RFI - updated flood investigation report

              Rainfall Depths
      Rainfall depths for events up to the 1% AEP were obtained from the BoM website and the
      Probably Maximum Precipitation (PMP) event was determined using the Generalised Short-
      Duration Method (GSDM) and the Generalised Tropical Storm Method Revised (GSTMR).
      Calculation sheet for the GSDM and GSTMR is provided in Appendix C. The rainfall depth for
      the 39%, 10%, 1%, 1% (2100) AEP and PMP events are summarised below in Table 3-2.
      Table 3-2: Rainfall Depths
         Duration            39% AEP            10% AEP            1% AEP   1% (2100) AEP        PMP
          (hours)
           0.25                26.2                 37               52.9        63.5            190
           0.50                36.9                52.7              76.5        91.8            290
           0.75                43.8                63.3              93.5       112.2            360
           1.00                49.1                71.7              107        128.4            420
           1.50                57.4                85.1              130         156             540
             2                 64.1                 96               149        178.8            630
           2.50                69.8                106               165         198             700
             3                 74.8                114               180         216             760
             4                 83.7                129               206        247.2            880
             5                 87.7                136               218        261.6            960
             6                 91.4                143               229        274.8            1020
            12                 98.3                154               249        298.8            1250




              Peak Flow Rates and Verification
      The peak flow rates estimated from the WBNM model have been verified against the rational
      method for the 1% AEP event, with the Bransby-Williams equation adopted to determine the
      time of concentration as recommended by the Department of Transport and Main Roads
      “Road Drainage Manual 2015”. The flow verification was completed in relation to two sub-
      catchments (1A and 2C). Details of the flow verification are presented in Table 3-3. Based
      on this assessment it can be seen a suitable verification has been achieved.
      Table 3-3: 1% AEP Peak Flow Verification
                Parameter                                  Value                            Value
              Sub-Catchment                                 1A                               2C
             Contributing Area                            29.8 ha                           156.0
          Time of Concentration                            38 min                            92 min
            Runoff Coefficient                               0.7                               0.7
            Rational Peak Flow                            9.1 m3/s                            31.2
             WBNM Peak Flow                               8.1 m3/s                            33.0
             Critical Duration                            720 min                           720 min


      3.3.1. TUFLOW Peak Flows

      A comparison of peak flows from the WBNM hydrology model to that estimated within the
      TUFLOW hydraulic model is presented below Table 3-4 at catchment 1A and 2C for the 1%
      AEP event. It can be seen that the TUFLOW hydraulic model estimates for these two
      catchments correlate well to the WBNM output, with peak flows from Catchment 1A slightly
      less in the hydraulic model and Catchment 2C slightly more. The critical duration also




21-0288-FS001-V3 | Flood Investigation | 60 Glenridge Drive, Cooroibah                                  |8