1909-13367 SRA referral response approved docs

                                        Figure 2 - XPSTORM model layout.

The XPStorm modelling program has utilised all the above data to give an estimate of peak flow rates
for each AEP storm event for the existing site in table 3.

               Table 3 – Existing & Developed site peak flow rates.

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

       1EY                           2610                                         3006

       0.5EY                         3837                                         3806

       0.2EY                         4239                                         4723

       10%                           5169                                         5628

        5%                           6397                                         6388

        2%                           7197                                         7207

        1%                           7786                                         7874


A comparison of the existing and developed peak flows as listed above indicates that there will be an
increase in peak stormwater flows from the developed site. From these comparisons it can be seen that
an On Site Detention (OSD) device is necessary to ensure that the peak stormwater flows exiting the
developed site do not cause a detrimental effect to downstream properties or infrastructure.

3. 4     O n sit e D et e nt io n ( O SD )

Runoff from the site is proposed to be captured and directed to one of three bioretention basins which
will act as OSD. Catchment 1 will drain to basin 1, catchment 2 will drain to basin 2 and catchment 3
will drain to basin 3. There will also be a fourth basin, however this will only treat a small portion of the
catchment for stormwater quality purposes. A highflow bypass will be in place for the fourth basin where
events in excess of the design event will bypass.

It is proposed that the combined flows from the three sub-catchments shall not exceed the peak flow of
the existing site. Table 4 details the proposed detention details.




                                         Stormwater Management Report                                      9
S19-007                                    122 & 144 Myall St, Cooroy