The essential requirement for designing of Storm Water Drainage system is the proper estimation of storm runoff to downstream drains or the point of disposal. It has a bearing on optimizing the cost of infrastructure as well as its performance. The parameters like rainfall intensity, imperviousness factor, runoff coefficient, recurrence period, climate change, and identification/zoning of drainage catchment play an important role.
Methods of Runoff Estimation
The following methods are generally used for runoff estimation for the design of urban storm water drainage systems.
1. Rational Method
2. Time Area Method
3. Unit Hydrograph Method
4. Rainfall-Runoff process simulation
Rational Method
The procedure for the estimation of storm runoff by the rational method is mentioned in the following steps:
Step 1: Obtain historical rainfall data of 30 years or more for the given project area
Step 2: Select a design return period
Step 3: Prepare the IDF curve for the above return period as per Chapter 3
Step 4: Demarcate the catchment
Step 5: Determine the time of concentration (tc) as described in section 4.4.1.6
Step 6: Determine rainfall intensity against the time of concentration from IDF curve
Step 7: Determine runoff coefficient (C) as described in section 4.4.1.5
Step 8: Calculate Peak flow by Rational formula as given in section 4.4.1.3
Peak flow Rational formula :
Where,
Qp : Peak flow at the point of design, m3/hr
C : Runoff coefficient, dimensionless
I : Average rainfall intensity should be taken for the duration of rainfall equal to the time of concentration, mm/hr
A : Catchment area, hectares
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Urban Infrastructure and Network Study notes for M. plan Sem-III
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