Appendix B - Amended Biome Consulting Response

        Since the Issue A report results have been updated to incorporate slight modification to the Central Pond.
        Updated results in Table 3-9 indicate that developed peak flows have been reduced below existing levels, which
        demonstrates no adverse impact.

As indicated in the results, peak flows have been reduced below existing levels at both discharge locations. Therefore,
the results demonstrate that the proposed detention basins are adequate to reduce peak flows to ensure no adverse
impact due to the development.

The Water Surface Levels (WSL’s) determined in each basin have been indicated in Table 3-10 below.

                        Table 3-10: Developed Mitigated Scenario Basin Water Surface Levels
                                       Northern Basin                             Central Basin
                AEP
                                WSL (m AHD)       Depth (m)1             WSL (m AHD)        Depth (m)1
                 63%               99.92              0.52                 100.28               0.68
                 39%              100.06              0.66                 100.50               0.90
                 18%              100.19              0.78                 100.64               1.04
                 10%              100.26              0.86                 100.75               1.15
                  5%              100.37              0.97                 100.96               1.36
                  2%              100.45              1.05                 101.00               1.40
                  1%              100.51              1.11                 101.03               1.43
 Depth noted is the height above the stormwater quality temporary ponding level, which is 99.4m AHD and 99.2m AHD for the
                                          Northern and Central Basin respectively.

The results above indicate that 300mm freeboard has been achieved to the top of each basin’s minimum embankment
level. Results also indicate that the major overflow weirs are only overtopped during the major storm events (i.e. during
the 2% and 1% AEP storms). During the 1% AEP storm, discharge across the major overflow weir is limited to a depth of
200mm. The depth velocity (dV) products reaches a maximum of 0.15m2/s for the major overflow weirs, which is lower
than the QUDM requirements.

        Since the Issue A report results have been updated to incorporate slight modification to the Central Pond.
        Updated results in Table 3-10 demonstrate that the same outcomes have been achieved as discussed above.

3.2.7     EXTREME EVENT SENSITIVITY CHECK

The major overflow/emergency weir for each detention basin has been designed to cater for extreme events, i.e. extreme
storms (i.e. 0.2% AEP storm event) and extreme blockage (100% blockage of basin pits/outlet pipes). To be conservative
the extreme 0.2% AEP storm event has been modelled with 100% blockage of the pits (considering weir flow only). The
0.2% AEP storm for 15 to 4320 minute durations has been modelled in XP-RAFTS with rainfall hyetographs generated
using the IFD parameters for Cooroy specified in Section 3.1.4. Results have been cross-checked with the 0.2% AEP
regional rainfall hyetograph for the 1440 minute storm duration adopted in Water Technology’s Cooroy Flood Study
(2013) model. The highest basin WSL was adopted from all these scenarios. Other than the rainfall parameters and basin
discharge values, all other modelling parameters remain the same as per the Developed Mitigated Scenario.

Table 3-11 below indicates 0.2% AEP basin WSL’s determined in XP-RAFTS.

                                  Table 3-11: Extreme Storm Event Weir Assessment
                                Parameter                         Northern Basin          Central Basin
               Minimum Top of Embankment Level (m AHD)                 100.8                 101.30
                Critical 0.2% AEP Storm Duration (minute)           45 minute              120 minute
                      0.2% AEP Basin WSL (m AHD)                      100.72                 101.26




16-001014-01B | STORMWATER MANAGEMENT PLAN                      COOROY RETIREMENT COMMUNITY, TRADING POST
(SWMP) | PALM LAKE RESORTS                                                                  ROAD, COOROY                  | 13