Flood Investigation & Stormwater Quantity Management Plan

Flood Investigation & Stormwater Quantity Management Plan
Proposed Residential Development at Doonella - Stage C

                                   Table 3.2: Developed Scenario Sub-Catchment Details
                    Sub-Catchment                               Area (ha)                   Fraction Impervious
                    A                                             2.42                              0.18
                    B1                                            3.46                              0.53
                    B2                                            1.55                              0.48
                    C1                                            1.61                              0.65
                    C2                                            1.73                              0.70
                    C3*                                           0.18                              0.80
               * - External sub-catchment of Sea Eagle Drive discharging minor design storm (pipe drainage) flows only to site.

For determining peak flows downstream of the site (for hydraulic modelling within the environmental corridor, refer to
Section 4) additional external sub-catchments over the adjacent residential development to the east and south have
been delineated for modelling of post development (mitigated scenario) conditions. Details of the external sub-
catchment areas and calculated fraction imperviousness is summarised in Table 3.3 below.
                                         Table 3.3: External Sub-Catchment Details *
                    Sub-Catchment                               Area (ha)                   Fraction Impervious
                    D1                                            0.43                              0.46
                    D2                                            4.99                              0.62
                    D3                                            1.82                              0.26
                           * - Sub-Catchments used only for the determination of peak flows for hydraulic analysis.

Refer to Drawing No. B09182.W-SK01 in Appendix D for the location and delineation of each developed scenario
sub-catchment area.

3.2      Existing Scenario Model
The existing scenario WBNM was created using the sub-catchment areas and fraction impervious values summarised
in Table 2.1 along with the follow input data adopted.
3.2.1    Rainfall Data
Design storm rainfall hydrographs were generated within WBNM using the log normal intensities and geographical
factors presented in Table 3.4 below. These values, sourced from the The Bureau of Meteorology Rainfall IFD Data
System website, are consistent with the Intensity Frequency Duration (IFD) data from Council’s DDPSP that have been
used in Rational Method as outlined in Section 3.2.4.
                                Table 3.4: Log Normal Intensities & Geographical Factors
                    Log Normal Intensities                       1 hr                 12 hr                 72 hr
                    2 Y ARI                                  52.03mm/hr            10.34mm/hr            3.46mm/hr
                    50 Y ARI                                 93.95mm/hr            24.00mm/hr            8.66mm/hr
                                                                  G                     F2                   F50
                    Geographical Factors
                                                                 0.11                  4.40                 17.6


3.2.2    Routing Factors
A stream lag factor of 1 was adopted for all sub catchments for the Existing Scenario.
3.2.3    Loss Model & Lag Parameter
As part of the WBNM Calibration various configurations of initial loss, continuing loss and lag Paramter were trialled
until a configuration produced peak flows consistent with those calculated using the Rational Method. Refer to
Section 3.2.4 for details on the WBNM calibration and Rational Method Calculations. Table 3.5 below summarises
the optimum Loss and Lag Parameters that resulted from the calibration process.

B09182.W-02A                                                                                                                      Page 4