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Pinpointing the cause of regular short duration flooding in Davao City. Part of Bachelor of Built Environment Third year internship research water management at Rotterdam University of Applied Science. Project Document name Version Organizations Website Author Date, location

Hydrologic model of Davao City A.03 4.2 HELP Davao, TropICS, Ateneo de Davao, DOST XI Vincent Oskam 4 January 2012, Davao, Philippines

Region XI Davao

Causes of short duration flooding in Davao city Using the hydrologic model as science based decision tool


I. INTRODUCTION Davao City is the biggest city on the Island Mindanao in the Philippines, with 244 000 hectares of land one of the largest cities in the world on land area (Davao City), located in the Davao bay it is a delta city. As third year student water management, this research has been part of the internship requirements of the Rotterdam University of Applied Science.

Most areas of Davao City are located higher than 5 m + MSL, the effect of tides is therefor expected to be confined to the coastal areas. Main cause of short term flooding is the discharge of intense rainfall up to 80 mm/hour or 120 mm/2 hours. As floods are reportedly of short duration, the drainage system on its whole must function. However, research shows that discharge capacity is reduced by lack of slope, siltation and damaged structures.

According to Dutch PUM expert M.M. Vierhout, who was asked to advise the city of Davao, many parts of the city are subject to waterlogging and flooding during heavy rainstorms. The flooding is said to be mostly of short duration (a few hours) and only shallow inundation (10 – 40 cm) (Vierhout, 2012).

Simulation of Rain Water Harvesting Systems show high investment with low revenue. While the mitigation effect on the storm water drainage is strongly limited by the tank size. Better and more effective investment in storm water drainage mitigation would be green roofs and other temporal water retention structures like multi-functional sport fields or parking areas.



This paper combines other researches to result in a comprehensive insight into causes of flooding in Davao City and advice on this issue. Sources of data are literature, calculations, field measurements and observations or combinations thereof. To make this paper more comprehensive, the results of extensive calculations or research are shown as is and refer to their original documents.

This paper first covers the quantity of hydrologic fluxes in chapter one (1). Analysis of the drainage system is followed in chapter two (2) and conclusions and advise are followed in resp. chapters three (3) and four (4).


Vincent Oskam, 2012

Table of Contents I.

Introduction....................................................... 2


Hydrologic fluxes ............................................. 4


Drainage system .............................................. 6


Conclusions ........................................................ 7


Advise ................................................................... 7


Works cited ........................................................ 8

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This paper would not have been possible without the help of several individuals and organizations. I would like to extend my gratitude to all whom made this research possible. All members of the HELP Davao network, the Department of Science and Technology, The TROPical Institute for Climate Studies (TROPICS) at Ateneo de Davao University. The University of Mindanao, Dr. Arquiza, her research team and students who made siltation levels known through extensive field work. Pag-Asa Davao who provided long term rainfall data. Professor Heikoop from the Rotterdam University of Applied Science who made this internship opportunity possible.

Flux Green roofs

Vincent Oskam, 2012

Flow rate divided by area Rooftops that have been altered to sustain vegetation Hectare 10 000 m2 2.471 acres m3 1000 liters Rainfall eq. Converted to mm, equal to the measurement of rainfall Return period Average statistical time between occurrences of event RIDF Rainfall Intensity Duration Frequency The intensity/duration ratio for rainfall events with certain return period RWHS Rain Water Harvesting System Contraption with the purpose of catching and storing rain water Slope Ratio of elevation difference between a length of area (m/m). Water square multi-functional public area, designed to temporarily retain water during intense rainfall

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Causes of short duration flooding in Davao city Using the hydrologic model as science based decision tool

1. HYDROLOGIC FLUXES To determine the cause of the short duration flooding, the hydrologic fluxes within the city are quantified.

(Oskam, Hydrologic model of Davao City, 2012).

HOUSEHOLD LOAD The most densely populated areas of Davao City are Agdao and Poblacion (Davao City, 2010). The Davao City Water District produces 1.37m3 water per service connection per day (Davao City Water District, 2012). Using a factor to control for households without connection the following formula is used to calculate the load of household water on the drainage system.

For Agdao and Poblacion it would respectively result in 3.37 mm and 2.63 mm rainfall equivalent a day. If assumed that 40% of the water is used between 6:00 and 8:00 in the morning it would be little more than 0.5 mm rainfall equivalent and this is probably an overestimation (Oskam, Hydrologic model of Davao City, 2012).





EVAPORATION Field measurements show that evaporation during the night is negligible and during the day the maximum measurement is 9 mm/day (Oskam, Hydrologic model of Davao City, 2012). Calculation using the energy balance results in a maximum of 3.7 mm/hour to 6.67 mm/hour. In practice most rainfall and flooding’s occur during the night or evening and otherwise cloudy days, the effect of evaporation on discharging the water from flooded areas is regarded unlikely (Oskam, Hydrologic model of Davao City, 2012).


TIDES Tides are often named as source of flooding in Davao City. Especially areas directly next to the shore flood during high tides. However, this is the sea itself flowing into the streets and only to be prevented by dikes and polder pumps or leveling up. Most parts of Davao City are elevated more than 5 m +MSL (Rodriguez, et al.) (Illustration 1.1). While the tides reach a maximum of 1.5 m and 2 m + MSL in respectively 55 and 18 days of the year (Mobile Geographics). A hydrologic head, caused by tides and rain, of 3 meters above sea tide level is unlikely

Vincent Oskam, 2012

Illustration 1.1 Elevation (Rodriguez, et al.)

Using satellite images and field observations, an infiltration capable area of 3% is estimated for Davao City (Oskam, Hydrologic model of Davao City, 2012). Sporadic soil sampling shows sandy soils. Field observations show puddles long after rainfall events. The infiltration capacity of the soil, based on literature (Natural resources management and environment department, 1988), is estimated on a maximum of 30 mm/hour. Correction for 3% infiltration area, results in 0.9 mm/hour rainfall equivalent (Oskam, Hydrologic model of Davao City, 2012).

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Causes of short duration flooding in Davao city Using the hydrologic model as science based decision tool

RAINFALL High frequency rainfall measurements are needed for accurate rainfall intensity and duration prediction, e.g. every half hour. The more than 60 years of rainfall data from Pagasa Davao only contains 6 hour frequency measurements. The available higher frequency gauges: Manila Observatory gauge in Shrine Hills and PREDICT gauges in Panabo and Bansalan, only measured for a few years or less. Accurate prediction of rainfall intensity is therefore not possible only approximation. The rain gauge data from Manila Observatory in Shrine Hills contains almost three years of data. Illustration 1.3 shows the Rainfall Intensity Duration Frequency (RIDF) curves for this station. The 80 mm in an hour and 110 mm in 2 hours matches the 80 mm/hour rainfall event from Panabo (Illustration 1.4).

Illustration 1.3 RIDF Manila observatory Shrine Hills (Oskam, Hydrologic model of Davao City, 2012)

Illustration 1.4 Design rainfall event (Oskam, Hydrologic model of Davao City, 2012)

A design rainfall of 80 mm/1-hour, 110 mm/2-hour is advised. The expected return period of this event is between 1 and 3 years. As infiltration and household load cancel each other out, correction is not necessary.

Illustration 1.2 Rain gauges Davao City (Oskam, Hydrologic model of Davao City, 2012)

Vincent Oskam, 2012

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2. DRAINAGE SYSTEM Best practice for calculating the load on drainage systems is using the Rational Method (Evans & Mara, 2011) (Vierhout, 2012) (World Health Organization (WHO)). For example computation for Obrero area results in 134.4m3/s for the total area (Oskam, Hydrologic model of Davao City, 2012).

SLOPES The required discharge capacity in Davao City is greatly reduced by the lack of sufficient slope. Many areas within Davao City central area have slopes less than 0.1 (Illustration 2.2) (Rodriguez, et al.).

According to the rational method, maximum load on the drainage system is dependent on the factors: slope, total surface area and maximum length of flow in the catchment area. Illustration 2.1 shows the maximum discharge load for several lengths of flow and slopes for areas of 10 hectares. Davao City, east of Davao River and south of C.P. Garcia highway, has a total area of over 3350 hectares.

Illustration 2.2 Reduced slope > 0.5 - >0.05 (Rodriguez, et al.)

Illustration 2.1 Calculation of Rational method for several slopes and length of flow for areas of 10 hectare

Vincent Oskam, 2012

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Causes of short duration flooding in Davao city Using the hydrologic model as science based decision tool REDUCED FUNCTIONING According to (Vierhout, 2012) (Oskam, Hydrologic model of Davao City, 2012) the designed discharge capacity of the Davao City Drainage is sufficient. However, actual discharge capacity is reduced by siltation, debris, clogging (Illustration 2.3) and damaged structures (Dr. Arquiza, et al., 2012) (Vierhout, 2012) (Oskam, Hydrologic model of Davao City, 2012).

Using the rational method to calculate drainage load, every 10 hectares has to discharge between 100 and 3000 liters/s. The required drainage capacity is greatly dependent on the slope of the area. Slope analysis shows many areas with little to very little slope. Although the design discharge capacity is sufficient, the actual capacity is reduced by siltation, lack of slope or even inverted slope caused by debris, damaged structures and siltation. Increasing discharge capacity alone does not seem an effective strategy. Due to lack of slope the drainage canals need to be increased and despite the extensive effort from task force drainage, siltation and clogging keeps occurring. A combined strategy of discharge and retention seem more constructive.

Illustration 2.3 Clogged drainage (Oskam, Hydrologic model of Davao City, 2012)

In an immense effort, students from the University of Mindanao have measured the bottom elevation of the drainage systems in Davao City, showing great amounts of siltation and inverted slope in the actual drainage system.

3. CONCLUSIONS Tides as well of household drainage load cannot be responsible for the majority of short duration flooding in Davao City. The rainfall can be up to 80 mm/hour or 120 mm/2 hours. This translates into a hydrologic load of 80 resp. 120 liters for every square meter.

Vincent Oskam, 2012

4. ADVISE Rain gauges To better define the required design capacity, more rainfall data is required. Increasing rain gauge density leads to better understanding of micro climates and Rainfall intensity duration. The TROPical Institute of Climate Studies (TROPICS) from Ateneo de Davao University already spearheads this requirement by creating and installing Automated Weather Stations (AWS). Wide spread support of this research and practice is advised. Reference: TROPICS – Ateneo de Davao com_content&task=view&id=69&Itemid=80

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Causes of short duration flooding in Davao city Using the hydrologic model as science based decision tool Pinpoint intervention hotspots Cooperate with University of Mindanao and use the data to pinpoint intervention hotspots in drainage dysfunction. Use the survey results, rational method and Manning or Chezy to calculate discharge/storage deficit. Reference: sh/books/diseases/cpd-annexe6.pdf Rain water harvesting and green roofs The mitigation effect of Rain Water Harvesting Systems is greatly reduced by the available storage tank size. Maximum mitigation effect is equal to the amount that can be stored in the storage tank. Example: a 1000 m2 building needs 80 m3 of storage to reduce an 80 mm design rainfall event (Illustration 4.1). For most buildings this is an unfeasible implementation (Oskam, Feasibility and revenue of Rain Water Harvesting Systems (RWHS) in Davao, 2012). Roof surface 500 m2 1000 m2 2000 m2 5000 m2 10000 m2

1 m3 tank 2.50% 1.25% 0.62% 0.25% 0.12%

1.5 m3 tank 3.75% 1.88% 0.94% 0.38% 0.19%

2 m3 tank 5.00% 2.50% 1.25% 0.50% 0.25%

Temporal water retention structures Water squares are a way of temporarily retaining water, while increasing livelihood (De urbanisten, 2006-2010). These structures retain water during intense rainfall when the storm water drainages system is overloaded. By retaining the water in safe locations, these structures prevent flooding. When the rainfall is over the water square is drained via the normal storm water drainage system. Water squares can be multifunctional areas, like sport fields and parks or parking areas. Water squares also help in areas affected by tidal back water in the drainages systems. They can retain water until the tide drops and then discharge the rain water. References: terpleinen e:115 Wadi’s Investigate implementation of public areas with lower inclination and vegetation that can retain water while it slowly infiltrates into the soil (illustration 4.2) (van Woerden & Schoor, 2011).

Illustration 4.1 actual drainage mitigation per tank size for several roof surface areas.

Investment in green roofs is probably far more effective in mitigating drainage loads. Green roofs are easy to implement on most types of rooftops, they retain water from (intense) rainfall events, reduce the heat island effect of cities and even reduce the energy needed to cool the building (United States Environmental Protection Agency) (Green Roof Philippines). References: (van Woerden & Schoor, 2011) @ Ateneo

Vincent Oskam, 2012

Illustration 4.2 Wadi’s (van Woerden & Schoor, 2011)

(van Woerden & Schoor, 2011) @ Ateneo

WORKS CITED Davao City. (2010). socio economic indicators 2010. City Planning and Development

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Causes of short duration flooding in Davao city Using the hydrologic model as science based decision tool coordinator, Davao City Investment Promotion Center, Davao City. Davao City. (n.d.). City planning and development office. Retrieved 09 19, 2012, from Office of the city planning and development coordinator: Davao City Water District. (2012). DCWD 2011-2012 info. Corporate Planning Department. Davao City: CREA. De urbanisten. (2006-2010). Water squares. Retrieved from De Urbanisten: olio=waterpleinen Dr. Arquiza, Dr. Cuizon, R., Engr. Relampagos, A., Dr. Tamayo, A., University of Mindanao Davao, & Students. (2012). Obrero drainage survey. Davao City. Evans, B., & Mara, D. (2011). Sanitation & Water Supply in Low-income Countries. Ventus Publishing ApS. Retrieved from Google. (2012). Davao. Retrieved 11 4, 2012, from Google Maps: Green Roof Philippines. (n.d.). Benefits. Retrieved from gory/benefits/ Han, D. (2010). Concise Hydrology. BOOKBOON. Han, D., & Ventus Publishing ApS. (2010). Concise Hydrology. Retrieved from Mobile Geographics. (n.d.). Tide table: Davao, Philippines 7.0833° N, 125.6333° E. Retrieved 10 31, 2012, from Mobile Geographics: calendar/year/1502.html Natural resources management and environment department. (1988).

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Irrigation water management, Annex 2 Infiltration rate and infiltration test. FAO. Retrieved December 12, 2012, from s8684e0a.htm Oskam, V. (2012). Feasibility and revenue of Rain Water Harvesting Systems (RWHS) in Davao. Hydrology for environment Life and Policy (HELP) Davao. Davao City: Ateneo de Davao university. Retrieved from Oskam, V. (2012). Hydrologic model of Davao City. Hydrology for Environment, Life and Policy (HELP) Davao. Davao: Ateneo de Davao university. Retrieved from Rodriguez, E., Morris, C. S., Belz, J. E., Chapin, J. M., Daffer, S., & et al. (n.d.). Shuttle Radar Topography Mission. Retrieved from NASA: United States Environmental Protection Agency. (n.d.). Green Roofs. Retrieved from Heat island Effect: greenroofs.htm van Woerden, H., & Schoor, E. (2011). Study on the Davao City Water Management. Davao: Ateneo de Davao. Vierhout, M. M. (2012). PUM expert's Final Report on Mission Drainage of Davao. Davao City, Philippines. World Health Organization (WHO). (n.d.). Annexe 6: Designing a simple drainage system. Retrieved 11 20, 2012, from aceh/wsh/books/diseases/cpdannexe6.pdf

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