# Find out the Quantity of Stormwater run-off in a Catchment Area

How to find out the quantity of stormwater run-off in a catchment area? Let’s take a mathematical example.

Mathematical Problem:

A catchment area of 20 km2 consists of two-thirds rural and one-third urban areas. The rainfall intensity in this area is recorded as 25 mm/hr. Find the quantity of stormwater run-off in the area in litre per second. ‘K’ for rural area is 0.30 and urban 0.50.

Solution:

Quantity of stormwater run-off

$Q = \frac{KiA}{360}$

Where,

• K = Coefficient of run-off
• i = Rainfall intensity (in mm/hr) = 25 mm/hr
• A = Catchment area in hector

Rural catchment area = $\left ( \frac{2}{3}\times 20 \right )$ = 13.3333 Km2 = 1333.33 hector.

Urban catchment area = $\left ( \frac{1}{3}\times 20 \right )$ = 6.6666 Km2 = 666.66 hector.

Now,

The quantity of storm water run-off from the rural catchment area

$Q = \frac{KiA}{360}$

Or, $Q = \frac{0.3\times 25\times 1333.33}{360}$ = 27.777 m3/sec = 27777 litres/sec

The quantity of storm water run-off from the urban catchment area

Q = $Q = \frac{0.50\times 25\times 666.66}{360}$ = 23.125 m3/sec = 23125 liters/sec