1714 IWMP_Report_Carramar_Version2 Oct 2018

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 0.82 hectare design catchment can
be seen in Figure 2 on page 6 of this report. The pre-development C10 runoff coefficient, for an
impervious fraction of 23 percent, is 0.744. The time of concentration, for sheet flow lengths of 60 to 70
metres at slopes of 3 to 4 percent through reasonably well-maintained grass and trees is 15 minutes. The
consequent pre-development peak flow rates are therefore calculated as follows:

Q100 = 0.00278 x 1.2 x 0.746 x 210 x 0.70 = 0.37 m3/sec
Q10 = 0.00278 x 1.0 x 0.746 x 145 x 0.70 = 0.21 m3/sec
Q1 = 0.00278 x 0.8 x 0.746 x 85 x 0.70 = 0.10 m3/sec

Post-Development:

The post -development cover and drainage pattern can be seen in Figure 2 on page 6 of this report. The
pre-development C10 runoff coefficient, for an impervious fraction of 66 percent, is 0.832. The time of
concentration, for sheet flow lengths of 20 to 25 metres at slopes of 3 to 4 percent through well-
maintained grass and trees, followed by 40 to 50 metres of allotment piping at 1 m/sec, is 10 minutes.
The consequent post-development peak flow rates are therefore calculated as follows:

Q100 = 0.00278 x 1.2 x 0.832 x 251 x 0.70 = 0.49 m3/sec
Q10 = 0.00278 x 1.0 x 0.832 x 173 x 0.70 = 0.28 m3/sec
Q1 = 0.00278 x 0.8 x 0.832 x 83 x 0.70 = 0.13 m3/sec




3.0     BROAD-BRUSH DETENTION VOLUME

The 43 percent in the catchment’s impervious fraction (from 23 percent to 66 percent) is about two
thirds of what would typically be expected for development of a medium-sized “Res A” lot. 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 0.70 hectare at
Cartwight Village should therefore be expected to require a detention volume in the order of:

Two-thirds of 250 x 0.70 = 117 cubic metres.




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      Brisbane Stormwater Management Pty Ltd Document File: Carramar.Doc