Revised Stormwater Management Plan

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



                           Table 12 - 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.

Refer to Site Plans in Appendix B for the proposed detention tank location and details.

6.6.1 Detention Tank

The flows up to 39% AEP will be directed to and will be detained within the proposed bioretention
basin and flow greater than the 39% AEP will be detained within the detention tank located at the
lowest point of the site in the south western corner. A diversion chamber prior to the bioretention
basin inlet will divert all flow above a 39% AEP event to the detention tank. Refer to Stormwater
Management Plan in Appendix B. The bioretention basin has a storage component 1.25 m deep
with a surface area of 115 m2, providing a storage volume of 143.75m3. The proposed detention
tank dimensions are approximately 5 m wide x 10 m long x 2 m high providing a maximum
detention volume of 100 m3. The combined detention volume provided between the bioretention
basin and the detention basin is 243.75 m3 with a minimum available depth of about 2.7 m. The
required detention is 171m3 peaking at a depth of 2.12 m in the 1% AEP event. The tank will slow
release via one (1) 325 mm orifice plate and a weir 1.5 m high and 0.45 m wide discharging into a
chamber and connecting to an existing stormwater network on site. Refer to the Stormwater
Management Plan in Appendix B for detail.

Table 13 summarises stormwater flows, storage volumes and stage height for the required
detention tank.
                               Table 13 - Post-Development Conditions – XP-RAFTS
                                 Stormwater Flows for proposed detention basin
                                                                 Basin                                  Peak
                                         Basin Inflow                            Peak Depth
       Basin               AEP                                  Outflow                                Storage
                                           (m3/sec)             (m3/sec)               (m)               (m3)
                           1%                0.724                   0.624             2.12              171
      Det_tank             10%               0.518                   0.349             1.78              122
                           39%               0.331                   0.211             1.45              73

XP-RAFTS modelling predicts that the proposed detention tank will reduce the post-development
peak stormwater flows for all critical storm events. Refer to Table 14 below for detail.

Covey Associates Pty Ltd                             11                                            Project No: 161968
Consulting Engineers                                                                                  100415 Rpt Iss B