updated flood investigation

              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 3-2. Based
      on this assessment it can be seen a suitable verification has been achieved.
      Table 3-2: 1% AEP Peak Flow Verification
                Parameter                                  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


      3.2.1. 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 3-3 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
      changed from 720min to the 120min, with the TUFLOW hydraulic model considered a better
      representation of flow routing within the catchment.
      Table 3-3: 1% AEP Peak Flow Comparison
                Parameter                                  Value                  Value
              Sub-Catchment                                  1A                     2C
             WBNM (Hydrology)                          8.1 (720min)           33.0 (720 min)
            TUFLOW (Hydraulics)                        7.5 (120 min)          33.5 (120 min)
                  Difference                                 -0.6                  +0.5


      A comparison of the peak 1% (2100) AEP flood levels from the NSC flood search and TUFLOW
      model results from this investigation is presented below in Figure 3-1. This highlights that the
      estimated levels are a close match across the site with a levels at the downstream boundary
      of the site slightly higher in this current investigation (which is considered to create a
      conservative outcome). As such it is considered that a suitable verification between the NSC
      regional flood model and the current investigation has been achieved.




21-0288-FS001-V1 | Flood Investigation | 60 Glenridge Drive, Cooroibah                                   |5