Attachment 7 - Flood Impact Assessment - Covey · 132005.638.06

         3.6     Pre-Burst Rainfall

         Rainfall losses adopted for the model are for complete storms. As such, pre-burst depths have been
         subtracted from the adopted initial losses to get the correct storm burst initial losses.
         From a review of the median and 75% pre-burst depths, it was concluded that the median pre-burst
         depths were not appropriate for this investigation, particularly given that the outcome of this
         investigation will determine minimum floor levels for residential properties. As such, adopting the 75%
         pre burst depths was considered more appropriate for this investigation.

         3.7     Areal Reduction Factor

         ARR 2019 guidelines recommend adopting Areal Reduction Factors (ARFs) for catchments greater than 1
         km2. Given that the associated catchment area modelled as part of this simulation is approximately 2.87
         km2, ARFs were adopted for this analysis. Refer to Table 3-1 for adopted ARFs.


                                               Table 3-1 - Areal Reduction Factors
                    Dur                                               AEP (%)
                   (min)          50             10                 1           0.5    0.2       0.1
                     20          0.957          0.955             0.948     0.946     0.944     0.943
                     25          0.962          0.958             0.952     0.951     0.948     0.947
                     30          0.965           0.96             0.955     0.954     0.951     0.95
                     45          0.972          0.969             0.961     0.959     0.956     0.954
                     60          0.976          0.973             0.964     0.962     0.959     0.957
                     90          0.981          0.977             0.966     0.965     0.96      0.958
                    120          0.984           0.98             0.967     0.965     0.961     0.958
                    180          0.987          0.983             0.969     0.966     0.962     0.959
                    270          0.99           0.986             0.975     0.972     0.969     0.966
                    360          0.991          0.989             0.982     0.981     0.978     0.977



         3.8     Manning`s Values

         Manning’s n roughness coefficients have been manually delineated for the entire contributing catchment.
         Refer to Appendix B for the Manning’s Roughness Coefficient Maps. Land uses and corresponding
         Manning’s roughness coefficients for the hydraulic model are as follows:
             • 0.018 – Roads
             • 0.075 – Trees / Bushland
             • 0.050 – Grassland
             • 0.300 – Roof
             • 0.030 – Unsealed road
             • 0.015 – Water body
             • 0.018 – Compost
             • 0.020 – Solar Panels
             • 0.030 – Landfill




Flood Impact Assessment                                                                                  Page 9 of 25
Project No 244035 | Ref No M25-0479Rpt – Issue A – January 2026