A presentation on the real reason why we were flooded during typhoon Gener and Habagat.

1. Lagmay et al., 2009. Earth
Vol. 55, No. 4

2. Tabios, 2009. National Hydraulic Research Center

3. Precipitation = Run-off +Abstraction +
Evapotranspiration
Abstraction:
Infiltration + Depression Storage + Interception
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 = 214,000,000 m3

4. Roughness Effect (shanties, stream
bed substrate)
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 n = roughness coefficient accounts for the roughness
of the channel or surface where the water is flowing
R = hydraulic radius
So = bed slope.
From (3) and (4), the velocity (V) is:
V = (1/n) R(2/3)So(1/2)

5. What can be done
• Precipitation is the primary determinant of
flooding and not the shanties
– With the volume of water, the floodplains will
surely be flooded with or without the houses
along the banks and in the waterways
• Engineering solutions, early warning systems
and reduction of vulnerability
– Flood drills, community preparation and disaster
management