How to calculate storm water runoff volume

There are a few complicated experimental formulae for calculating Run-Offs of catchments which are not strictly accurate and give unreliable results. The rational method has been evolved in the form of the following general equation for calculating the flood discharge of Run-Off an area.

Q=  (R.A.P)/36

Where Q is total Run-Off in Cu.Metrs/Second

R is Intensity of Maximum rainfall in Centimeter /Hour.

A   is drainage area in hectares contributing to Run-Off

P is Factor of imperviousness

Example

Suppose length of catchment is 1900 ft having width of 40 ft for expected Run-Off in drainage area ,considering rainfall depth as given by weather department is 5cm in peak hour of rain intensity.   

  Length of drainage Area= 1900        

Width of strip causing Run-Off = 40 Ft.

Area of Run-Off = 1900 x 40 = 76000 Sft

1 Hectares = 107639 Sft

A= Converting Sft Area into Hectares = 0.70 Hectares

R= Max. Intensity of Rain fall depth = 05 Cm/Hour

P= This value varies according to nature of Ground surface.

We have following factors and we must choose one as per condition of our area, Factor no. 4 matches with our case.

1- Steep bare rock = 0.90

2- Rock steep but wooded = 0.80

3- Plateaus lightly covered, ordinary ground bare = 0.70

4-Densely built-up areas of cities with metallic Roads

and paths = 0.70 to 0.90

5- Residential areas not densely built up with metallic roads =0.50 to 0.70

6- Residential areas not densely built up with Un-metallic roads =0.20 to 0.50

 

4-Densely built-up areas of cities with metallic Roads

and paths = 0.70 to 0.90

P= Average value (0.80)

Q= (R.A.P)/36

Putting values in above formula

Q= (5 Cm/Hour x 0.70 Hectares x 0.70)/36 = 0.068Cubic. Meter

So, we have Discharge of 0.10 Cum = 2.4 Cubic Feet /Sec which is also equal to (0.068x1000) = 68 Liters/Second

Note: Rainfall data will be taken from local departments as per acctual locations in the world.