Why is Metro Manila Vulnerable to Flood

We have learned from our previous post (bit.ly/PRHydrodynamics) that Pasig River connects Manila Bay and Laguna Lake, and that the natural flow of water is into the Bay. However, aside from the Laguna Lake, flows conveyed by the Pasig River come from various tributaries. Its biggest tributary is the Marikina River with a catchment area of around 546 sq. km. During heavy storm events, Marikina River can discharge high volumes of water in a short span of time, leading to flashy (steep) hydrographs. This is due to Marikina River Basin’s large dendritic drainage pattern, which means multiple streams covering a wide area eventually converge to a single stream in a narrow valley. Coupled with the generally rounded shape of the Marikina River Basin upstream of the Upper Marikina River, this drainage pattern results to the flashy hydrographs as the flows from different areas tend to arrive at the outlet more simultaneously than it would in other basin shapes and river network patterns.

When Typhoon Ulysses hit last November 2020, flows exceeding 2500 cms were recorded along Marikina River. That is around 50 times the rate at which Angat Dam supplies water to Metro Manila and Bulacan. It has been the heaviest event since Tropical Storm Ondoy in 2009, in which a flow of over 1800 cms was recorded. Part of these flows directly discharge to Pasig River, inundating adjacent lands and communities. The existence of Manggahan Floodway somehow helps alleviate this flood by first diverting some of the waters to Laguna Lake before discharging to Pasig River (you may read more on this here: http://bit.ly/PMLDB3_Flooding). A long-term hydrologic simulation shows that the Marikina River contributes around 81% of the discharges of Pasig River to Manila Bay in terms of annual average discharges.

Another major tributary that drains to Pasig River is the San Juan River which has a catchment area of 86 sq. km. Based on the long-term simulation, it contributes about 15% of the Pasig River discharges. Increased flooding from the San Juan River Basin only affects Manila City which is the most downstream city within the Pasig-Marikina River Basin.

Based on a long-term hydrologic simulation, the Pasig-Marikina River Basin yields an average annual flow of 36.7 cms. This flow rate can fill an entire olympic-sized swimming pool in just about 1 minute. The mean seasonal flows are 17.5 cms during the dry season and 60.6 cms during the wet season. The exchanges of Manila Bay and Laguna Lake including the backflows of Pasig River, and the attenuating effect of the Manggahan Floodway are factored in the Project’s hydrodynamic and hydraulic models, respectively. Model calibration resulted to a Nash-Sutcliffe efficiency of 0.56, which signifies a good fit between the simulated and observed flows.

A synthetic flow duration curve was derived based on the 25-yr hydrologic simulation. The resulting dependable runoff or the flow that is equaled or exceeded 80% of the time is 2.3 cms. This is enough to meet the estimated daily average domestic water consumption of households in Quezon City. The dependable flow is often used as basis for flow diversion (hydropower, water supply, etc.) requirements, where 10% of which is allocated as environmental discharge or the required minimum flow that must be retained in rivers to sustain aquatic flora and fauna. The very steep flow duration curve represents tremendously high flows during storm events that are far from the baseflow or its usual flows. This is consistent with the flashy hydrographs seen in the discharge plots. The average domestic water consumption of a household member is assumed to be 90 liters/day.

Based on the hydrologic model, the largest component which is 43% of the water balance, is classified as surface runoff. The high surface runoff is consistent with the flashy hydrographs that resulted from the model. This can be attributed partly to the high basin slopes especially within the Upper Marikina River Basin (outlet located at the junction of Marikina River and Manggahan Floodway) and the highly urbanized downstream areas. The percolation and groundwater subsurface flow are also relatively lower. These can also be due to the high percentage of land covered in urban or built-up areas especially those areas within Metro Manila (36.6% of the Pasig-Marikina River Basin area). The urban land covers can also affect the water quality of the rivers, especially that of the Pasig River. Since these areas are where the populace resides and where factories and industrial establishments are located, these are also expected to produce the highest amount of wastes that can go into the river systems. The San Juan River for example, with a catchment that is 97.8% covered in urban land, is considered one of the most polluted waterways in Metro Manila.

The Pasig-Marikina River Basin is peculiar compared with almost all the other basins within the Manila Bay Watershed. The usual watershed or river basin is defined as an area of land bounded by a drainage divide of topographic highs and drains to a single outlet. Since the Pasig-Marikina River Basin is connecting two bodies of water (Manila Bay and Laguna Lake) and can flow to either of these depending on the season, it already deviates from the single-outlet definition. The flows also come not only from the headwaters of Marikina and San Juan River Basins but also from Laguna Lake itself, which has its own catchment. These flows coming from the Laguna Lake which enter the Pasig River through the Napindan Channel affect both the quantity and quality of water in the Pasig River. The basins of Manggahan and Taguig, which drain to the Manggahan Floodway and the Napindan Channel respectively, are also highly urbanized and can also be adding more pollution into the Pasig River.