Green the gap

GREEN THE GAP

A catalogue presenting three contextual landscape-based stormwater solutions supporting the social life of Ho Chi Minh City, Vietnam

Cl imate c h a n g e & its impac t on H o C hi M i nh C i t y

Introduction Climate change is threatening many cities all over the world and Ho Chi Minh city is no different. Rapid urbanization and destruction of natural habitats has made the city vulnerable to flooding caused by sea level rise and typhoons. The city was built on old marshland and coastal wetlands, thus the land is subsiding. These three challenges are making the city extra vulnerable to the impacts of climate change. This catalog presents three alternative landscape-based stormwater solutions to mitigate flooding, Urban Heat Island effect and enhance the social life of Ho Chi Minh City. The three contextual solutions can be modified and adapted to fit another city in the same climate zone, dealing with some of the same challenges.

A low-lying vulnerable urban fabric With HCMC located on the northern fringe of the Mekong Delta and a large part of the urban fabric is built on old marshland. Approximately 60% of the city is at an elevation 1 m above sea level and 45% of the urban fabric is 0+1 m above sea level (Katzschner et al. 2016). Not only does this mean that the urban fabric is vulnerable to the aftermaths of flooding, but because of decreasing groundwater levels the land is also subsiding (Katzschner et al. 2016). Almost half of the wards in HCMC are experiencing frequent flooding during the rainy season. The future effects of climate change will intensify the heavy monsoon rains, increase SLR and storm surges will be more intense and frequent (Katzschner et al. 2016).

A vulnerable city HCMC is between the top 5 cities in the world, where the civil society is being largely exposed to flooding.

Studies have found that from 1958 to 2007 there is an increase in temperature across the world. The average temperature has increased with 0.5-0.7 ยบC (Katzschner et al. 2016). Although there is not a clear pattern of an increase in precipitation in that period, the intensity of typhoons has in increased and occur more often the past years (Katzschner et al. 2016). Furthermore, the sea level along the Vietnam coast have risen with an annual rate of 3 mm (Katzschner et al. 2016) an oceanographical station in the north of Vietnam shows that the sea level has risen 20 cm from 1958 to 2007 (Katzschner et al. 2016).

The consequences of the rapid urbanization From 1989 to 2006 the amount of impermeable surfaces in the urban area has doubled (Katzschner et al. 2016). The rapid urbanization of the city transformed natural habitats such as forests and wetlands with permeable surface, into urban areas with roads and buildings. Due to the amount of areas that have lost its function as a natural infiltration and evaporation, that prevented part of the city to flood, has led to an increase in surface runoff. Furthermore, the urban development in the southern and western part of HCMC is built on old wetland and riverbanks, decreasing the storage for the natural water network, and occupying areas that used to work as natural retention areas and floodplains (Katzschner et al. 2016). Hence the increase in precipitation and the intensity and frequency of the monsoon rains, will increase the surface runoff in the sealed off urban areas (Katzschner et al. 2016).

A flooded street in Ho Chi Minh City Image source: https://bobandsuewilliams.com/explore/Ho-Chi-Minh-City-flood/

Reference: Katzschner, A., Schwartze, F. and Schmidt, M. (2016) Part I Introduction in: Sustainable Ho Chi Minh City: Climate Policies for Emerging Mega Cities, Springer p.1-3.

The desig n

Contextual design solution based on Detention, conveyance and evaporation The design proposals are based on three fundamental landscape-based stormwater solution principles; Detention, Conveyance and Evaporation.

The three landscape-based stomwater solutions are contextual solutions emerging from the wishes and needs from the locals, gathered through field research and informal interviews. Furthermore, the field observations have contributed to the understanding of the morphology and praxeology of the streets in HCMC. Understanding the space and social conduct has been an important factor, in order to design a solution that does not intervene with people’s daily life and livelihood an improves the quality of lived experience after a stom event.

Designing for the people of Ho Chi Minh City The contextual designs are exemplified in the case area, with illustrations and renderings. The solutions are not designed exclusive for the case area but are solutions that can be adapted and implemented in most areas in HCMC. It has been a crucial design criterion, that the solutions can be implemented in the built-up environment, in ways that provide safety and beneficial spaces for low-income families without compromising the use of space. Thus, a community-based adaptation approach plays an important role for the design of sustainable and feasible solutions in order to create an integrated design, in terms of use, aesthetics and simple technology.

The proposed design solutions supports the social life of HCMC by enhancing the livability in dense urban areas.

Detention

Conveyance

Evaporation

The desig n - Co n c e p t fo r t he S w al e Evaporatio n The Swale is the vegetated stormwater solution with evaporation as its primary function. The size of the swale can be modified, so it fits the surroundings it is implemented in. It is designed with 40 cm of mulch, so the plants can thrive, an underground reservoir with chippings, and an under-drain to convey the water, to a detention basin. It is encouraged to plant species that can be found in the HCMC Region, to ensure the plants will thrive. Examples of such species can be found next to the illustration of the swale (see opposite page).

There was a wish from that locals that more trees should be planted in the urban area, because they provide shade and cooling effect. It is important to note that if trees are planted in the swale, they will need a separate plant bed with a root barrier ensuring the roots not to damage the under-drain.

Implementation The example illustrates the Swale implemented along the larger roads. It will become a dynamic

green division between vehicles and pedestrians and create a safer space for the people using the sidewalks for various activities, which is an important part of the urban culture.

The main function of the Swale is evaporation and evapotranspiration. The vegetation will have a cooling effect and reduce the UHI effect. The solution will also add aesthetic value and increase safety in the streets and neighborhoods. If the Swale is being implemented over a longer stretch, it is suggested that chippings made from recyclable building material, can be laid out perpendicular to the Swale, so people and motorbikes can access the sidewalk and to make sure that the Swale does not become a barrier.

It was observed that most of the larger roads have crossfall and therefore the swale is placed in the side of the road. If enough space is available, it is encouraged to implement the swale in both sides, in order to decrease the surface runoff.

1:200 Trees e.g Albizia julibrissin, Melaleuca quinquenervia and Terminalia catappa

Vegetation e.g Saccharum officinarum and Rhodomyrtus tomentosa

Under-drain ø 400 mm

Illustration of the Swale

Shophouse

Sidewalk 4m

Concrete corner stones Mulch 40 cm Reservoir Under-drain

Road 10-15 m

Sidewalk 4m

Office building 1:200

Private rooftops with planters

Social sidewalk with small businesses, street vendors and parking for motorbikes The swale is a green division between the people and the traffic, creating a safe space for pedestrians and the street vendors. A resevoir is implemented to increase the water capacity and an under-pipe in the bottom to convey the stormwater away

Shophouses and office buildings with private rooftops.

The shophouse is a typical building typology that can be seen in most areas in HCMC

Trees are plantet on the side of the swale towards the sidewalk. The trees provide shade for the people on the sidewalk, add capacity for evaporation, in addition it can reduce the UHI effect

Swale with vegetation that can be found in the HCMC region. A permeable path made of stones connects the street with the sidewalk, so pedestrians and motorbikes can cross

Isometric illustration of implementation of the Swale in street typology L

Rendering of the Swale

The desig n - Co n c e p t fo r t he Tr enc h Mul ti-func tio n a l The Trench is a multi-functional landscape-based stormwater solution supporting the social life of the sidewalks in HCMC. Through field observations and discussing needs and wishes with the locals, it became very clear that there was a need for a solution that decreases the amount of surface runoff, but at the same time does not take up space that is used by the streets vendors, restaurants, small shops, parking of motorbikes, and other sidewalk activities.

To accommodate the wishes and needs of the locals this solution also propose trees and it should again be noted that if trees are planted in the trench, they will need a separate plant hole with a root barrier ensuring the roots not to damage the under-drain.

Implementation The size of the trench can be modified so it fits the street and needs in the area and is solely replacing part of the sidewalk, not taking up any space from the streets.

The trench is constructed of chippings, made from reused construction material, to make the solution more sustainable. It is important to note that the size of the chippings has to be the same size, in order to make sure the water can penetrate. As an experiment, bamboo is used as underground stabilizers to ensure the road and sidewalk structure does not collapse. This solution also has an under-drain, to make sure that the water can be conveyed during heavy rainfalls.

1:100

Trees e.g Albizia julibrissin, Melaleuca quinquenervia and Terminalia catappa Chippings made from recycled building material. All stones must have the same grain size to ensure penetration

Underground stabilizer constructed with locally grown bamboo Illustration of the Trench Under-drain ø 400 mm

Office building

Bamboo Stabilizer Plant hole Reservoir Under-drain

Street 8m

Sidewalk 3m

Shophouse 1:100

Shophouses and office buildings with private rooftops.

The shophouse is a typical building typology that can be seen in most areas in HCMC

The trench is constructed with recycled building material, and makes it possible for people to still use the space for various activities. The underground reservoir increases the water capacity and an under-drain is installed to convey the water from the system to a detention basin

Private rooftops with planters

Trees planted on the side of the trench towards the sidewalk to provide shade for the people on the sidewalk

Isometric illustration of implementation of the Trench in street typology M

Rendering of the Trench

The desig n - Co n c e p t fo r t he R i l l Conveyanc e The third and last solution is a rill, designed for the many narrow alleys in the dense urban environment. The alleys are between 1 and 3 m wide, and are most often used by residents, pedestrians and motorbikes.

The Rill is a narrow landscape-based stormwater solution, and the main function is to convey the water away from the streets. To increase the capacity, an underground reservoir and an under-drain is installed. If the rill is implemented near a trench or swale, it would be feasible to connect the rill to one of the two solutions, in order to covey to water to a solution with more capacity.

Implementation The structure of the Rill is very is similar to the structure of the Trench. It was observed that many

of the alleys incline towards the center of the alley, hence the solution is suggested to be implemented in the middle of the alley. The chippings are made of recycled building material. It is again important that the size of the chippings are the same, to ensure penetration.

To make the solution more sustainable and because there are few to no heavy vehicles in the alleys, it is suggested that local timber, such as bamboo, can be used as an underground stabilizer. Bamboo was seen used as scaffolding throughout various construction sites and it has a high tensile strength.

Using it as an underground stabilizer, would be an experiment and HCMC and Vietnam could become known as an innovative region or country experimenting with sustainable, locally produced materials for climate adaptation.

1:100

Chippings made from recycled building material. All stones must have the same grain size to ensure penetration

Underground divider made of locally grown bamboo

Under-drain ø 400 mm

Small manufacture

Bamboo stabilizer Reservoir Under-drain

Shophouse 1:100

Shophouses and office buildings with private rooftops.

The shophouse is a typical building typology that can be seen in most areas in HCMC

Small manufactures and enterprises can often be seen in between residential shophouses

An underground reservoir to increase the water capacity and an under-drain is installed, to convey the water away

Private rooftops with planters

Narrow alley, mostly used by pedestrians and motorbikes

Rill constructed in recycled building material, to make it more sustainable. The chippings all have the same grain size, to ensure water is able to penetrate, and pedestrians and motorbikes can easily navigate through and in the space

Isometric illustration of implementation of the Rill in street typology S

Rendering of the Rill

The desig n - Ca n a ls Detention

Tan Hoa Lo Gom canal dist. 6

The canals in HCMC make up approximately 16% of the total area (Nigel et al. 2016) and can be an asset to the landscape-based stormwater solutions. It is suggested that the canals are used as detention basins for the stormwater, by connecting the landscape-based stormwater solution to nearby canals.

It was observed during the fieldwork that many of

the canals had extra capacity. Instead of relying on wetlands in the periphery of the city, the canals could be a temporary detention basin for the stormwater within the build-up environment, slowly discharging the water to a wastewater treatment plant, or conveyed to a wetland in the periphery of the city if the water is clean enough.

The desig n - Ov e r p a s s e s Detention

Rendering of an underground detention basin

According to the locals, the government has for the past year initiated a new green strategy underneath overpasses. Throughout the city, you can find small green oases, with paths leading you through green groundcover and smaller trees, and in the tropical climate it works as a cooling spot in the city.

To my knowledge this very unique for HCMC, and they have turned an urban space that is often occupied by homeless people and a space that can be unsafe, into a green oasis, that invites people in to enjoy the benefits of the shade and vegetation.

The space over ground is already being utilized, and in connection to the landscape-based stormwater solutions, it is suggested that an underground enclosed concrete detention basin can be installed to utilize the space underground. These underground detention basins can be constructed in different sizes, and it is another way of using the space in the build-up environment for detaining the water, until it can be discharged to a wastewater treatment plant, or if the water is clean, a natural or constructed wetland.

The val ues o f la n d s c a p e - bas ed s t or m w at er s ol ut i ons

In recent years, many cities all over the world have experienced serious consequences of climate change, and the need for resilient cities are of utmost importance. Asia is like many other parts of the world facing the threats of climate change, and frequently flooded cities has become a wellknown phenomenon. With the rapid urbanization that is examplified by this focus on HCMC, there is a pressing need to create solutions that, while mitigating flooding, ensure a sustainable development and create a livable city.

Blue and green infrastructure focusses on mobility, biodiversity, safety and recreation and are designs for the human scale, making people an integral part of the solution. By exposing people to a landscape-based stormwater solution, you also create awareness. People become one with the raindrop, and see how it travels, what it affects and what it can do. It can create awareness on how it affects

the environment and how it will affect future generations.

It is about creating simple solutions, looking at natural waterways, and combining functionality, efficient technology with high aesthetics.

When designing landscape-based stormwater solutions, it is of utmost importance to work with the local environment and a community-based adaptation, to ensure a sustainable solution that supports the urban lifestyle and a solution that does not compromise with the social conduct. There is a competition for space in dense urban environments, thus multifunctional natural systems that can integrated in space and time are important. Dynamic landscapes can add adaptability and flexibility to the urban environment, help mitigate the UHI effect, and decrease the surface runoff - thereby mitigating the flooding of people’s homes and livelihood.

Bishan park, Singhapore - A project upgrading the capacity of the Kallang river and transforming the surroundings into a park Image source: http://www.landezine.com/index.php/2012/06/kallang-river-at-bishan-ang-mo-kio-park-by-atelier-dreiseitl/

Contact Pernille Vad Normark Landscape Architect MDL vadpernille@gmail.com +4529872037