Callaghan and Toth IWMP Report revised

system, and providing a total detention volume of 82 cubic metres, is therefore shown in Figure 4 on page
12 of this report. A description of the ecoAID system is shown in Appendix A hereto.
2.0 PRE- AND POST-DEVELOPMENT RUNOFF RATES BY THE RATIONAL METHOD

The Rational Method has been used in this study as a sanity-check on the flow rates predicted by the Urbs
models.

Pre-Development:

The pre-development topography, cover and drainage pattern of the 0.5144 hectare design catchment can
be seen in Figure 2 on page 5 of this report. The pre-development C10 runoff coefficient, for an
impervious fraction of 32 percent, is 0.72. The time of concentration, for a sheet flow length of 55 metres
at slopes of 3 to 4 percent through reasonably well-maintained grass and trees is 14 to15 minutes. The
consequent pre-development peak flow rates are therefore calculated as follows:

Q100 = 0.00278 x 1.2 x 0.72 x 210 x 0.514 = 0.26 m3/sec
Q10 = 0.00278 x 1.0 x 0.72 x 145 x 0.514 = 0.15 m3/sec
Q1 = 0.00278 x 0.8 x 0.72 x 85 x 0.514 = 0.07 m3/sec

Post-Development:

The post -development cover and drainage pattern can be seen in Figure 3 on page 6 of this report. The
pre-development C10 runoff coefficient, for an impervious fraction of 71 percent, is 0.84. The longest
flow time, for sheet flow lengths of 25 to 30 metres at slopes of 3 to 4 percent through well-maintained
grass and trees, followed by 50 metres of allotment piping at 1 m/sec, is 12 minutes. The roof and carpark
areas, occupying a substantial proportion of the design catchment, will have a response time of 6 to 7
minutes. An overall concentration time of 9 to 10 minutes has therefore been adopted. The consequent
post-development peak flow rates are therefore calculated as follows:

Q100 = 0.00278 x 1.2 x 0.84 x 245 x 0.514 = 0.35 m3/sec
Q10 = 0.00278 x 1.0 x 0.84 x 170 x 0.514 = 0.20 m3/sec
Q1 = 0.00278 x 0.8 x 0.84 x 97 x 0.514 = 0.09 m3/sec




3.0   BROAD-BRUSH DETENTION VOLUME

The 39 percent increase in the design catchment’s impervious fraction (from 32 percent to 71 percent) is
equivalent to about two thirds of what would typically be expected for development of a medium-sized
“Res A” lot (60% increase). Given that medium-sized “Res A” development generally requires about 250
cubic metres of detention volume for each hectare of development platform, the proposed extension of
facilities into the 0.51 hectare design catchment should therefore be expected to require a detention
volume in the order of:

Two-thirds of 250 x 0.51 = 85 cubic metres.



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