Flooding, the urban poor and mass housing

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Giovanni Tapang, Ph.D.

Giovanni Tapang, Ph.D.

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.

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3 Comments

  1. The flood problem cannot be solved without understanding the hydrologic process:

    1.) Precipitation (rainfall intensity) exceeds the rate of absorption of natural ground and the vegetation cover (infriltration) and the environments’ capacity to evaporate (evapo-transpiration) its ambient water/moisture thus producing excess rainwater in the surface (surface run-off)

    2.) Due to natural terrain (topography), surface run-off and infiltrating water (groundwater) will flow to the lowest portion either to the sea/ocean or basins (lakes/ponds/). The transportation mechanism will be the following:
    2a.) For surface run-off, by drainage (gravity flow) through natural conduits and waterways (i.e. rivers and its tributaries)
    2b.) For groundwater, by percolation to the aquifer/aquitard/aquifuge.

    3.) Flood occurs when either or in any combination of the following conditions are satisfied:
    3a.) The intensity of rainfall exceeds the capacity of natural or catchment basins (eg.lakes, flood plain, dams / reservoirs) to contain the volume thus the overflow of surface run-off.
    3b.) The intensity of rainfall exceeds the rate of infiltration/percolation to the ground, and it also exceeds the capacity of the aquifer/aquitard/aquifuge to store water, thus surface run-off.
    3c.) The rate of flow of surface run-off to natural conduits exceeds the capacity of the waterways to discharge/drain to the basins/lakes or open sea/oceans.
    3d.) Extreme change in weather patterns such as strong storm surges, and increasing mean sea level.

    The parameters therefore for flood to occur are:

    A.) Capacity to contain surface run-off
    Factors:
    -Insufficient reservoirs
    -Lack of retention ponds/lakes

    B.) Capacity to store surface run-off
    Factors:
    -Insufficient water sheds and vegetative cover (deforestation, indiscriminate land conversion, etc.)
    -Surface run-off not percolating to the ground aquifer/aquitard/aquifuge
    -Ingress of saline water to the groundwater aquifer/aquitard/aquifuge due to grounwater drawdown or pumping from development areas (eg. residential subdivision using groundwater pumps)

    C.) Capacity to discharge surface run-off
    Factors:
    -Inefficient sewer/stormwater network and system
    -Diminishing natural waterways due to economic development (eg. riverside development, informal settlers)
    -Clogged waterways either by garbage or siltation

    D.) Climate Change
    Factors:
    – Change in weather patterns (extended and/or wetter rainy season,extended and/or drier/hotter summers)
    – Rising sea levels
    – etc.

    =================================

    S O L U T I O N

    Having defined the parameters for flood to occur, the structural solution therefore should address each or in any combination of the following the parameters.

    A.) Capacity to contain Surface Run-off
    – Typical solutions are as follows:
    a.1) Provision for dams and reservoirs
    a.2) Provision for retention ponds & lakes

    B.) Capacity to store
    -Typical solutions are as follows:
    b.1) Watersheds and vegetative cover
    b.2) Artificial Recharging wells
    b.3) Swales

    C.) Capacity to discharge
    -Typical solutions are as follows:
    c.1) separate network for each of the stormwater, residential sewer and industrial sewer lines.
    c.2) increasing waterways capacity either, or in any combination of the following: dredging the river bed, widening the waterways, raising the embankment by providing dikes or levees.
    c.3) reduce the potential of siltation and scouring either or in any combination of the following: bioswales along embankment, baffle chutes to reduce velocity,riprap/gabions/rock armours along embankment,etc.
    c.4) diversion for other purposes such as irrigation, power generation, water supply, transportation, etc.

    D.) Climate Change
    – Typical solutions include:
    d.1) Scenario Planning
    d.2) Improved forecasting techniques
    d.3) Infrastructure adaptation

    The above solutions should be part of a comprehensive water management framework in a regional or national scale and should be incorporated in the land use zonation planning,natural hazard & disaster risk mitigation plan, infrastructure development plan, and economic development plan of the region concerned. This blue print can then be replicated to the rest of the archipelago.

  2. this is a old perenial problem. But I think one of the most practical is to send this squatters in the rural areas and go into farming where they will have food in the table and a small house to live. the country has numerous idle lands. what are the squatters doing in metro manila living in a animal like conditions and even if they have a job, it does not pay much to support themselves thats why they end up homeless. the government can help them allocate some idle lands, provide them with training in farming and as payment for the government land and assistance can share the profits of their produce. its better than living in crowded Metro Manila and maybe tempted to commit crimes. in the provinces they can have more quality life and they can be independent.

    • Benjamin Vallejo Jr on

      Why not send the Manila’s elite and the middle classes to the provinces so that they can invest in industries or establish lending facilities that will ensure job creation or business start ups? Sending the urban poor to the provinces is easier said than done. It will only work if the urban poor have tenure to lands in their provinces. Their tenure can make them sufficient if they enough capital to start their own businesses. But they need capital and that could be provided by micro credit.