapter 6.

5.2.2 Determining the suitable peri-urbanization pattern In studying the peri-urbanization effect on the urban area, whether it is environmental or socio-economical, many studies have used the scenario planning method (Nilsson, Pauleit, Bell, Aalbers, & Nielsen, 2013).

Thus, it can be said that scenario planning has always been presented in urban planning. It has aided stakeholders (officials, developers, agencies, landowners, the general public, etc.) in identifying possible development options and make informed decisions for the future by comparing and assessing different plausible stories and/or standards.

However, rather than the forecasting of any particular futures, scenario planning is an approach for generating and testing development strategies and plans in an uncertain future to facilitate informed decision-making (O’Brien, 2000). It systematically explores the predeterminations and uncertainties of a situation and their impacts on the core issue to create a plausible future (Cork, Dealny, & Salt, 2005). Furthermore, due to its hypothetical nature about the uncertainties of the future, scenario planning suits the context of wide regional planning and visioning very well (Choy, Sutherland, Gleeson, dodson, & Sipe, 2008).

Since this study’s objective is to propose a coherent picture of a suitable development pattern for the peri-urban areas that can aid in flood control in the whole city, scenario planning is an appropriate method to use. Therefore, different scenarios peri-urbanization patterns of Boeung Cheung Ek will be proposed and the effect that each scenario has on Phnom Penh’s flood risk situation will be analysed to provide an evident-based conclusion.

Detailed explanation of the method for this part is given in Chapter 7.

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Chapter 6 Peri-urban contribution to flood management

6.1

Flood susceptibility mapping method

In creating the flood susceptibility maps, many flood-inducing components are considered for spatial distribution of the hazardous areas, and they correspond to the hydraulic, hydrology, geomorphology, and land use characters. Basic parameters are precipitation rate, stream density, stream network, and elevation (Choubin, Moradi, Golshan, Adamowski, Sajedi-Hosseini, & Mosavi, 2019). Other factors are geology as well as slope, curvature, stream power index, and topographic wetness, which are a part of geomorphology (Tehrany, M., Pradhan, B., & Jebur, M., 2014).

Land use land cover (LULC) dynamic is concerned with the pervious/permeable or permeable/impermeable surface of a land use type and it has big influence on hydrological processes such as infiltration, surface runoff, evaporation, and evapotranspiration so it will be considered as an input as well (Beven & Kirkby, 1979). It was also indicated that land use relates directly to soil moisture condition since it controls the amount and the time of precipitation reaching the soil surface (Kourgialas & Karatzas, 2011) so level of porosity will also be used as an impact factor. In this study, LULC dynamic will be referred to as ‘landscape coverage’.

However, as mentioned in Chapter 3, flooding issues in Phnom Penh are primarily due to overland flow hence hydraulic factor will not be used in this analysis. Neither will the geology be considered as the geology map of the Cambodia indicates that Phnom Penh is composed entirely of young alluvium (A-Fig 6.1) (ODC, 2016).

Therefore, in this literature, flood risk map will be used to determine the contribution of the peri-urban area to urban flood management by mapping the extent of urban flooding in two scenarios: flood risk in Phnom Penh before Boeung Cheung Ek’s development and flood risk in Phnom Penh after Boeung Cheung Ek’s development, business-as-usual development pattern (BAU). The following flood conditioning factors will be considered in this thesis (descriptions of data used are illustrated in Table 6.1 while flood-inducing criteria along with their ranges and rankings are shown in Table 6.2): -

Hydrology criteria like rainfall and stream power index.

-

Spatial criteria which include elevation, distance to stream, and stream density, slope, and curvature.

-

Land use land cover criteria.

-

Permeability criteria are derived from the sealing level/impervious level of land use type within Phnom Penh combined with the topographic wetness.

Table 6.1 Description of data used and derived

Data

SRTM digitial elevation model

Descriptions

Derivables

Source: USGS EarthExplorer (07/08/21) SRTM 1 Arc-Second Global Cloud cover — 0% - 1% Spatial resolution — 30m Temporal resolution — 2014 (Manual editing of the DEM data for post-development scenario)

Landsat8 images

Source: USGS EarthExplorer (17/08/21) Landsat8 OLI TIRS Level 1 Cloud cover — 0% - 1% Spatial resolution — 30m Temporal resolution — 15/01/2021

PERSIANN-CCS

Source: CHRS (09/11/21) PERSIANN-Cloud Classification System Spatial resolution — 0.04° or 4km Temporal resolution — 2010 - 2020

Elevation Slope Curvature Distance to stream Stream density Stream Power Index Topographic Wetness Index

Land Use Land Cover Landscape coverage level (Manual editing of the LULC classification for post-development scenario)

Annual rainfall

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30

6.2

Weighted sum model

Weighted sum model, also known as weighted linear combination and weighted overlay (Malczewski & Rinner, 2015), is a multi-criteria decision-making/decision-analysis method in which the best alternative will be determined based on the evaluation of different inputs (Fishburn, 1967) (Triantaphyllou E. 2000). It has been used in extensively in Geographic Information System (GIS) for multi-criteria decision making in various fields due to its applicability to a wide range of decision and management situations. Some of the application domains of this method include, but are not limited to, environmental management, urban and regional planning, suitability model for hydrology and water resource, as well as susceptibility maps for natural hazards (Malczewski & Rinner, 2015).

In a similar vein, the weighted sum model will be used in order to analyse the different criteria and create a flood susceptibility map. The weight given to each criterion is usually proportional to the criteria’s importance to the problem at hand. In this case, there are ten flood-causing factors so each will be given a ten percent weight andand that will sum all criteria to one hundred percent. The impact of each flood-inducing element will be categorized into five different risk hazard levels: very high, high, moderate, low, and very low.

The tools in ArcGIS that correspond to the weighted sum model are Weighted Sum (Spatial Analyst and Image Analyst) and Weighted Overlay (Spatial Analyst). The schema of this flood susceptibility weighted sum model is shown in Fig 6.1. i

ii

Hydrological criteria

- Precipitation - Stream power index (SPI)

iii

Spatial criteria

Land-use/land-cover

- Elevation - Slope - Curvature - Distance to streams - Stream density

- Very high flood risk (5) - High flood risk (4) - Moderate flood risk (3) - Low flood risk (2) - Very low flood risk (1)

- LULC dynamic

iv

Permeability criteria

- Landscape coverage - Topographic wetness Index (TWI)

Reclassify Reclassifying the attributes of each criteria

Weighted Sum Model Weighting each criteria in order of importance

- Precipitation = 10% - SPI = 10% - Elevation = 10% - Slope = 10% - Curvature = 10% - Distance to streams = 10% - Stream density = 10% - Land-use/land-cover =10% - Landscape coverage= 10% - TWI = 10%

Flood susceptibility maps (before and after BCE development) Fig 6.1 Schema of Phnom Penh’s flood susceptibility mapping, before and after BCE development

6.3

Flood conditioning factors

As the Weighted Sum and Weighted Overlay tools use raster data, the data of the raster for each flood-inducing element were processed from their raw raster forms in ArcGIS Pro. Then, they will be reclassified using the Reclassify tool (Spatial Analyst) into the five categories and ranked into the five flood risk levels.

The dataset used and their derivatives are listed in Table 6.1. while the ranges and ratings of all flood 31

Ch6 Flood susceptibility analysis

susceptibility components can be seen in Table 6.2.

Table 6.2 Flood susceptibility ranges and rating Flood causative criteria

Flood susceptibility class ranges and raitings of flood risk Unit

Very high (5)

High (4)

Moderate (3)

Low (2)

Very low (1)

Precipitation

mm

2,465 - 2748

2,403 - 2,465

2,385 - 2,403

2,352 - 2,385

2,234 - 2,352

Stream Power Index (SPI)

level

3.54 - 18.46

2.01 3.53

-2.70 -2.00

-3.64 - -2.69

-11 - -3.63

Elevation

m

-11.00 - 8.62

8.62 - 11.96

11.97 - 14.83

14.84 - 17.70

17.71 - 111

Slope

degree

0.01 - 54.79

54.80 - 69.45

69.46 - 75.73

75.74 - 79.92

79.93 - 88.99 Convex

(i) Hydrological criteria

(ii) Spatial criteria

Curvature

type

Concave

-

Flat

-

Distance to stream

m

<200

200 - 500

500 - 1,000

1000 -1,500

>2,000

Stream density

sqm

549.34 - 983.31

398.39 - 549.33

300.28 - 398.38

221.03 - 300.27

21.02 - 221.02

type

Water bodies

Medium density

Low density

Wetlands

Agriculture and

(iii) Land-use/land-cover LULC dynamic

High density

Bare lands

urban green

(iv) Permeability criteria Landscape coverage

type

Water bodies

Medium density

High density Topographic Wetness Index (TWI)

level

12.30 - -0.18

Low density

Wetlands

Agriculture and

-4.50 - -3.40

-7.75 - -4.49

Bare lands -1.95 - -0.17

-3.41 - -1.94

urban green

6.3.1 Factors before BCE development I. Spatial processing Elevation data of Phnom Penh (A-Fig 6.2a) was directly extracted from the DEM file and then classified into 5 flood risk categories (A-Fig 6.5a). Lower areas are more probable to suffer from flooding as water flow from higher regions into this area (Luizzo, Sammartano, & Freni, 2019).

Meanwhile, slope information (A-Fig 6.5b) can be generated from the DEM using the Slope tool (Spatial Analyst). Slope is relative to flow of water from high altitude to low altitude, with steep slopes increasing runoff due to less time for infiltration, so the flood risk level will increase from low to high degree (Mahyat, Lalit, & Farzin, 2019) (Pham, Jaafari, Prakash, Singh, Quoc, & Bui, 2019).

In case of curvature, the three types, concave (positive values), flat (value 0), convex (negative values), were used. The curvature of Phnom Penh was derived from Curvature tool (Spatial Analyst) before being reclassified to fit the flood hazard level (A-Fig 6.5c).

II. Hydrological processing The information for distance to stream, drainage density, stream power index (SPI), topographic wetness index (TWI) can all be extracted from the DEM data obtained from USGS through spatial analyst and hydrological processing in ArcGIS using the Spatial Analyst toolset, Hydrology toolset, and ArchHydro extension toolset. First, the catchment of Phnom Penh must be delineated as the process of catchment delineation yields many raster data that are relevant to the determination of distance to stream, drainage density, stream power index (SPI), topographic wetness index (TWI). The procedure of delineating catchment is as follows: fill sink → flow direction → flow accumulation → stream definition → stream segmentation → catchment grid delineation → catchment polygon processing → drainage line processing. An illustration of catchment delineation and drainage line of Phnom Penh can be seen in the appendix figure A-Fig 6.3a.

The data for distance to stream (A-Fig 6.5d) was derived from stream segmentation raster by using the Euclidean Distance tool (Spatial Analyst). It was classified so that higher flood risk corresponds to areas that are closer to the natural streams because they have higher chances of flooding (Mahyat et al., 2019).

In the case of stream density (A-Fig 6.5e), this data was developed by utilizing the Line Density tool (SpaCh6 Flood susceptibility analysis

32

tial Analyst) on the stream segmentation data. As a high density of drainage indicates higher surface runoff, which means an increase in the likelihood of flooding (Paul, Saha, & Hembram, 2017), the data was categorized so that high drainage density equates to a higher risk of flooding.

Stream power index layer (A-Fig 6.3f) and topographic wetness index layer (A-Fig 6.5g) were developed from the flow accumulation and slope raster with the following equations (Moore, Grayson, & Ladson, 1991): SPI = As*tanB and TWI = ln(As/tanB) where As is the specific area of the catchment (flow accumulation raster, in m2m-1) and B is the slope radiant gradient (slope raster, in degrese).

It is expected that higher topographic wetness index value indicates higher chances of flooding and areas with low stream power index will experience more flooding (Moore et al., 1991), so the flood hazard level was classified and ranked accordingly.

III. LULC and landscape coverage criteria In terms of land-use/land-cover detection, Landsat8 data was used. In this study, land use classification was made based on the available data source with respect to the types of the first level’s land use classification system with remote sensor data proposed in 1976 (Anderson, Hardy, Roach, & Witmer, 1976) and the land-use/land-cover change nomenclature provided by Earth Observation for Sustainable Development (EO4SD, 2019). Thus, the land use in this study was classified into seven categories: water bodies, agriculture and urban green, wetlands, bare land, low density urban fabric, and high density urban fabric. Land use land cover information was obtained by using the Image Classification Wizard (Image Analyst and Spatial Analyst) with supervised classification. Accuracy assessment was done with a classified LULC map that was crosschecked with Google Maps, ground truth data, and LULC map by EO4SD. The LULC classification system is presented in A-Table 6.1, the LULC map of Phnom Penh pre-development is shown in A-Fig 6.4a, and the LULC Cohen Kappa’s Coefficient accuracy assessment is in A-Table 6.2, along with a brief explanation.

Urban built-up greatly affects the natural catchment and its hydrology as it alters the permeable landscape into an impermeable one. Typically, a higher development density equals to a pervious landscape surface percentage which generates more runoff (Erickson, 1995). Low-density development, usually residential units, was estimated to have 20-49% impervious surface while medium density density, also typically residential units, typically have 50–79% impervious surface and high-density urban development, including mixed-use units, has up to 80–100% impervious surface (A-Table 6.3). Land use without plants coverage such as barren plots were estimated to have moderately susceptible to flood since there is nothing to control and prevent water from overflowing onto the surface. Meanwhile, vegetation and flooding have a negative relationship, so vegetated areas have lowest the risk of flooding (Guan, Sillanpää, & Koivusalo, 2015) (Ulah & Zhang, 2020).

The LULC’s flood risk (A-Fig 6.5h) and the landscape coverage’s flood risk (A-Fig 6.5i) were reclassified into five levels of flood risk using this logic.

6.3.2 Factors after BCE development For flood-triggering factors of Phnom Penh after BCE development, the DEM raster data needed to be edited to fit with the ING City’s development plan before they can be processed and conditioned for flood risk mapping. As the Boeung Cheung Ek area is being reclaimed for development (STT, 2020), the elevation data for this area is modified to mirror that of the city center as it is typical for land reclamation development in Phnom Penh to in-fill to the level of the surrounding area. Spatial and hydrological processing and classification of flood risk of the flood conditioning components will follow the above procedure (A-Fig 6.6a – g).

As for LULC and landscape coverage elements, the classified LULC raster (A-Fig 6.4b) was directly

33

Ch6 Flood susceptibility analysis

edited to correspond to the ING City’s masterplan (Fig 4.4). However, ING Holdings is not transparent with the density of their development nor the sealing level so it was assumed that peri-urbanization will follow the business-as-usual pattern (BAU). Thus, the landscape coverage level of the developed scenario of Boeung Cheung Ek will follow one proposed in the previous section.

The modified elevation of Phnom Penh is presented in A-Fig 6.2b, changed catchment delineation and drainage line is in A-Fig 6.3b, and the LULC of Phnom Penh post-BCE development is in A-Fig 6.4b. Maps of the LULC and landscape coverage’s flood risk of Phnom Penh after BAU development of Boeung Cheung Ek are show in A-Fig 6.6h – i.

6.3.3 Rainfall The annual rainfall data obtained from the open source CHRS is for the whole of Cambodia so, first, the data from 2011 to 2020 was clipped to Phnom Penh’s parameter, by using the Clip Raster tool (Data Management). Then, they were merged into a single raster data with the tool Mosiac to New Raster (Data Management) then reclassified to five flood risk.

Phnom Penh’s flood susceptibility rainfall criteria map can be seen in A-Fig 6.7.

6.4

Results

6.4.1 Increased flood risk As can be seen from Fig 6.2a and Fig 6.2b and Table 6.3 (a-c), urbanization of the Boeung Cheung Ek peri-urban arena causes an increase in flood risk within the whole city. It is of note that it is not only the west side of the riverbank that is affected by this development, but the southeastern side as well, as can easily be detected in the area around Platinum City (depicted as satellite city No.7 on the maps).

Overall (Fig 6.2a, Fig 6.2b, and Table 6.3a), it is forecasted that the development of Boeung Cheung Ek area into the ING City satellite city project will decrease the city’s low flood risk by 24.95%, which is a drop of 58.55 km2 of flooding area. However, other levels of flood risk will experience a rise, especially the very high flood risk which sees a sharp increase of 24.70 km2. That’s an additional 11.21 km2, totalling the very high flood risk area after the peri-urbanization project to 56.60 km2, making overall very high flood risk area equal to 8.3%. Areas that are highly prone to flooding will also rise by 20.39%, adding another 11.24 km2 to existing high flood rise aresa, making up to 66.36 km2. High flood risk of Phnom Penh after Boeung Cheung Ek’s peri-urban development will be at 9.70%. Areas that are moderately predisposed to flood is projected to increase by 12.95%. That is an extra 34.36 km2 that will become more at risk, doubling the moderate flood risk area to 299.62 km2 which takes up to 43.9% of the city. Meanwhile, low flood risk will also increase by a small margin of 2.18%, extending another 1.83 km2, and making the overall low flood risk area of the city to be 85.79 km2.

Phnom Penh’s urban core is shown to largely be moderately susceptibly to flooding in the before and after Boeung Cheung Ek’s development and the increase flood risk within the urban core is not very noticeable (Fig 6.2a, Fig 6.2b, and Table 6.3b). 92.32% of the entire urban core currently has moderate susceptibility rate and it is expected to rise to 98.60%. Despite the high percentage, only an additional 1.87 km2 is expected to become moderate flood risk area.

The same cannot be said about the peri-urban perimeters (Fig 6.2a, Fig 6.2b, and Table 6.3c) as it is projected to experience all level of flooding except the low flood risk areas. Up to 56.92 km2 is expected to be very highly susceptible to flooding, making up to 11.28% of the peri-urban area extremely like to experience flooding. That is an increase of 11.36 km2 in the areas that are very inclined to flooding. Meanwhile, high flood risk areas will increase by 11. 33 km2 which will result in a total of 66.65 km2, putting up to 12.54% of Phnom Penh’s peri-urban interface at risk. The Peri-urban extent that are moderately susceptible to flooding will jump from 212.81 km2 to 231.05 km2, accounting for 43.50% of the overall flood risk. Very low flood risk areas are Ch6 Flood susceptibility analysis

34

Fig 6.2a Flood risk map of Phnom Penh before BCE’s development

expected to expand as well, increasing by an extra 1.04 km2, resulting in 81.25 km2, or 15.29%, of the city that is very unlikely to flood.

The study area will likely suffer from more flooding as well (Fig 6.3a, Fig 6.3b, and Table 6.3d), in post-development scenario, up to 0.21 km2 is predicted to be very high flood risk area. That is an increase of 100% in very high flood susceptibility zone since, in pre-development scenario, none of the Boeung Cheung Ek area is very highly prone to flooding. Furthermore, an additional 0.85 km2, or 14.70%, will become highly at risk of flooding, making the total highly susceptible to flood risk areas of the ING City to be 6.63 km2. That is 30.41% of the study area that is highly prone to flooding. Moderate flood risk will also increase by 2.70 km2, which is a further in crease of 40.76%, measuring overall moderate flood risk at 9.32 km2, which takes up to 42.75% of the study area.

6.4.2 Changes in urban hydrology Results show that altering the elevation and LULC of Boeung Cheung Ek has drastic consequences as the natural urban hydrology of the area is completely modified after development.

Urban stream networks shifts from within the Boeung Cheung Ek peri-urban area into the nearby peri-urban area on the right side of it (A-Fig 6.3a and A-Fig 6.3b). This means, that natural drainages is expected to be forcefully move from ING City to be housed in the BTP satellite city (depicted as satellite city No.7 in

35

Ch6 Flood susceptibility analysis

Fig 6.2b Flood risk map of Phnom Penh after BCE’s development

Fig 2.7, Fig 6.2a and Fig 6.2b). The stream density in the BTP satellite city area will increase and its density will become dramatically concentrated (A-Fig 6.5e and A-Fig 6.5e) which consequently increases its stream power index (A-Fig 6.5f and A-Fig 6.6f) and the topographic wetness index (A-Fig 6.5g and A-Fig 6.6g).

In addition, the natural drainage lines of the whole city will elongate due to the ING City project. This can be clearly seen in A-Fig 6.3b where Boeung Cheung Ek peri-urbanization will cause the stream network to extend from near the area and to intrude into the urban core. However, this hydrological change is not limited to the southwestern part because the northwestern area of Phnom Penh is also expected to undergo similar alteration. The natural streams will not only become longer but will also branch out and deviate to the surround area as well.

6.5

Discussion

6.5.1 Cause of increased flood risk Rainfall is a crucial variable in causing flood risk in Phnom Penh as flood susceptibility maps of the before and after Boeung Cheung Ek peri-urbanization shows that the most northeastern part and the southwestern part of the city that receive the highest amount of rain (A-Fig 6.7) are most vulnerable to flooding (Fig 6.2a and Fig 6.2b). These cases are to be expected as these areas have very high stream density as well as very high topographic wetness index (A-Fig 6.5g) and very low stream power index (A-Fig 6.f) which are all indicators Ch6 Flood susceptibility analysis

36

Fig 6.3a Flood risk and LULC maps of BCE after development

Fig 6.3b Flood risk map of BCE after development

37

Ch6 Flood susceptibility analysis

Table 6.3a Flood risk area in all of Phnom Penh, before and after BCE development Flood susceptibility before development SqKm

Flood susceptibility after development

Change in flood susceptibility

%

SqKm

%

SqKm

%

Very low (1)

83.96

12.27

85.79

12.53

+ 1.83

+ 2.18

Low (2)

234.62

34.28

176.08

25.73

- 58.54

- 24.95

Moderate (3)

265.36

38.76

299.62

43.77

+ 34.36

+ 12.95

High (4)

55.12

8.05

66.36

9.70

+ 11.24

+ 20.39

Very high (5)

45.39

6.63

56.60

8.27

+ 11.21

+ 24.70

Total

684.45

100

684.45

100

-

-

Table 6.3b Flood risk area in the urban core area, before and after BCE development Flood susceptibility before development

Flood susceptibility after development

Change in flood susceptibility

SqKm

%

SqKm

%

SqKm

%

Very low (1)

-

-

-

-

-

-

Low (2)

2.23

7.68

0.41

1.40

- 1.82

- 81.61

Moderate (3)

26.78

92.32

28.65

98.60

+ 1.87

+6.98

High (4)

-

-

-

-

-

-

Very high (5)

-

-

-

-

-

-

Total

29.05

100

29.05

100

-

-

Table 6.3c Flood risk area in the peri-urban area, before and after BCE development Flood susceptibility before development

Flood susceptibility after development

Change in flood susceptibility

SqKm

%

SqKm

%

SqKm

%

Very low (1)

80.21

15.10

81.25

15.29

+ 1.04

+ 1.29

Low (2)

137.21

17.93

95.24

17.93

- 41.97

- 30.58

Moderate (3)

212.81

43.50

231.05

43.50

+ 18.24

+ 8.57

High (4)

55.32

12.55

66.65

12.54

+ 11.33

+ 20.48

Very high (5)

45.56

10.72

56.92

10.71

+ 11.36

+ 24.93

Total

531.12

100

531.1

100

-

-

Table 6.3d Flood risk area in the study area, before and after BCE development Flood susceptibility before development

Flood susceptibility after development

Change in flood susceptibility

SqKm

%

SqKm

%

SqKm

%

Very low (1)

-

-

-

-

-

-

Low (2)

9.40

43.11

5.64

25.87

-3.76

-40

Moderate (3)

6.62

30.36

9.32

42.75

+2.70

+40.76

High (4)

5.78

26.51

6.63

30.41

+0.85

+14.70

Very high (5)

-

-

0.21

0.96

+0.21

+100

Total

21.80

100

21.80

100

-

-

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38

of vulnerability to flooding.

Despite this, results show that more than 90% of Phnom Penh’s urban core of are under moderate flood risk even though not all of it experiences moderate risk rainfall, let alone heavy risk rainfall. Additionally, the elevation (A-Fig 6.5a) and slope (A-Fig 6.5b) of the urban core show a general resistance against flooding as well. However, LULC of the urban core indicates very a high susceptibility to flood risk as the majority of Phnom Penh’s urban built-up is concentrated here (A-Fig 6.4a) to cater toward the high density population there. Landscape coverage’s flood risk also points toward very high flood risk since most of the urban built-up within the urban core has a very high sealing level (A-Table 6.1).

Meanwhile, the project (ING City) within the Boeung Cheung Ek study area is predicted to become at greater risk of flooding even though stream network and stream density will be diverted from the area after peri-urbanization (A-Fig 6.3b and A-Fig 6.6e). The area’s topographic wetness index will also lower (A-Fig 6.6g) and its stream power index will rise (A-Fig 6.6f), signs that indicate increased resilience against flooding. The elevation and slope of Boeung Cheung Ek post-development also suggest that this area should be less prone to flooding but the result revealed otherwise. However, the shift in land use in this area due to the ING City (Fig 6.3a and Fig 6.3b) is drastic. Land use will dramatically change from agriculture and wetland that once encouraged water retention, to those of urban built-up and water resources that are very prone to flooding. Therefore, it can be concluded that Boeung Cheung Ek’s surge in flood susceptibility after peri-urbanization is not only determined by the changes in spatial and hydrological feature but can also be attributed to the change in LULC and landscape coverage. The change in land usage from that of pervious ones to impervious ones will decrease the site’s permeability, resulting in the exacerbation of the area’s flood risk.

It is clear that there is a correlation between LULC and pervious landscape cover and flood risk. Therefore, it can be said that land use land cover and landscape cover are vital flood-inducing factors and should be seriously considered in peri-urbanization efforts if the peri-urban area is to continue to maintain its flood protection quality.

6.5.2 Landscape coverage as main flood protection factor It was claimed that Boeung Cheung Ek is able to control flooding within Phnom Penh because of its large size and depth (STT, 2019) (STT, 2020). However, as can be seen in A-Fig 6.3, before development, Boeung Cheung Ek area is densely packed with stream network with a higher stream power index (A-Fig 6.5f) and a lower topographic wetness index (A-Fig 6.5g) than the post-development scenario. The urban stream density within this area is extremely high, suggesting a great water retention capacity.

However, the hydrologic alteration of Boeung Cheung Ek peri-urban arena in the post-development scenario is severe as stream network (A-Fig 6.3a) and stream density (A-Fig 6.6e) will all but disappear from the area. The natural drainage networks will relocate to the adjacent peri-urban area on the left and this will cause the area to experience a decline in stream power index and an increase in topographic wetness index, making it become more susceptible to flooding. Stream density shifts southward and spread not only into the nearby area but into the adjacent urban core area as well. A majority of the area in which the streams shift into is shown to change from low flood risk to moderate flood risk (Fig 6.2b). This suggests that rather than the depth and size, it is the intricate web of natural drainage and the permeability criteria of the area that retain rainfall and runoff and facilitate their travel to the water network downstream.

The reason for this major alteration in the urban hydrology of Boeung Cheung Ek is the change in elevation by land reclamation as well as the change in land usage. These are the consequences of peri-urbanization.

It can be said that the modified elevation of the Boeung Cheung Ek is a fixed element as land reclamation is currently ongoing and it is a peri-urbanization practice that has persisted since the birth of Phnom Penh.

Another component that is unlikely to change is the proposed land use zoning of the area because this area will be developed one way or another.

39

Ch6 Flood susceptibility analysis

Existing studies have proven that the main flood-related problem that arose from urbanization is the change in the natural urban hydrology. The development of peri-urban arena changes land use and permeable cover which greatly affect the flow of water and disrupt the effectiveness of the urban watershed. Thus, the only uncertain flood conditioning factor is the landscape coverage of the study area, which is the pervious and impervious surfaces on each land plot.

6.5.3 Conclusion The results show that peri-urbanization and flooding are interconnected as it is proven that development in the peri-urban area will worsen the overall flood risk within the city. It can be reliably concluded that the existing landscape interface of the peri-urban area contributes to flood management of the whole of Phnom Penh. Therefore, it is crucial that the existing landscape interface of the peri-urban parameter be maintained, restored, and/or amplified and utilized as a flood protection asset.

The flood-inducing factor that was deemed to be capable of turning a developed peri-urban project into a flood management tool is the permeability ratio of each land use type. This permeability ratio is the main uncertain factor in the ING City development project and it is also generally proven to affect an area’s capability to manage rainfall, runoff and the flow of water.

Ch6 Flood susceptibility analysis

40

Chatper 7 Suitable peri-urbanization pattern

7.1

Green infrastructure Over the past decades, as urbanization continues to expand and urban stormwater impact on environment and civilization worsens even further, the management of urban drainage and urban water cycle has become incredibly important (Chocat, et al., 2001) (Fletcher, et al., 2013). The field of sustainable urban drainage management, also known as green infrastructure, has rapidly evolved over time to try and keep up with the intense urban development around the world. Its objectives and scope of practice have also since diversified to be inclusive of many disciplines since its conception (Fig 7.1a) (Fig 7.1b). However, despite the variations in terminology and focus, at its core, all types of stormwater management techniques are an integrated water infrastructure that works with nature and aims to manage urban flooding caused by, but are not limited to, rainfall, runoff, and storm events. The key components of these practices remain largely the same (Fletcher, et al., 2013) (NJStormwater, 2021) and they are: To limit urbanization disturbance in the site by maintaining the site’s pre-existing features and managing its impervious cover.

To mitigate hydrological alterations and preserve the natural drainage network and local environment.

-

To improve water quality and limit pollutants.

Previous studies on urban stormwater management have shown that these techniques are able to effectively mitigate negative hydrological impacts caused by urban flooding (Burns, Fletcher, Walsh, Ladson, & (Source: Fletcher, et al., 2014) Hatt, 2012) (Carter & Jackson, 2007) (Jia, Lu, Yu, & Chen, 2012). It was also found that when these practices are combined into the developed fabric of the peri-urban area, there is a significant increase in flood control measures as total runoff volume and peak flow rate reduce (Guan, Sillanpää, & Koivusalo, 2015) (Petrucci, Rioust, Deroubaix, & Tassin, 2013). Furthermore, these stormwater management practices have the potential to restore the pre-development total runoff volume and the key elements of natural flows for some small events through combined regulations. Moreover, these techniques are applicable and effective Fig 7.1a Evolution of urban drainage management

in every development stage, whether in fully urbanized or yet-to-be urbanized area (EPA, 2012).

Fig 7.1b Urban drainage practices classification, their focuses, and specificities (Source: Fletcher, et al., 2014)

Some of the urban drainage management techniques with these potentials are the infiltration-based approach and the retention-based approach which alter the permeability of a plot via the manipulation of pervious and impervious surface of a land plot (Burns, Fletcher, Walsh, Ladson, & Hatt, 2012) (Carter & Jackson, 2007) (Jia, Lu, Yu, &

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42

Chen, 2012) (Fletcher, Andrieu, & Hamel, 2013). These approaches have their advantages and disadvantages but a combination of both can imitate the rural hydrology of the pre-existing peri-urban area and curb flooding. The infiltration-based method can deal with percolation loss caused by land cover changes while the retention-based methods can address peak flows and runoff volume. Therefore, this section will introduce the infiltration-based and retention-based stormwater management technologies sampled from practices in North America and Australia and deemed to be suitable for the developing peri-urban fabri.Then they were group into three categories: -

Attached: stormwater management tools that are used in with buildings and/or infrastructures.

-

Detached: techniques that are free standing, independent from any architecture.

-

End-of-pipe: techniques that are able to hold great volume of rainfall and runoff.

All three types allow water to percolate in some variation with the detached type and end-of-pipe type usually being connected to the groundwater below. However, the end-of-pipe stormwater management tools are the traditional techniques that encourage the most infiltration into the soil.

I. ‘Attached’ green infrastructure type a. Green roof -

Also called rooftop garden.

-

It is a vegetative cover grown on the roof that must have plants, soil, drainage layer, roof barrier and irrigation system. Rainfall is captured and temporarily stored before being slowly released as runoff through the downspots.

-

Suitable land use type: all.

-

(USEPA, 2021b), (Grownyc, 2015), (Shafique & Kim, 2017)

Fig 7.2a Green roofs Source: (USEPA, 2021b) & Thammasat University

b. Living wall -

Also called bio-wall, green wall, and eco-wall.

-

Stormwater management techniques where plants are modularly grown and integrated with the facade of a building.

-

Suitable land use type: all.

-

(CDR, 2021)

c. Rainwater harvesting -

Also called rain barrel, rain cistern, or rain tank.

-

It is a water collecting and reusing technique where rainwater from an adjacent building flowsinto a container before excess water is redirected to a garden or drainage system. It is often used in combination

43

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with techniques from the ‘detached’ types.

-

Suitable land use type: all.

-

(Grownyc, 2015), (USEPA, 2021a), (USEPA, 2021b)

Fig 7.2b Living wall with infiltration trench

Fig 7.2c Rain barrel

Source: (CDR, 2021)

Source: (USEPA, 2021a)

II. ‘Detached’ green infrastructure type a. Enhanced tree pit -

Also called planter boxes.

-

Similar to rain garden but with vertical walls and includes an underground storage chamber to maximize stormwater retention.

-

Suitable land use type: all, especially streetscape in high density land use.

-

(MPCA, 2021), (Grownyc, 2015), (Melbourn Water, 2005) (Shafique & Kim, 2017), (USEPA, 2021b)

Fig 7.3a Enhanced tree pit Source: (Grownyc, 2015)

b. Permeable pavement -

Also called porous paving.

-

A hard surface that allows stormwater runoff to seep into the underlying soils and groundwater or to be removed by a subsurface drain. Permeable pavings can come in a variety of permeable concrete, filter fabric, gravels, or wooden planks.

-

Suitable land use type: all.

-

(Grownyc, 2015), (USEPA, 2021b)

Fig 7.3b Permeable pavement Source: (Grownyc, 2015)

c. Bioretention cell -

Also called bioretention basin or rain garden.

-

Shallow basin for upland areas with vegetation that are constructed for detention, infiltration, and treatment of rainwater and runoff.

-

Suitable land use type: all.

-

(Melbourne Water, 2005), (MPCA, 2021), (NYDEC, 2001), (USEPA, 2021b)

Fig 7.3c Rain garden Source: (USEPA, 2021b)

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44

d. Bioswale -

Open channel or vegetated buffer strip.

-

Artificially vegetated drainage path designed to capture and treat rainwater and runoff within dry or wet cells before conveying it downstream.

Essentially the same as a bioretention cell but designed and constructed in linear form and primarily used to slow down runoff as it flows into other stream networks.

-

Suitable land use type: all land use type, especially streetscape.

Fig 7.3d Dry swale and wet swale Source: (USEPA, 2021b)

-

Dry swales, also called grass swale, are designed to capture and treat runoff but prevent standing water to form within the area.

-

Wet swales, has a permanent pool of shallow water, equiped with native wetland vegetation, that intersect with the groundwater and exhibits similar characters to wetlands.

-

(Melbourne Water, 2005), (MPCA, 2021), (NYDEC, 2001), (USEPA, 2021b)

III. ‘End-of-pipe’ green infrastructure type a. Stormwater pond -

Constructed retention basin that collects, stores and, treats runoff and recharges the groundwater table.

-

Land use type: medium rise and low rise residential units, public and green spaces, water resources

-

Dry pond also called detention pond, doesn’t retain water all year round, instead temporarily holds excess stormwater before the water is drained into the soil beneath or flows into nearby lower depression.

-

Wet pond, also called retention pond, has a standing pool of water, sometimes used in combination with dry pond. This mixture of permanent pool and an extended detention storage above that pool, providing a greater water capture capacity and better rate control.

-

(Melbourne Water, 2005), (MPCA, 2021), (NYDEC, 2001)

Fig 7.3d Dry swale and wet swale Source: (MPCA, 2021)

b. Constructed wetland -

Also called stormwater wetland.

-

It imitates the hydrology of a natural wetland and can be used as a mitigation step for natural areas lost to urbanization. It is an ‘endof-the-pipe’ green infrastructure type used to control urban runoff by holding standing water on its surface and saturated water below the soil. It and was proven to be a useful measure in handling increased runoff volume in urbanized area.

-

Land use type: low rise residential, public and green spaces, water resources

-

(Melbourne Water, 2005), (MPCA, 2021), (Nguyen et al., 2014), (NYDEC, 2001)

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Ch7 Suitable peri-urbanization pattern

Fig 7.4b Constructed wetland Source: Qunli stormwater wetland park

c. Infiltration trench -

Also called infiltration basin, dry wells, filter strip, and underground infiltration systems.

-

It is an ‘end-of-the-pipe’ green infrastructure type that is used to capture and temporarily store runoff before allowing it to percolate into the soil below. It is shown to be effective in urbanized areas with elevated runoff volume. It can be used in combination with other stormwater management techniques mentioned above.

-

Land use type: All.

-

(Melbourne Water, 2005), (MPCA, 2021), (NYDEC, 2001)

Fig 7.4c Infiltration trench Source:(MPCA, 2021)

7.1.1 Green infrastructure movement in Phnom Penh As illustrated in Chapter 3 and Chapter 4, Phnom Penh’s stormwater management and flood control effort are entirely dependant on grey infrastruce. Future projects, mainly currently undertaken by JICA, are also focused on robust infrastructure aimed at funnelling water out of the city. The coping strategies and adaptations in the peri-urban fringe cannot be considered as integrated sustainable urban drainage system either since these interventions treat water as nuisance, getting away from the flood rather than integrating it into the developing urban fabric.

However, recognizing the need limitation of traditional grey infrastructure and the need for a new model of stormwater management to be introduced into the urban area, the Global Green Growth institute (2019) published a report recommended measures for flood protection. This report proposed many sustainable urban growth concepts and instruments which are vital to the development of bankable green city projects that are also resilient to the changing climate. The suggestions for minimizing flooding that are related to the urban hydrology are: -

Usage of green building techniques that will maximize rainwater capture on site.

-

Sustainable stormwater management techniques that maximize water retention and infiltration should be applied in flood-prone areas and integrated into the existing drainage system.

-

Protection/preservation of agricultural hinterland, lake, stream, and wetland system.

-

Restoration of natural hydrological system for multi-function purposes beyond simple water retention.

It is clear from these proposed methods that maximizing porous land cover of an urban development and the application of stormwater management techniques into the projects are vital to the flood protection of Phnom Penh.

7.2

Scenario planning

Scenario planning is a qualitative and strategic tool that can systematically explore a range of possible futures. It involves the examination of certain and uncertain components of a situation where the ‘unknowable’ elements will be studied to see how they will unfold and how they will influence the future. This will enable the creation of multiple comprehensive pictures for the future that can be used to test plans and strategies (O’Brien, 2000).

This method involves multiple steps (Cork et al., 2005), and they are: 1. Focal issue: the main research question that will be answered by using this scenario planning method.

It involves the identification of elements that brought about changes in the past and could bring about changes in the future.

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46

2. Certainties and uncertainties: concrete factors that will cause differences need to be separated from the uncertain ones as the uncertain components will be studied and used as drivers to create various scenarios.

3. Development options: identified uncertainties will need to be analyzed and explored so that scenarios can be created by playing around with them.

4. Roadmap or storyline: descriptive narratives need to be developed to “tell the story” of each scenario.

5. Testing: each scenario will need to be tested against existing strategies and plans.

Typically, three scenarios will suffice, with the middle scenario being the most likely future to happen (O’Brien, 2000).

7.3

Focal issue & scenario driver

As can be seen in the results and discussion in Chapter 6, the landscape coverage of the peri-urban area was determined to be a major flood control factor that can maintain the flood protection quality of the peri-urban area.

Coincidentally, landscape coverage of a site is shown to be a key variable in stormwater management technologies as the relationship between impervious surface ratio and stomwater runoff is undeniable (MSSC, 2005) (FISRWG, 2001) (Fig 7.5).

Furthermore, the pervious and impervious cover is a quantifiable development currency that can be managed, regulated, priced, mitigated, and traded and Fig 7.5 Relationship between impermeable surface and runoff it has been widely used by watershed planners and (Source: FISRWG, 2001) storm-water engineers for various purposes related to hydrology (Goetz, Wright, Smith, Zinecker, & Schaub, 2003) (Jantz, Goetz, & Jantz, 2005). Moreover, the degree of alteration in permeable and impermeable surface can be reliably forecasted as it changes in response to land use and zoning categories (Cappiella & Brown, 2001) (Slonecker & Tilley, 2004). Hence, landscape coverage was clearly an appropriate driver and used to derive different development patterns of the Boeung Cheung Ek area.

With landscape coverage as the driver for development of different scenarios in mind, existing landscape cover guidelines were sampled for possible narratives to create options for scenario planning. The schema of scenario planning to determine a suitable peri-urbanization pattern is illustrated in Fig 7.6.

7.3.1 Landscape coverage guidelines In this study, landscape cover is defined as the green component of a plot that is pervious/permeable and allows for water retention and percolation and is connected to a hydrology system. It is not limited to only porous surface on the ground area but also includes any water-absorbing surface on and within the built environment as well. Meanwhile, impermeability or imperviousness refers to any area that is not “green”, or can be seen as “grey”, which includes roads, parking lots, sidewalks, rooftops, and other impermeable surfaces of the urban landscape (Schueler, 1994).

I. Cambodia’s guideline to landscape coverage In an attempt to ensure the quality, efficiency, equity, and sustainability of developments in the city, mu47

Ch7 Suitable peri-urbanization pattern

Focal issue / main problem Altered hydrology caused by: - Elevation change - Land use land cover change - Landscape coverage change

Certainties

Uncertainties / drivers

- Elevation - Land use

Landscape coverage

Landscape coverage guidelines - Sub-Decree No.42 (Government’s mandated guideline on pervious and impervious cover ratio) - Impervious Cover Model (ICM) (Pervious and impervious surface percentage that indicate the urban stream’s hydrological effectiveness in stormwater management)

Green infrastructure techniques

(Consideration for water sensitive urban design)

Scenarios A

Existing landscape coverage regulation

Baseline: landscape coverage as mandated by the government in Sub-Decree No.42, no requirements for green infrastructure techniques

B

Mild landscape coverage

Improve post-development peri-urban’s hydrology beyond urban drainage: mild green infrastructure in private realm, moderate green infrastructure in public realm

C

Moderate landscape coverage

Preservation of post-development’s hydrology: moderate green infrastructure in private realm, rigorous green infrastructure in public realm

D

Intense landscape coverage

Attempt to restore post-development hydrology to pre-existing condition: rigorous green infrastructure in private and public realm

Testing 1. Generate flood susceptibility map of each scenario 2. Compare the flood risk of scenario B, C, D with scenario A (baseline) and the BAU scenario (after BCE’s development scenario in Chapter 6)

Results Best scenario: the development option that maintains the landscape interface of the peri-urban area but still allows for dense urban intensification Fig 7.6 Schema of scenario planning to determine a suitable peri-urbanization pattern

nicipality, and towns, the Royal Government of Cambodia promulgated Sub-Decree No. 42 in 2015 (RGC, 2015a) (RCG, 2015b). Another goal of this law is to encourage urban green growth that not only help reduce global climate change but also have the potential to prevent and control hazards in the urbanized area.

In this Sub-Decree, the government outlined the floor area ratio (FAR), the building coverage area (BCR), and the pervious cover percentage in accordance with the land use type within the urbanized area. This information was derived and illustrated in Table 7.1. It is of note that in Sub-Decree No.42, the government regulated pervious surface to the ground cover only which is unlike the permeable surface that is defined in this study that also allows for porosity to be integrated into the buildings and infrastructures.

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Table 7.1 Guideline on land use type, FAR, BCR, and landscape coverage in urban area of Cambodia

Land use type

FAR

BCA

Landscape coverage Percentage

Range

Average

≤ 50%

15 - 50%

32.5%

≤ 75%

7.5 - 25%

16.25%

≤ 60%

12 - 40%

26%

10.5 - 35%

22.75%

7.5 -25 %

16.25%

9 - 30%

19.5%

12 - 40%

31%

9 - 30%

19.5%

≤ 60%

12 - 40%

31%

≤ 50%

15 - 50%

32.5%

≤ 10%

27 - 90%

58.5%

35 - 75%

55%

Residential units - Discontinuous low rise

≤ 1.5

- Continuous low rise - Medium rise

≤ 3.0

- High rise

≤ 5.0

- Mixed residential

≤ 5.0

≤ 65%

- Commercial

≤ 12.0

≤ 75%

- Mixed commercial

≤ 10.0

≤ 70%

Industrial units

≤ 3.0

≤ 60%

Mixed-use units

≤ 10.0

≤ 70%

Transportation units

≤ 3.0

Administration & services units

≤ 5.0

Cultural & religious units

≤ 2.0

Commercial units

Tourism units

≤ 5.0

Other units Public space & green space units

≤ 0.3

Restricted land use units - Agriculture, forested area - Water resource - Protected area

≤1.5

≤ 30%

≥ 30% of the remaining plot area

≥ 50% of the remaining plot area

II. Impervious cover model (ICM) The impervious cover model was initially used by Schueler as a management tool that diagnoses the severity of future urban stream problems in urban subwatersheds. These issues range from hydrological and water quality to habitat and bio indicator (Schueler, 1994). In 2004, the ICM was used to classify and manage urban streams and urban hydrological health. The following year, ICM was introduced as the key variable in stormwater management practices, with added potential to manage stormwater runoff downstream (MSSC, 2005). It should be noted that the application scale of ICM is not limited to subwatershed but also used on the watershed level and is is usually used on on suburban or currently developing areas (Schueler, Farley-McNeal, & Cappiella, 2009).

According to the ICM (Schueler, 1994) (Schueler et al., 2009), the hydrology of a small scale subwatershed (around 5 to 50 sqkm) will start to decline once there are more than 10% of impervious surface. The general predictions of ICM can be found in Table 7.2.

Table 7.2 Impervious and pervious cover index of ICM

Stream type

Stream quality

(SS) Sensitive streams

Excellent

Transition from sensitive to impacted streams (IS) Impacted streams Transition from impacted to nonsupporting streams (NS) Nonsupporting streams

Good

Fair

Transition from nonsupporting to urban drainage (UD) Urban drainage

49

Poor

Impervious cover Range Average 0 - 5% 2.5%

Pervious cover Range Average 95 - 100% 97.2%

5 - 10%

7.5%

90 - 95%

92.5%

10 - 20%

15%

80 - 90%

85%

20 - 25%

22.5%

75 - 80%

77.5%

25 - 60%

42.5%

40 - 75%

57.5%

60 - 70%

65%

30 - 40%

35%

70 - 100%

85%

0 - 30%

15%

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As illustrated in the table, sensitive streams (SS) that continue to have near perfect hydrologic functions are within watersheds that have more than 95% pervious cover. Manwhile, impacted streams (IS) that adequately perform its hydrologic functions typically have 80% - 90% porous landscape cover. Once the permeable surface drops below 40% and hovers around 75%, the streams become nonsupporting (NS) and stream degradation will occur. Continued reduction in permeable surface and prolonged impairment on the streams will affect its recovery and predevelopment hydrological function might not be possible. Once pervious cover within the area is reduced to less than 30%, streams become urban drainage (UD) and they are sometimes turned into enclosed storm drains or completely reclaimed (Schueler, 1994) (Schueler et al., 2009).

7.4

Scenarios

The land use zoning in the peri-urbanization project of Boeung Cheung Ek is diverse, varying from residential type to administration and commercial. The development is predominantly residential in nature with another majority given to commercial uses. Water resources is also a somewhat big part while green spaces are minor in comparison to the other land use type.

Land use in ING City was classified into the land use types set out by the Cambodian government (derived from Fig 4.4 and Table 7.1) in the following list: -

Low-rise residential → Discontinuous low rise (It is uncertain if every low-rise residence will be discontinuous in nature, but the Villa Town project proposed in the first phase of ING City indicates that low rise housings will mostly be in the form of villas in individual parcels (ING Holdings, 2021))

-

Medium-rise residential → Medium rise

-

High-rise residential → High rise

-

Mixed-use → Mixed-use

-

Central business district (CBD) → Mixed commercial

-

Administration office, government center, ISPP (private international school), primary school, secondary school, and high school, police station, hotel, fire station, electrical substation, electricity station, stormwater treatment plant, and water supply → Administration and service units

-

Bus interchange → Transportation units

-

Green space and heritage park → Public space and green space units

-

Lake/river → Restricted land use (water resources)

-

Resettlement land → Other units

Landscape coverage of Sub-Decree No.42 were used to created the ‘existing regulation’ scenario. While three other scenarios were proposed by suggesting landscape coverage ratios based on this scenario and the ICM input into the different land use types of ING City. The ‘existing regulation’ development option was used as a baseline for the peri-urbanization project of ING City and the flood susceptibility of Phnom Penh in this scenario was the reference for the flood suscpetibility of the three other proposed scenarios.

7.4.1 Existing regulation scenario It is unclear whether ING City, the peri-urbanization project of Boeung Cheung Ek, was approved before or after the passing of Sub-Decree No.42. The time of its approval is important because if this peri-urbanization project was approved before Sub-Decree No.42 was put into place, it will be exempted from meeting the requirements set out by this regulation. However, in this scenario, it is assumed that the permeability of this project will follow the regulation set out by the government. Flood susceptibility of this scenario is determined through the averaging of the ranges of perviousness that were proposed in Sub-Decree No.42.

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50

The existing regulation scenario will serve as the baseline for comparison. The landscape coverage indices and their flood risk rating of this scenario are illustrated in Table 7.3 while visual of the landscape coverage ratio in each land use type for this development pattern can be viewed in Fig 7.7. The map demonstrating the flood risk of each land use type in this scenario is shown in A-Fig 7.1a.

As demonstrated in the table, if the peri-urban area is to be developed according to the mandated guideline, the maximum permeable landscape cover of every land use type, except the public and green space unit, will cap at 75%. This means that in the existing regulation scenario, nearly all the land use of ING City has the potential to be nonsupporting stream type (NS), which was established to be at risk of being incapable of fulfilling their hydrologic functions of water retention and infiltration (Schuler, 1994) (Schuler et al., 2009).

Moreover, the average pervious cover of every land use type, except the public and green space unit, consistently falls below 57.5%. This low pervious landscape coverage puts the hydrology of most of the land use type as urban drainage which has lost of hydrological function.

Table 7.3 Landscape coverage index and flood risk level of existing regulation scenario

Land use type

Landscape coverage % Range

Average

- Discontinuous low rise

15 - 50%

32.5%

- Continuous low rise

7.5 - 25%

16.25%

12 - 40%

26%

10.5 - 35%

22.75%

7.5 -25 %

16.25%

Flood susceptibility

Stream type (Average)

Residential units

- Medium rise - High rise - Mixed residential

High risk (4)

Trans. from NS to UD

Very high risk (5)

UD

Commercial units - Commercial

9 - 30%

19.5%

Industrial units

12 - 40%

31%

High risk (4)

Trans. from NS to UD

Mixed-use units

9 - 30%

19.5%

Very high risk (5)

UD

12 - 40%

31% High risk (4)

Trans. from NS to UD

Moderate risk (3)

UD

- Mixed commercial

Transportation units Administration & services units Cultural & religious units Tourism units

15 - 50%

32.5%

27 - 90%

58.5%

35 - 75%

55%

Other units Public space & green space units Restricted land use units - Agriculture, forested area - Water resource - Protected area

7.4.2 Mild landscape coverage scenario The mild landscape coverage scenario is built upon the existing regulation. However, it attempts to elevate the hydrology of ING City’s land use from nonsupporting stream type to the impacted stream type (IS). This is to encourage the application of green infrastructure into the study area, as recommeded by GGGI (2019) to integrate sustainable urban drainage system into flood prone area. This stream type was shown to have good hydrologic functions which can aid in stormwater management (Schuler, 1994) (Schuler et al., 2009).

To reach this feat, the pervious surface per watershed area must ranges between 80% to 90%, the permeable surface range of impacted stream type watershed. Furthermore, to ensure that overall pervious cover is substantially better and more effective than the existing regulation scenario, the minimum landscape coverage percentage is also increased. Hence, the maximum landscape coverage of each land use type is increased to 90% of the plot area for this scenario. The minimum pervious landscape cover that developers 51

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must meet is the average percentage of the existing regulation. With these numbers in consideration, the overall hydrology of ING City will at least be in the nonsupporting stream type. It is expected that the hydrology of the peri-urban area will still be functional in this scenario, but it is still at risk of stream degradation.

To achieve this scenario, it is recommended that stormwater management tools of the public realm (streets, parks, plazas, etc.) be of moderate intensity while mild stormwater management techniques can be applied in the private realm. Suggested integrated urban stormwater management tools in this scenario are: -

Public realm: permeable pavement, enhanced tree pit, stormwater pond, and infiltration trench.

-

Private realm: green roof, rainwater harvesting, permeable pavement, and infiltration trench.

The landscape coverage indices and their flood risk rating of this scenario are illustrated in Table 7.4 while visual of the landscape coverage ratio in each land use type for this development pattern can be viewed in Fig 7.8. The map demonstrating the flood risk of each land use type in this scenario is shown in A-Fig 7.1b.

Table 7.4 Landscape coverage index and flood risk level of the mild scenario

Land use type

Landscape coverage %

Flood susceptibility

Stream type (Average)

Range

Average

- Discontinuous low rise

50 - 95%

72.5%

Low risk (2)

- Continuous low rise

25 - 95%

60%

Moderate risk (3)

40 - 95%

67.5%

Low risk (2)

35 - 95%

65%

25 - 95%

60%

Residential units

- Medium rise - High rise - Mixed residential Commercial units - Commercial

Moderate risk (3)

30% - 95%

62.5%

Industrial units

40 - 95%

67.5%

Low risk (2)

Mixed-use units

30 - 95%

60%

Very high risk (5)

40 - 95%

67.5%

50 - 95%

72.5%

90 - 95%

92.5%

75 - 95%

85%

- Mixed commercial

Transportation units Administration & services units Cultural & religious units Tourism units

Trans. from NS to UD

Low risk (2)

Other units Public space & green space units Restricted land use units - Agriculture, forested area - Water resource - Protected area

Trans. from IS to SS Moderate risk (3)

IS

7.4.3 Moderate landscape coverage scenario As state by the GGGI (2019), on top of the application of sustainable urban drainage system, protection of existing natural wetlands, lakes, and canals is a must in the flood management effort of Phnom Penh.

Coincidentally, the peri-urban area was proven to be a major player in the flood control of the city as well. As ING City is developing the peri-urban parameter of Boeung Choeung Ek, which is a zone that is dense with wetland systems and proven to be an effective flood control tool, it is vital that the hydrological asset of this area is preserved.

Therefore, to protect and restore the post-development urban hydrology of ING City, land use types should transition from impacted stream type to the sensitive stream type. Moreover, to ensure that the overall hydrological function of ING City in this scenario will be of higher quality than the existing regulation option,

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52

the minimum value of the landscape coverage in this development option will be based on the maximum landscape coverage percentage of the existing regulation scenario.

Hence, the maximum pervious percentage in this scenario is increased to 95%, to meet with the maximum 95% permeable surface per watershed that was set out by the ICM. The minimum value of the landscape coverage in the moderate scenario is the maximum landscape coverage percentage of the existing regulation development option. With these numbers in consideration, the public and green space unit has the potential to become sensitive stream type land use that can nearly perfectly function like their pre-development stream type. The hydrology of the urban built-up land use in this scenario becomes nonsupporting stream, which is still functional but at risk of stream degradation if the permeable surface fall below the threshold.

The stormwater management technique in the private and public realm will be more rigorous than the mild scenario in an effort to restore the urban hydrology. Private realm will be as stringent as possible and the recommended green infrastructure tools for the moderate landscape coverage scenario are: -

Public realm: bioretention cell, bioswale, constructed, permeable pavement, enhanced tree pit, stormwater pond, and infiltration trench.

-

Private realm: bioretention cell, green roof, rainwater harvesting, permeable pavement, and infiltration trench.

The landscape coverage indices and their flood risk rating of this scenario are illustrated in Table 7.5 while visual of the landscape coverage ratio in each land use type for this development pattern can be viewed in Fig 7.9. The map demonstrating the flood risk of each land use type in this scenario is shown in A-Fig 7.1c.

Table 7.5 Landscape coverage index and flood risk level of the moderate scenario

Land use type

Proposed landscape coverage %

Flood susceptibility

Proposed range

Proposed average

- Discontinuous low rise

50 - 95%

72.5%

Low risk (2)

- Continuous low rise

25 - 95%

60%

Moderate risk (3)

40 - 95%

67.5%

Low risk (2)

35 - 95%

65%

Moderate risk (3)

Stream type (Average)

Residential units

- Medium rise - High rise - Mixed residential Commercial units - Commercial

25 - 95%

60%

- Mixed commercial

30 - 95%

62.5%

Industrial units

40 - 95%

67.5%

Low risk (2)

Mixed-use units

30 - 95%

62.5%

Moderate risk (3)

40 - 95%

67.5%

50 - 95%

72.5%

90 - 95%

92.5%

75 - 95%

85%

Transportation units Administration & services units Cultural & religious units Tourism units

Moderate risk (3)

NS

Low risk (2)

Other units Public space & green space units

Very low risk (1)

Trans. from IS to SS

Restricted land use units - Agriculture, forested area - Water resource - Protected area

Low risk (2)

IS

7.4.4 Intense landscape coverage scenario Studies have shown that the manipulation of pervious surfaces allow green infrastructure to imitate pre-ex-

53

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isting hydrological condition and control flooding (Burns, Fletcher, Walsh, Ladson, & Hatt, 2012) (Carter & Jackson, 2007) (Jia, Lu, Yu, & Chen, 2012). GGGI (2019) also encourages the restoration of the natural hydrology of an urban area to its pre-existing state as well. So, in this scenario, each land use type will be given the potential to imitate the pre-existing hydrology of the study area. This gives ING City the opportunity to transform into the sensitive stream type which have near perfect hydrological function. To ensure that the overall hydrologyof the ING City in this development option can rise above that the nonsupporting stream type, the average landscape coverage of the moderate scenario will serve as the minimum for this scenario.

Thus, the maximum landscape coverage of the intense scenario is 100%, which is the maximum value of sensitive stream type that was shown to fulfill its hydrological functions nearly perfectly. The minimum landscape coverage of this scenario is based on the average value of the moderate scenario. With these numbers in consideration, the overall hydrology of the urban built-up land use in this scenario can be considered as the impacted stream type which can perform its hydrological functions sufficiently. Meanwhile, restricted land use is in the transition from impacted stream type to sensitive stream type and the public and green space units are fully functioning sensitive streams.

To ensure successful realization of the intense landscape coverage development option, stormwater management techniques must be as stringent as possible. Recommended green infrastructure types are: -

Public realm: bioretention cell, bioswale, constructed wetland, permeable pavement, enhanced tree pit, stormwater pond, and infiltration trench.

-

Private realm: living wall, stormwater pond, constructed wetlands, green roof, bioretention cell, rainwater harvesting, permeable pavement, and infiltration trench.

The landscape coverage indices and their flood risk rating of this scenario are illustrated in Table 7.6 while visual of the landscape coverage ratio in each land use type for this development pattern can be viewed in Fig 7.10. The map demonstrating the flood risk of each land use type in this scenario is shown in A-Fig 7.1d.

Table 7.6 Landscape coverage index and flood risk level of intense scenario

Land use type

Proposed landscape coverage % Proposed range

Proposed average

72.5% - 100%

86.25%

60% - 100%

80%

67.5% - 100%

83.75%

65% - 100%

82.5%

60% - 100%

80%

Flood susceptibility

Stream type (Average)

Residential units - Discontinuous low rise - Continuous low rise - Medium rise - High rise - Mixed residential Commercial units - Commercial - Mixed commercial

62.5% - 100%

81.25%

Industrial units

67.5% - 100%

83.75

Mixed-use units

62.5% - 100%

81.25%

67.5% - 100%

83.2.5%

72.5% - 100%

86.25%

92.5% - 100%

96.25%

85% - 100%

92.25

Transportation units Administration & services units

Low risk (2)

IS

Cultural & religious units Tourism units Other units Public space & green space units Restricted land use units - Agriculture, forested area - Water resource - Protected area

SS Very low risk (1)

Trans. from IS to SS

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54

Fig 7.7 Visuals of the existing regulation scenario (baseline for comparison)

Fig 7.8 Visuals of the mild scenario

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Fig 7.9 Visuals of the moderate scenario

Fig 7.10 Visuals of the intense scenario

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7.5

Testing and results

7.5.1 Overall flood risk of each scenario This section will present the overall flood risk results of Phnom Penh in each development options.

As can be seen from Fig 7.11a-d and table 7.7a, all the proposed scenarios will generally fare better than the business-as-usual peri-urbanization practice in terms of flood management of Phnom Penh. In every scenario, all flood hazard levels except low flood risk are lower than the flood susceptibility levels in the business-as-usual development model. Additionally, in all the scenarios, flood susceptibility downstream of the Boeung Cheung Ek peri-urban area is anticipated to be much less than BAU situation as well.

The very low flood hazard area of the city in the existing regulation, mild, and moderate permeability scenarios, is projected to be at 82.96 km2, which is only 1 km2 more than the current situation. Meanwhile, in the intense permeability scenario, very low flood risk area is projected to be measured at 83.00 km2. The numbers are much more favorable than the very low flood risk area of the business-as-usual situation, which predicted to expand up to 85.79 km2.

In terms of low flood risk, the predisposed area in intense permeability scenario is the lowest, at 232.42 km , while the affected area in moderate permeability scenario is anticipated to be 232.46 km2. The existing regulation scenario will have the largest area, measuring at 232.23 km2. These numbers are similar to the current low flood risk area, sizing at 234.62 km2. However, they are much worse than the situation in BAU peri-urbanization approach which is predicted to be at only 176.08 km2. However, the low flood risk area becoming greater may be a good think as it means a reduction in moderate, high, and very high flood risk zone.

2

As for the moderate flood hazard level, liable areas in the moderate and intense permeability scenarios are expected to be at 263.60 km2, the lowest out of the proposed scenarios. Moderate flood risk in the mild permeability comes second, at 263.91 km2, while existing regulation will have the highest value at 264.62 km2. Compared to the projected moderate flood risk area of 299.62 m2 of the business-as-usual situation, these numbers are promising and very close to the current moderate flood risk area which is 265.36 km2.

Areas that are highly vulnerable to flooding in mild, moderate, and intense permeability are expected to be the lowest, sizing at 55.15 km2. In the existing regulation scenario, high flood risk area will be 55.36 km2.

That is only 0.24 km2 more than the current situation but 10 km2 less than the high flood risk area of the business-as-usual scenario.

Very high flood susceptibility area in every scenario is anticipated to measure at 50.28 km2. these numbers are much more favorable than the business-as-usual development pattern which is predicted to expand up to 56.60 km2. That is 11.21 km2 more than the current high flood risk area, which valued at 45.39 km2, and very high compared to the additional 4.89 km2 of the very high flood risk of all scenarios.

7.5.2 Flood risk of each scenario in ING City This part will describe the flood risk results of ING City (study area, BCE) in each development options.

In the study area in particular, Fig 7.8a-d and Table 7.7b, flood risk results of all scenarios point to a more promising flood control asset than the business-as-usual situation. Perviousness of the mild permeability, moderate permeability, intense permeability, will prove to be the most effective flood management tool.

In the business-as-usual development model, 0.21 km2 of the area is expected to be very highly prone to flooding. However, in the proposed scenarios, very high flood risk will be nonexistent, just like the current situation. Furthermore, the highly flood-prone area of each scenario is also close to the 5.78 km2 of the current situation with moderate and intense permeability scenario being the lowest, 5.83 km2, the mild permeability coming second at 5.86 km2, and existing regulation being the highest with 6.04 km2.

Moderate flood risk of each scenario vary but will still better than the business-as-usual, which is expected 57

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Table 7.7a Flood risk area in all of Phnom Penh in every scenario Current situation

BAU situation

Existing Regulation

Mild scenario

Moderate scenario

Intense scenario

SqKm

SqKm

SqKm

SqKm

SqKm

SqKm

Very low (1)

83.96

85.79

82.96

82.96

82.96

83.00

Low (2)

234.62

176.08

231.23

232.46

232.46

232.42

Moderate (3)

265.36

299.62

264.62

263.91

263.60

263.60

High (4)

55.12

66.36

55.36

55.15

55.15

55.15

Very high (5)

45.39

56.60

50.28

50.28

50.28

50.28

Total

684.45

684.45

684.45

684.45

684.45

684.45

Table 7.7b Flood risk area in the study area in every scenario Current situation

BAU situation

Existing Regulation

Mild scenario

Moderate scenario

Intense scenario

SqKm

SqKm

SqKm

SqKm

SqKm

SqKm

Very low (1)

-

-

-

-

-

-

Low (2)

9.40

5.64

9.20

9.88

10.06

10.38

Moderate (3)

6.62

9.32

6.56

6.06

5.91

5.59

High (4)

5.78

6.63

6.04

5.86

5.83

5.83

Very high (5)

-

0.21

-

-

-

-

Total

21.80

21.80

21.80

21.80

21.80

21.80

to size up to 9.32 km2, 2.7 km2 more than the current situation. In the intense permeability scenario, only 5.59 km2 of the area are projected to suffer from moderate flood risk while up to 5.91 km2 of the study area in the moderate permeability scenario will be at moderate flood risk. 6.06 km2 of the study area in the mild permeability scenario is anticipated to be liable to moderate flood risk while up to 6.56 km2 in the existing regulation scenario proposed by the government.

In terms of low flood risk, results show that, in every proposed scenario except the existing regulation, the study area will have more area with low flood susceptibility. In the existing regulation permeability scenario, only 9.20 km2 of the area are predicted to suffer from low flood risk while up to 9.88 km2 of the study area in the mild permeability scenario will be at low flood risk. 10.06 km2 of the study area in the moderate permeability scenario is likely to be liable to low flood risk while up to 10.38 km2 in the intense scenario. These numbers are all higher than the anticipated 5.64 km2 low flood risk area of the business-as-usual scenario.

7.6

Discussion

7.6.1 Suitable peri-urbanization pattern In their attempt to propose a landscape cover guideline that could reduce the side effects of climate change and has the potential to endure hazards, the Royal Government of Cambodia is somewhat successful. As the results of this study has exemplified, peri-urban development that exercises the land use type’s pervious surface ratio mandated by the government will be able to reduce the flood risk of Phnom Penh to nearly the current flood risk level. However, ICM indicates that the Sub-Decree No.42’s landscape coverage index maybe severely limited in maintaining the flood protection asset of the peri-urban area since the average percentages show that, overall, ING City’s land use will fall into the urban drainage stream type. This means that, post development, the peri-urban area that once hydrologically function as water retention and infiltration basin is likely to be incapable of fullfilling its original hydrologic function. In addition, the proposed Ch7 Suitable peri-urbanization pattern

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Fig 7.11a Phnom Penh’ s flood risk map (‘existing regulation’ BCE’s development scenario)

Fig 7.11b Phnom Penh’ s flood risk map (‘mild’ BCE’s development scenario)

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Fig 7.11c Phnom Penh’ s flood risk map (‘moderate’ BCE’s development scenario)

Fig 7.11d Phnom Penh’ s flood risk map (’intense’ BCE’s development scenario)

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60

landscape coverage scenario is unsuccessful in providing a balance between urban built-up and the natural landscape interface since most land use types disproportionately favor impervious cover. Thus, the landscape coverage ratio implemented by the government is deemed insufficient.

On the other hand, the mild, moderate, and intense landscape coverage scenarios are shown to provide

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Fig 7.12a ING City’s flood risk map (existing regulation scenario)

Fig 7.12b ING City’s flood risk map (mild scenario)

Fig 7.12c ING City’s flood risk map (moderate scenario)

Fig 7.12d ING City’s flood risk map (intense scenario)

Ch7 Suitable peri-urbanization pattern

a slightly better resilience against flood risk while also maximizing pervious cover per land use type. The mild scenario will ensure that the hydrology within each land use will be within the nonsupporting stream type which could potentially fullfil its original role as a stormwater management asset. In the moderate development option, the maximum proposed permeable surface of urbanized land use type can reach up to 95%, allowing the natural urban stream to reach impacted stream type. Although the hydrology of the urbanized land use type is still stuck in the nonsupporting stream type, the natural land use can become impacted stream type which can adequately fullfil pre-development hydrological functions. Meanwhile, intense scenario demands that the urban built-up land use type dedicate a minimum average of 75% and maximum average of 80% of their land cover to percolation purposes. These numbers illustrate that in the moderate and intense development options, ING City has the potential to maintain pre-development flood control capacity.

Clearly, the intense landscape coverage scenario is the best scenario from a flood control stand point as it tries to ambitiously ensure that the post-development landscape is restored to its pre-development stage.

However, the realization of this development option is somewhat far-fetched due to the current nature of urbanization in Phnom Penh itself. Urban development in the city is primarily dominated by the conversion of natural land use to impervious urban built-up. In addition, the ING City project also favors impervious cover over pervious cover in their development plan so strict mandate to maximize.

More importantly, the biggest challenge in realizing the intense landscape coverage scenario in the peri-urban development is the lack of regulation for integrated sustainable urban drainage system in the current urban development governance. Despite the GGGI’s urge for implementation of water urban sensitive design (GGGI, 2019), there are no official mandates or manuals for developers to apply green infrastructure techniques in their projects. The Sub-Decree No.42 (RCG, 2015) is the only existing regulations that attempts encourage water sensitive urban design that can manage stormwater. Clearly, the current absence of legal binding power hinders the implementation of the intense landscape coverage scenari.

Nonetheless,the government has begun to develop many long-term plans to create a more sustainable building sector in Cambodia (GGGI, 2021). In fact, GGGI and CamGB (Cambodia’s Green Building council) recently entered into an agreement that will focus on the sustainable green growth of buildings and community in Cambodia. So, it can be said that even though the intense landscape coverage scenario is currently not applicable, it can benifit future developments. ` To conclude, although the intense development option is the most favorable, it is not the most plausible peri-urbanization pattern in the current situation. Under the current urban development climate, the moderate landscape coverage scenario is deemed to be the most sensible peri-urbanization pattern. Meanwhile, once more rigorous laws on water sensitive urban design are authorized, the intense landscape coverage scenario can be applied in future peri-urban development not only in Phnom Penh but also in cities within Cambodia.

7.6.2 Expected challenges One of the issues that is projected to challenge the implementation of the moderate landscape coverage scenario is the fact that Phnom Penh is predicted tol continue heavily depend on grey infrastructure for flood control. As explored in chapter 3, future flood protection projects that were approved within Phnom Penh are robust grey infrastructure types that focuses on evacuating water out of urbanized area and into outlier zones.

The main strategy for flood control is to rely on drainage pipes, drainage channels, and pump stations to move runoff to other undeveloped peri-urban arena for water retention. However, this study has established that grey infrastructure alone will be an insufficient flood management tool due to its reliance on the peri-urban arena. So, Phnom Penh’s modus operandi of funneling floodwater out to the edge area will no longer serve as an appropriate flood control measure. In fact, it will put additional strain on the developments in the peri-urban area as the urban projects in this zone will have to handle the runoff from other parts of the city on top of ensuring its own flood protection. Moreover, urban development projects in the peri-urban parameter cannot solely count on grey infrastructure since it must maintain its landscape interface so that it can serve as a flood control tool for Phnom Penh. That being the case, floodwater in the peri-urban area should no longer Ch7 Suitable peri-urbanization pattern

62

be treated as an obstacle but should instead be embraced in the urban fabric.

Another challenge that is expected to prevent the moderate permeability scenario from being effectively realized is the ineffective flood protection adaptations on the local scale. Although the local’s low-cost and ingenuous efforts to live with rainfall and runoff in the peri-urban area are commendable, they are too disorderly and entirely unsustainable. Coping strategies and adaptations to flooding in the community vary from one location to another and are too individualistic due to the lack of policies and regulations. They are too incohesive and disconnected, making them inconsequential in the grand scheme. Moreover, these small-scale flood protection solutions are focused on keeping the floodwater away from their daily life facilities, treating the water as a nuisance.

Additionally, due to the lack of coherent policies and regulations, the urban drainage system of Phnom Penh as a whole has become disconnected and incohesive. New urban projects in the peri-urban fringe operate independent drainage systems that tamper with the city’s already fragile flood management system, further exacerbating the flood risk. Thus, there is a need for a cohesive and coherent drainage practice that can blend with the existing drainage network of Phnom Penh and does not interfere with the city’s flood management.

So, in essence, in order for the peri-urban area continue to be a flood control asset of Phnom Penh, an in-situ stormwater management technique needs to be introduced into the developments in the peri-urban arena. The green infrastructure tools must be cohesively integrated into the dense urban land use and work seamlessly with the existing traditional drainage system.

7.6.3 Vision As mentioned in Chapter 4, the peri-urbanization of Boeung Cheung Ek is somewhat flexible. Therefore, it is likely that the intensification of pervious cover as well as the introduction of the stormwater management techniques are welcome additions. Furthermore, increased in green landscape coverage and usage of sustainable urban drainage system will alleviate peri-urbanization projects’ vision for creating sustainable and climate-resilient communities. These visions align with Cambodia’s attempt to combat weather hazards and climate risks.

Hence, this study will make several crucial suggestions concerning green infrastructure techniques to implement to achieve the water sensitive built environment: -

Large public spaces and water bodies should not merely function as a lake but as a constructed wetland. Firstly, leaving the water surface as is make it highly susceptible to flooding, which is undesirable.

Secondly, the Boeung Cheung Ek peri-urban area has always acted as a natural water treatment system for Phnom Penh and the water remaining lake within the ING City will most likely continue to serve this purpose. Therefore, it is only logical that it be transformed from a single use water body into a multi-used stormwater wetland that not only act increase water retention capability but also function as an end-ofpipe stormwater tool with options for recreations as well.

-

Techniques such enhanced tree pit, bioretention cell, permeable pavement, inflitration trench, green roof and living wall should be introduced into every land use type, especially in the mixed-use, mixed commercial, and high density residential area. Meanwhile, the low and medium density residential land use type could benefit to employ the rainwater harvesting techniques.

-

Dry swale and dry pond are most suitable in the residential areas because they are more favorable as the watery aspect of wet swale and wet pond raises various concerns in this land use type.

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Fig 7.13a Public/green space and water resources

Fig 7.13b Mixed-used / mixed commercial

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64

Fig 7.13c Low rise residential

65

Ch7 Suitable peri-urbanization pattern

Chapter 8 Summary

8.1

Summary of findings

This research studied the extent in which the peri-urban area, especially the Boeung Cheung Ek area, contributes to the flood protection system of Phnom Penh by doing a flood risk mapping. It also utilized scenario modeling methods to identify possible development options for the peri-urban area. These scenarios were then analyzed with flood risk analysis to determine to the best urbanization pattern that is suitable for the balancing the anticipated dense urban built-up of developed peri-urban fabric while maintaining the area’s flood management value. (The conceptual framework of this thesis is illustrated in Fig 8.1.)

8.1.1 Peri-urban’s contribution to flood management Two flood susceptibility maps were created to investigate how much the peri-urban area contributes to flood control of Phnom Penh. Two scenarios were studied: before BCE’s development (current situation) and after BCE’s development (business-as-usual situation). The breakdown of this map showed that if the peri-urban parameter is allowed to be encroached upon with the usual urbanization practice, the flood risk of Phnom Penh will increase exponentially.

Land reclamation will severely change the Boeung Cheung Ek’s elevation which will drastically affect the natural stream network and stream density within the study area. These changes are not limited to the study area as the stream network across the city are projected to elongate in response to the topographical modification. Even the parts of Phnom Penh that is on the other side of the rivers are influenced by the alteration made in Boeung Cheung Ek. Moreover, the shift from agricultural and rural landscape to an urban fabric will have a bad impact on the LULC and landscape coverage of the area. These landscape interface elements were identified as the major attributes that can increase or decrease flood risk within the city. Moreover, previous research indicated that impervious cover can greatly affect the flow of water and alter the watershed’s capability in terms of water management. Flood susceptibility mapping also illustrated that the amount of pervious and impervious surface in the peri-urban development is the biggest uncertain factor of the project. Hence, it was concluded that the landscape coverage ratio is the main flood conditioning factor.

The identification of landscape coverage ratio as the main flood control factor allows for different development options to be explored even if the spatial and hydrological property of the peri-urban area are permanently altered in preparation for large scale peri-urbanization.

8.1.2 Suitable peri-urbanization pattern Three scenarios were developed and then compared to the existing regulation scenario, the development option proposed by the government. These scenarios are: mild, moderate, and intense landscape coverage scenarios (Table 8.1). They alter the pervious and impervious surface ratio of the Boeung Cheung Ek study area in an attempt to identify the most fitting landscape coverage index that will maintain porose interface but still allow for dense urban built-up in the peri-urban arena.

Compared to the business-as-usual peri-urban development approach, in the existing regulation scenario and in the proposed scenarios, Phnom Penh will have relatively lower flood risks. Results indicate that if Boeung Cheung Ek follows any of the proposed development options, there will be a significant decrease in very high and high flood prone area compared to the normal post-development situation. Flood susceptibility downstream of the peri-urban area in every scneario is anticipated to be a lot lower than the business-as-usual development model as well. Moreover, the expected flood susceptibility area of each hazard level in the proposed scenarios is anticipated to be at a similar scale as the flood prone area in the pre-development scenario.

Favorable flood susceptibility results of the mild permeability, moderate permeability, and intense landscape coverage scenarios indicate that the pervious surface ratios proposed in these scenarios are much 67

Ch8 Summary

Research background 1

2

Flood issue - Rely on aging and insufficient, grey infrastructure - Also relies on the peri-urban area for water retention Peri-urbanization problem - High pressure to develop the urban fringe - Unsustainable peri-urbanization practice (land reclamation) is linked to increased flooding

Research objectives 1

Provide evident-based reasonings that the peri-urban area is a major flood management tool that should be preserved

2

Suggest a suitable peri-urbanizationi practice that allows for intense development but still maintains the flood management value of the area

Study area (BCE) - Allegedly the major flood control asset of Phnom Penh - Currently being reclaimed for development

Peri-urban area’s contribution to flood management

1

Developing the BCE peri-urban area with the business-as-usual approach will exponentially increase Phnom Penh’s flood risk

Altered spatial and hydrological factors - Elevation: affects stream network, stream density, TWI, and SPI - Land use change: affects landscape coverage’s flood risk

Flood control factor - Landscape coverage ratio

Stormwater management techniques (Retention-based & detention-based green infrastructure techniques) 2

Suitable peri-urbanization pattern Landscape coverage is the key to achieving a sustainable peri-urbanization approach that can preserve pre-development hydrology and allows for intense peri-urban development

Scenarios ‘Mild’ scenario: introduction of sustainable urban drainage system ‘Moderate’ scenario: preservation of the post-development’s urban hydrology ‘Intense’ scenario: restore post-development’s hydrology to pre-development condition

Expected challenges - continued dependency on grey infrastructure - disconnected & incoherent urban drainage system

Fig 8.1 Conceptual framework of study on the peri-urban contribution to flood management

Ch8 Summary

68

more suited for flood control than the landscape coverage ratio proposed by the government in the existing regulation scenario. Although the intense scenario has the a slightly better outcome, the moderate scenario was deemed to be the most application peri-urbanization pattern in the current urban development climate.

The intense landscape coverage scenario can be implemented in future developments once there are more stringent regulations flood flood protection.

Some anticipated challenges were taken into consideration and they are: continued reliance on grey infrastructure, disconnected urban drainage system that tampers with the city’s already weak flood control system, and rainfall and runoff being treated as obstacles. These are existing issues that could potentially crop up within the peri-urbanization projects and interfere with the implementation of the moderate scenario.

To ensure that the scenario can come to fruition, two integrated urban drainage system were suggested to be implemented: retention-based and detention-based green infrastructure techniques. These techniques are: -

Public realm: bioretention cell, bioswale, permeable pavement, enhanced tree pit, stormwater pond, constructed wetland, and infiltration trench.

-

Private realm: green roof, living wall, stormwater pond, bioretention cell, rainwater harvesting, permeable pavement, and infiltration trench.

This standardized landscape coverage ratio and suggestion for green infrastructure to supplement grey infrastructure will open new avenues for further discussion on sustainable peri-urbanization. It will also encourage new water sensitive urban design approaches that fits the developing urban patterns where the local environment still needed to be preserved.

Table 8.1 Proposed landscape coverage ratio for all three scenarios Land use type

Mild scenario

Moderate scenario

Intense scenario

Improve peri-urban area’s hydrology beyond urban drainage

Preserve the peri-urban area’s post-development hydrology

(mild green infrastructure in private realm, moderate green infrastrcutre in public realm)

(Moderate green infrastructure in private realm, stringent green infrastructre in public ream)

Restore the peri-urban area’s post-development hydrology to its pre-development function (Stringent green infrastructure in both private and public realm)

Range

Average

Range

Average

Range

Average

32.5 - 90%

61.25%

50 - 95%

72.5%

72.5% - 100%

86.25%

26 - 90%

58%

40 - 95%

67.5%

67.5% - 100%

83.75%

19.5 - 90%

54.75

30 - 95%

62.5%

62.5% - 100%

81.25%

31 - 90%

60.5%

40 - 95%

67.5%

67.5% - 100%

83.2.5%

32.5 - 90%

61.35%

50 - 95%

72.5%

72.5% - 100%

86.25%

58.5 - 90%

74.25%

90 - 95%

92.5%

92.5% - 100%

96.25%

55 - 90%

72.5%

85%

85% - 100%

92.25

Residential units - Discontinuous low rise - Medium rise - High rise Mixed commercial Mixed-use units Transportation units Administration & services units Other units Public space & green space units Restricted land use units

8.2

75 - 95%

Further studies

This study only investigated the effect of peri-urbanization within the city. However, there is also evidence suggesting that the area downstream of the developing peri-urban parameter will be greatly impacted by this event. Hence, future research can discuss peri-urbanization in the regional city context by analyzing the changes in hydrology downstream.

Alternatively, further studies can be conducted within the peri-urban arena after the introduction of the stormwater management techniques introduced in this thesis. Hydrological and spatial changes within the Boeung Cheung Ek should be examined so that better design alternatives and policies can be created to ensure that the peri-urban’s flood protection asset is properly preserved and utilized.

69

Ch8 Summary

Furthermore, this research is limited to the environmental aspect of flood susceptibility and water sensitive peri-urbanization patterns. Social and economical aspects of flood risk and sustainable urban drainage systems in peri-urban areas should be explored in order to study these subjects so that a holistically sustainable peri-urban development option can be identified.

Ch8 Summary

70

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Appendix

Fig 4.4 ING City’s conceptual land use masterplan

A-Fig. 4.1 ING City’s 2017 conceptual land use masterplan

A-Fig 6.1 Geology map of Cambodia

77

Appendix

A-Fig 6.2a Elevation before BCE’s development

A-Fig 6.3a Catchment and natural stream network before BCE’s development

A-Fig 6.4b LULC before BCE development

A-Fig 6.2b Elevation after BCE’s development

A-Fig 6.3b Catchment and natural stream network after BCE’s development

A-Fig 6.4c LULC after BCE development

Appendix

78

Land use land classification of Phnom Penh in this study is conducted with the Image Classification (Image Analyst tool) and utilized Landsat8 OLI TIRS Level 1 data, in 30m, dated on 2021/01/15.

First, seven bands of the Landsat8 data were composited into a single data by using the Composite Band function (Data Management tool).Then, training samples were drawn and qualified by using the Training Samples Manager (Image Analyst tool) to reflect current land use of Phnom Penh. This is done with crossreferencing with existing maps, Google Earth map, and ground-truth data. Finally, land use land cover classification is performed by using the maximum-likelihood algorithm of supervised classification for pixel clusters based on the classification system presented in A-Table 6.1.

A-Table 6.1 LULC classification system for use with remote sensor data

Urban fabric or Residential

Urban or built-up land

Artificial surfaces

Industrial and commercial units Non-residential urban fabric Road and rail network and associated land Airport areas

Other urban or built-up area & barren land

Construction sites Reclaimed land Empty land without current use Transitional land, waiting for development

Urban greenery Agriculture & cropland

Non-artificial surfaces

High density continuous urban fabric (sealing level (SL) > 80%) Medium density discontinuous dense urban fabric (SL 50% - 80%) Low density discontinuous urban fabric (SL 10% - 50%)

Industrial, commercial & services, transportation, communitication, utilities

Bare land

Agriculture & urban green

Forest land

Parks, promenades Outdoor sport and leisure facilities Cropland, pasture, orchards, groves, vineyards, nurseries and other ornamental horticultural areas Deciduous, evergreen, mixed forest land

Wetland

Natural and seminatural land, including wetland

Water

Water

Forested wetland and nonforested wetland

Stream, canal, lake, reservoir, bay and estuary

(1) Source: (Anderson et al., 1976) (2) Source: (EO4SD, 2019) (3) Cambodian Government LULC Classification A-Table 6.2 Cohen’s Kappa (Accuracy assessment of Phnom Penh’s LULC detection) Class

Water Bodies High Density Medium Density Low Density

Bare Land

Wetlands Agriculture Urban Green Total User Accuracy

Kappa

Water Bodies

68

0

0

0

0

0

0

0

68

1

0

High Density

0

40

2

0

0

0

0

0

42

0.952380952

0

Medium Density

0

4

66

2

0

0

0

1

73

0.904109589

0

Low Density

0

0

6

68

1

0

1

2

78

0.871794872

0

Bare Land

0

0

2

1

23

0

0

0

26

0.884615385

0

Wetlands

1

0

0

0

0

48

1

0

50

0.96

0

Agriculture

0

0

0

1

0

1

103

1

106

0.971698113

0

Urban Green

0

0

0

1

0

2

1

56

60

0.933333333

0

73

24

51

106

60

503

0

0

0

0.938369781

0

0

0

0.928214643

Total

69

44

76

Product Accuracy

0.985507246

0.909090909

0.868421053

Kappa

0

0

0

0.931506849 0.958333333 0.941176471 0.971698113 0.933333333 0

0

0

0

0

An accuracy assessment was also performed to ensure that the LULC classification is as accurate as possible. Assessment begins with verification of detected results against actual condistion based on random 79

Appendix

points generated by ArcGIS program. Then, an error matrix was conducted to define overall accuracy while a product matrix was deteremined from the random points. Cohen Kappa coefficient was automatically determine to judge the accuracy of land use detection.

Cohen’s Kappa coefficient is a statistic that is used to measure inter-rater reliability for categorical items (McHugh & Mary, 2012). The accuracy range is as followed: - 0.01– 0.20 = none to slight (0 - 4%) - 0.21– 0.39 = minimal (4 - 15%) - 0.40– 0.59 = weak (15 - 35%) - 0.60– 0.79 = moderate (35 - 63%) - 0.81– 0.89 = strong (64 - 82%) - 0.91–1.00 = almost perfect (82 - 100%) - Percent agreement, 61% = problematic, almost 40% of the data in the dataset represent faulty data As the Cohen’s Kappa coefficient of the LULC classificiation is 0.92, an almost perfectly accurate assessment, reclassificatioin was not implemented. The LULC detection was directly used to derive the LULC dynamic and permeability criteria of Phnom Penh before and after BCE’s peri-urbanization.

A-Fig 6.5a Elevation

A-Fig 6.6a Elevation

A-Fig 6.5b Slope

A-Fig 6.6b Slope

Appendix

80

81

A-Fig 6.5c Curvature

A-Fig 6.5c Curvature

A-Fig 6.5d Distance to stream

A-Fig 6.6d Distance to stream

A-Fig 6.5e Stream density

A-Fig 6.6e Stream density

Appendix

A-Fig 6.5f Stream Power Index

A-Fig 6.6f Stream Power Index

A-Fig 6.5g Topographic wetness index

A-Fig 6.6g Topographic wetness index

A-Fig 6.5h LULC

A-Fig 6.6h LULC

Appendix

82

A-Fig 6.5i Landscape coverage

A-Fig 6.6i Landscape coverage

A-Fig 6.5 (a - i) Spatial and hydrological flood inducing factors, before BCE’s development A-Fig 6.6 (a - i) Spatial and hydrological flood inducing factors, after BCE’s development

A-Fig 6.7 Rainfall flood inducing factor of Phnom Penh (2011 - 2020)

83

Appendix

A-Fig 7.1a Flood susceptibility ranking of ‘existing regulation’ landscape coverage

A-Fig 7.2a Flood susceptibility ranking of mild scenario

A-Fig 7.3a Flood susceptibility ranking of moderate scenario

A-Fig 7.4a Flood susceptibility ranking of intense scenario

Appendix

84