1909-13367 SRA referral response approved docs

Table 5 shows the mitigating effect of the OSD on the developed sites peak flows. The mitigated site
shall decrease peak stormwater flows (when compared with the existing site) for all design events.
Therefore ensuring that the downstream environment will not experience an increase in peak
stormwater flow from the proposed development.

          Table 5 – Mitigated peak flow rates from the developed site

                                                  Peak Flow Rate (L/s)
 AEP Storm
   Event
                        Combined Existing Catchment               Combined Mitigated Catchment

       1EY                            2610                                    1652

       0.5EY                          3837                                    2021

       0.2EY                          4239                                    2063

       10%                            5169                                    2281

        5%                            6397                                    3051

        2%                            7197                                    4734

        1%                            7786                                    5516



The developed site has changed the proportions of area draining to each of the LPOD. In the existing
case approximately 71.2% of the total site is considered to drain LPOD 1 with the remaining 28.9%
draining to LPOD 2. The developed case splits this with approximately 56.6% draining to LPOD 1,
16.5% draining to LPOD 2 and 26.9% draining to LPOD 2. Despite this the peak flows discharging to
each LPOD have not been increased for any event up to the 1% AEP event.

3. 5     W ate rw a y St ab il it y

The proposed development is proposed LPOD shall discharge to existing waterways within Cooroy golf
course. This triggers the need for waterway stability within Appendix 2 of the SPP. Table B of Appendix
2 of the SPP states that the design objective for waterway stability management shown below.
“Limit the peak 1-year ARI event discharge within the receiving waterway to the pre-development peak
1-year ARI discharge” – SPP Appendix 2 Table B.

As shown below in table 6 the proposed stormwater detention modelling has shown that the 63.2% AEP
event peak discharge (equivalent to 1 year ARI) has been reduced to each waterway. It is therefore
deemed that the development is meeting the waterway stability design objective.




                                        Stormwater Management Report                                11
S19-007                                   122 & 144 Myall St, Cooroy