Revised Stormwater Management Plan

                              STORMWATER MANAGEMENT PLAN
Client: Hass Holdings ATF The Quasar Trust                                   Location: Taylor Court, Cooroy

6.3.6 Manning’s Values

Manning’s values varied due to the differing catchment characteristics / land uses and were
applied independently to the catchment. The values were generally adopted as follows:

     Pervious Area, n = 0.032; and
     Impervious Area, n = 0.018.

6.3.7 Catchment Slope

The Equal Area method was used to determine the catchment slope, which is summarised in
Table 8. The catchment slope in the developed case is unchanged from the existing case scenario.

6.3.8 Storm Durations

Standard storm durations of 15, 20, 25, 30, 45 60 and 90 minutes were modelled to estimate the
peak discharge.

6.4 Calibration

6.4.1 Rational Method

Rational Method calculations were undertaken for the site for calibration of the existing case XP-
RAFTS model – Table 10.
Variable         Value        Units
Area:            0.924        ha
Tc               5.9          minutes
C10              0.68         fraction

                                   Table 10 - Rational Method Calculations
                            Variable                1% AEP   10% AEP    39% AEP
                            Freq Factor              1.2       1.0        0.85
                            Intensity (mm/hr)       299.5     205.2      149.1
                            Discharge (m3/s)        0.628     0.358      0.221

The Rational Method Calculations for the 1%, 10% and 39% AEP events generally reflect results
from the XP-RAFTS model. Thus, the XP-RAFTS peak flows have been adopted for hydrological
modelling and detention sizing purposes. A comparison of the results is provided in Table 11.

                           Table 11 - XP-RAFTS and Rational Method Comparison
                                         Rational       XP-RAFTS       Difference
                             AEP          (m3/s)          (m3/s)           (%)
                              1%          0.628           0.630            0.4
                             10%          0.358           0.357            -0.4
                             39%          0.221           0.219            -1.1

6.5 Design Discharges Unmitigated

6.5.1 Peak Durations

Temporal patterns from Australian Rainfall and Runoff, Volume 2, (1987) have been adopted for
each storm duration. A range of storm durations were analysed to determine the peak discharge
for the catchment. Table 12 presents the flows generated using XP-RAFTS from design storms at
the proposed development site for the existing and developed (unmitigated) case scenarios.
Covey Associates Pty Ltd                         10                                      Project No: 161968
Consulting Engineers                                                                        100415 Rpt Iss B