TarmiziIsmail 1 Q = Peak flow (m 3 /s); C = Runoff coefficient (Table 2.5 MSMA 2 nd Ed) i = Average rainfall intensity (mm/hr) A = Drainage area (ha). 360 A i C Q = Assumptions used in the Rational Method are as follows: • The peak flow occurs when the entire catchment is contributing to the flow; • The rainfall intensity is the uniform over the entire catchment area; and • The rainfall intensity is uniform over a time duration equal to the time of concentration, t The Rational Method is not recommended for use where: • The catchment area is greater than 80 ha (TxDOT, 2009); • Ponding of stormwater in the catchment might affect peak discharge; and • The design and operation of large and more costly drainage facilities are to be undertaken, particularly if they involve storage.
Using Rational Method procedure to calculate a 20 yrar ARI peak
discharge from a subcatchment area of 40.7ha in Wangsa Maju, Kuala
Lumpur. Also develop the runoff hydrograph using the RHM for drain
AB on 5 and 10 min durations design storms
