D.3 Design hydrograph

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 Q_p 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 I_e.A, (D.5)

where

C_s' = 2t_e/(2t_e + t_d), (D.6)

t_e = the critical storm duration as determined by trial and error

I_e = the average rainfall density for the critical storm duration t_e and a return period T years.