Stormwater Management Plan

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

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 11 summarises stormwater flows, storage volumes and stage height for the required
detention tank.
                           Table 11 - 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 12 below for detail.

     Table 12 - Comparison of Pre & Post-Development (mitigated) Stormwater flows in XP-RAFTS
                                                  Existing                    Mitigated
          Catchment          AEP         Critical      Peak Flow         Critical   Peak Flow
                                        Duration         (m3/s)         Duration      (m3/s)
                             1%          25 min              0.630       25 min        0.624
            Site Area        10%         25 min              0.357       25 min        0.349
                             39%         60 min              0.219       25 min        0.211

7.0 CONCLUSIONS AND RECOMMENDATIONS
As described in Table 5, the stormwater treatment measures selected for the proposed
development achieve the performance criteria set by the NSC and by the State Planning Policy
(2014). Annual load reductions of TSS, TP and TN between the unmitigated and mitigated MUSIC
model scenarios have been met or exceeded. It is considered that a conservative approach has
been taken as no routing in the drainage links has been adopted.

This site based SWMP has demonstrated that the proposed development, when incorporating the
proposed detention strategies, can be undertaken with non-worsening to the quantity of runoff to
downstream environments.


Covey Associates Pty Ltd                         10                                       Project No: 161968
Consulting Engineers                                                                         100415 Rpt Iss A