|Stormwater Drainage and Land Reclamation for Urban Development (HABITAT, 1991, 94 p.)|
|Annex I: STORMWATER DRAINAGE DESIGN PROCEDURES|
|D. FLOOD ESTIMATION|
In the application of the modified rational formula in cases where a design hydrograph is required, the form of the hydrograph shown in figure D.4 should be used.
In figure D.3, the peak discharge Qp is found from equation D.1. For detention ponds where the critical storm duration is greater than the time of concentration, then the hydrograph shown in figure D.5 should be used. This situation generally occurs when the average outlet discharge of the pond is less than 50 per cent of the peak inflow.
Figure D.2. Run-off coefficients for urban catchments
Figure D.3. Run-off coefficients for rural catchments
Figure D.4. Design hydrograph - rainfall duration equals time of concentration
Figure D.5. Design hydrograph - rainfall duration exceeds time of concentration
In figure D.5 the peak discharge Q'p is found by
Q'p = F C C's Ie.A, (D.5)
Cs' = 2te/(2te + td), (D.6)
te = the critical storm duration as determined by trial and error
Ie = the average rainfall density for the critical storm duration te and a return period T years.