by Catherine Abon, Msc. AGHAM – Advocates of Science and Technology for the People
Flooding occurs whenever a watershed receives more water than it can handle. There are two types of flooding, fluvial and local flooding. The former is caused by a certain flow that exceeds the capacity of a stream or drainage channel hence it overtops its natural or artificial banks and the latter happens when water accumulates in a local depression and/or due to poor drainage system (See Note #1).
Precipitation is the primary determinant of flooding and not the shanties
The meteorological events—typhoon Gener and Habagat rainfall in August 5-9 brought around 500 mm in just 24hrs. This amount even surpassed the rainfall from 2009 Typhoon Ondoy. It must be noted therefore that the main cause of flood is the sheer volume of rainfall from these meteorological events.
The long duration of rainfall made the ground saturated enough, thereby decreasing the rate of infiltration and consequently increasing the amount of run-off such that the sheer volume of rainfall on Aug. 6-7 produced a huge volume of water that flooded Marikina and Metro Manila. For the Marikina River Basin with an area of 535 km2, assuming that the run-off coefficient is 0.8 (means 80% from precipitation is translated to run-off) a 500 mm rainfall would have produced an estimated volume of 214,000,000* m3 (214 million cubic meters or 214 billion liters) of water flowed in the Marikina River. With this volume of water, the floodplains will surely be flooded with or without the houses along the banks and in the waterways.
The presence of the structures will not change the volume of the floodwater although these will account for an increase in the roughness of the surface. The physical effect of this increase in roughness is the a decreased water velocity along the river (See note #2).
Velocity decreases as the roughness increases. Estimates of roughness range from 0.035 for smooth channels and up to 0.060 for rough channels. Water velocity increases by a factor of 1.7 when the roughness is decreased from the maximum to the minimum. A flood velocity then of say 2.3 m/s at roughness 0.035, when the roughness increases to 0.060 slows down to 1.3 m/s. The houses in the waterways may account for higher factor of velocity impediment would not be high enough to be the primary cause of flooding especially for a fluvial torrent of this magnitude. Moreover, urban poor homes are usually built with stilts and with narrow alleyways (not a total blockade) where the flowing water
eventually finds its way and flow towards an exit. On the other hand, it is local flooding that will be exacerbated by the clogged waterways and esteros where the high amount of rainfall together with local depressions and low-lying areas result to restricted drainage in some parts of the Metro.
The Philippines is geographically situated in a region where most of the typhoons pass through and experience meteorological conditions that bring rain (NE Monsoon or Amihan, SW Monsoon or Habagat, and the Intertropical Convergence Zone or ITCZ). Flooding is therefore a perennial hazard.
As far as studies are concerned, there have been studies since 1980’s on flood hydraulics of the Marikina River (Tabios, 2009). We now have institutions equipped with modeling and computing capabilities for rainfall prediction and flood forecasting but it seems that they have not yet been fully maximized and that our “preparation” remains insufficient.
Green group calls for flood drills and disaster awareness
A more sustainable plan, not only in the urban areas but equally important in the entire watershed unit (the upper watershed of Marikina River Basin includes the foot of the Sierra Madre Mountain ranges where logging and quarrying are being done) should be put in place. Engineering measures could also be done but the safety and welfare of the people such as affordable and decent housing and jobs should be given priority consideration. Instead of drastic measures being proposed to remove shanties, community flood drills should be immediately conducted before more rains come in the remaining months of the year.
Kalikasan Partylist calls on the government to allocate resources and funds needed for local disaster response systems to properly act. There should be consistent monitoring and updating of areas which are vulnerable to widespread flooding or flash floods (especially in low-lying areas or communities near large dams and rivers) and landslides (especially in geohazard zones and deforested areas). All support must be given to ensure prompt and proper warning, evacuation and rescue of communities in danger, especially among the urban and rural poor.
Kalikasan Partylist earlier urged the public to remember three major steps to prepare for the coming rains. First, to be aware of the risks and hazards in your local area. Second, to inform your communities of these risks. These can take the form of discussions or a public map of the vulnerable areas of local flooding in your vicinity. Lastly, we call on the public to take to action to reduce flood risk that will involve your local barangays, businesses, homeowners and most especially the youth such as proper waste management and disposal.
Kalikasan Partylist is a progressive political party formed to protect the environment and defend the people’s welfare. It is comprised of individuals and various sectors which support pro-people, pro-Filipino and pro-environment policies, reforms and programs. It is one of the first green political parties in the Philippines.
References: Ms. Catherine Abon, +639208406054
Contact: Lisa Ito-Tapang, Public Information Officer at 09178179955.
Address: # 26 Matulungin Street, Barangay Central, Diliman, Quezon City 1100
Facebook: Kalikasan Partylist
*Run-off Volume Calculation:
Run-off coefficient = 0.8
Marikina watershed area = 535 km2 = 535000000 m2
Rainfall (24 hours) = 500 mm = 0.5 m
Volume = 0.5 x 535000000 = 267500000 m3
Run-off volume = 267500000 x 0.8 m3 = 214000000 m3
To understand what happens to the rainwater that fell on a certain watershed, one needs to look into the
hydrologic cycle which is given by the following equation:
Precipitation = Run-off +Abstraction + Evapotranspiration
Where Abstraction includes:
Infiltration + Depression Storage + Interception
The right-hand side of the equation (1) are the processes that occur once the rainwater reached the surface and
these are governed by the physical characteristics of a watershed such as initial conditions, land-use, and soil
type among others. The amount of run-off (the component of rainwater that causes flooding) is highly affected
by soil type and land-use. Sandy soils can lead to faster infiltration rate as compared to clayey soils. In terms
of land-use, thick vegetation can increase interception and infiltration while paved roads and cemented grounds
can lead to minimal or even no infiltration.
This can be easily seen in the discharge (Q) equation. The quantity to express the volume of water that passes through a stream channel at a certain time interval is discharge (commonly referred to as Q with unit of m3/s).
This can be expressed simply as
Q = A*V
Where A is cross-sectional area (m2) and V is velocity (m/s).
This is similar to Manning’s equation as:
Q = (1/n) AR(2/3)So(1/2)
Where the quantity n which is roughness coefficient accounts for the roughness of the channel or surface where the water is flowing, R is hydraulic radius, So is bed slope.
From (3) and (4), the velocity (V) is:
V = (1/n) R(2/3)So(1/2)