Appendix B - Amended Biome Consulting Response

3.1.6     MODEL VERIFICATION CHECK - RATIONAL METHOD

A verification check of the XP-RAFTS model was undertaken by comparing model generated peak flows with Rational
Method calculations. A Rational Method calculation was undertaken for the overall catchment to Discharge Location B.
Refer to Appendix B for Rational Method details. A comparison is provided in Table 3-4 below.

                         Table 3-4:   Existing Peak Flow Comparison at Discharge Location B
                                          Rational Method     XP-RAFTS Peak            Difference
                             AEP
                                          Peak Flow (m3/s)      Flow (m3/s)                (%)
                             63%                1.32               1.14                   -14%
                             39%                1.80               1.71                    -5%
                             18%                2.55               2.54                    0%
                             10%                2.96               3.02                    2%
                             5%                 3.69               3.69                    0%
                             2%                 4.82               4.28                   -11%
                             1%                 5.27               4.95                    -6%


As indicated in the table above, XP-RAFTS peak flows vary from QRM, but are generally lower than the QRM peak flows.
The average variance is 5%. Therefore the model parameters adopted from the council report in Section 3.1.2 are
considered appropriate.

3.2       DEVELOPED MITIGATED SCENARIO
The Developed Mitigated Scenario was undertaken to demonstrate no adverse impact as a result of the proposed
mitigation measures.

3.2.1     CATCHMENT CHARACTERISTICS

Developed catchments were delineated and split based on preliminary earthworks. A Fraction Impervious value of 70%
was adopted for all sub-catchment areas as per the Concept SWMP Report (No. B14158.W-03A dated November 2015).
Catchment details have been updated to reflect minor changes undertaken during detailed design. The vectored slope
remains consistent with the corresponding existing catchment, as no significant changes under developed conditions
were expected. A summary of the developed catchment details are provided in Table 3.5 below.

                             Table 3-5:    Developed Mitigated Scenario Catchment Details
                                            Downstream          Sub-
          Overall            Sub-                                            Fraction
                                                Sub-         Catchment                       Vectored Slope (%)
         Catchment        catchment                                         Impervious
                                             Catchment        Area (ha)
        North           A1                 A2                   0.98            0.70                0.7%
                        A2                 A_OUTLET             1.51            0.70                2.9%
        Central         B1                 B2                   2.13            0.15                1.7%
                        B2                 B3                   2.38            0.70                2.5%
                        B3                 B4                   2.06            0.70                7.1%
                        B4                 D                    1.05            0.38                4.0%
        South           C1                 C2                   1.32            0.22                0.8%
                        C2                 C2                   2.02            0.80                2.6%
                        C3                 D                    2.93            0.70                6.0%
        Central &       D1                 D2                   0.27            0.63                4.4%
        South           D2                 D_OUTLET             1.25            0.70                4.0%
                        D3                 D_OUTLET             0.58            0.70                4.0%



16-001014-01B | STORMWATER MANAGEMENT PLAN                    COOROY RETIREMENT COMMUNITY, TRADING POST
(SWMP) | PALM LAKE RESORTS                                                                ROAD, COOROY                 |9