Metro Manila came to a standstill a few weeks ago when heavy downpour caused simultaneous flooding in major roads such as Taft and Espana. The Metro Manila Development Authority (MMDA) and the Department of Public Works and Highways (DPWH) pointed fingers at each other as who is to blame with regard to the delayed implementation of the flood reduction projects promised August last year.
One common reaction of both agencies was to blame urban poor residents along the esteros for the flash floods that happened. The DPWH was pointing out the high density of households along waterways as the main reason for the flooding and as such, these households should be removed in order to solve the flooding problem in Manila.
In general, water seeks the area of lowest elevation as it flows from high areas because of gravity eventually flowing out to the sea. As it flows down, water encounters resistance which would slow down the drainage process. This resistance can come from different
factors such as surface roughness or constrictions within the drainage basin.
The drainage basin is composed of the whole area where rainwater would fall and be collected. The water in the whole catchment area would be eventually discharged to a main drainage system such as a river or esteros. The rain that falls within this watershed would usually divide into three parts: run-off, infiltration and evapotranspiration.
Evapotranspiration would be the water that evaporates and returns to the atmosphere, mostly in open areas. Infiltration is the part of the rainfall that is absorbed by soil and the surrounding areas while runoff is what is usually seen as the water flowing over land. This runoff is usually channeled into rivers or drainage systems.
Flooding occurs when the runoff is larger than the capacity of the drainage system to handle. In urban areas, runoff typically increase than its pre-urbanization volumes since infiltration is reduced by larger built up areas and roads brought about by urbanization. As most cities would be nearly completely built up, infiltration in those areas are greatly reduced thus converting most of the rainfall into runoff. In cities, this runoff would have to be channeled into sewers that bring this to esteros or rivers for proper drainage.
We have to also account the height of the land area in the city above sea level. If the area of land is below sea level, water from land will not drain naturally to the sea. This would explain local flooding of areas that are sinking or naturally lower than the sea. Metro Manila and its surrounding areas are reported to be sinking due to too much ground water extraction which contributes to the flooding that occurs.
However, the flashfloods, as we usually term them, that occur when our streets fill up rapidly with water after it rains is more often than not is because there is too much rainwater that can be drained out of the area using pipes and the sewers. The curb inlets are usually small in size or filled with detritus. When this happens, street level flooding occurs because the surface runoff cannot reach the drainage pipes or culverts.
When we blame the flash floods entirely on constricted esteros, we are assuming that the flooding is due to the back flow arising from the volume of water flowing out of the full esteros and rivers. If the flooding occurs rapidly after it rains and the water remains on the streets, it is more probable that it is the street drainage and culverts that are full. Some residents of the community reported that the rivers and esteros were still flowing and not yet full right after it rained as major roads and arteries were already flooded a few weeks ago.
Eventually water (and all the floating garbage) would flow into the esteros. That is why we see large garbage islands in our esteros and rivers. These are not all products of the communities along the rivers. They are mostly ours.
The focus on “cleaning” the esteros is one-sided and unfair to the communities that are already hard put to find decent housing. It seems the “perfect” solution for government because it produces direct visual results. Yet it might not solve flooding in the Metro.
In relocation areas such as Montalban, there are no stable services for electricity and water despite the promises to the relocatees. In some areas, they are placed near the drainage areas of the Marikina watershed and nearby rivers. They also live near the Marikina faultline. From their original places near waterways where they are exposed to flooding hazards, they are transplanted to areas with even greater hazards from flooding, earthquakes and landslides.
Risk is a product of the hazards present in the area, the exposure of our communities to these hazards and their vulnerability. Instead of trying to reduce risk by removing the communities by demolishing their homes, the government should instead move to decrease their vulnerability by providing stable jobs, decent housing and support to these poor communities.