Stormwater Management Plan

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


Variable         Value           Units
Area:            0.924           ha
Tc               5.9             minutes
C10              0.68            fraction

                                      Table 8 - 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 9.

                           Table 9 - 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 10 presents the flows generated using XP-RAFTS from design storms at
the proposed development site for the existing and developed (unmitigated) case scenarios.

                           Table 10 - XP-RAFTS Design Discharges (Unmitigated)
                                                         Existing                  Developed
                                                                  Peak                       Peak
                                                Critical                       Critical
                  Catchment           AEP                         Flow                       Flow
                                               Duration                       Duration
                                                                 (m3/s)                     (m3/s)
                                      1%           25 min        0.630         15 min       0.724
                      Outlet          10%          60 min        0.357         25 min       0.518
                                      39%          60 min        0.219         25 min       0.331

The peak flows discharging from the site have increased in the developed case scenario due to the
resulting increase in impervious area. To reduce the flows to meet non-worsening conditions, a
detention tank is proposed to increase detention capacity on site and reduce the offsite flows.

6.6     Stormwater Detention

The pre-developed and post-developed (mitigated) conditions were modelled in XP-RAFTS to
demonstrate that stormwater flows will not be increased with the inclusion of an appropriately
designed storage device. The increases in peak flows have been mitigated on site through the use
of a detention tank. Detention provided by the bioretention basin to treat water quality will also be
utilised on site.


Covey Associates Pty Ltd                             9                                           Project No: 161968
Consulting Engineers                                                                                100415 Rpt Iss A