Flood Investigation & Stormwater Quantity Management Plan

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

The results indicate that the WBNM produces peak flows that are generally consistent with those determined by the
Queensland Rational Method. While the 2 year ARI flows from the WBNM are lower than calculated by the Rational
Method its significance on major flows within the environmental corridor and the immunity of proposed residential
allotments is negligible.
The adopted lag parameter, initial and continuing losses and configuration of the WBNM is therefore considered
appropriate.
3.2.5    Existing Peak Flows
The existing scenario WBNM, configured with the fraction impervious values as presented in Table 3.1, was run. The
results of the existing scenario WBNM and comparison to peak flows determined at the same location by JF&P (2008)
are presented in Table 3.7 below.
                                       Table 3.7: Existing Scenario Peak Flows
                                                                 Peak Flow (m3/s)
                   ARI
                                                      Existing WBNM            JF&P (2008) *
                   2                                        0.89                  0.91
                   5                                        1.42                  1.28
                   10                                       1.75                  1.51
                   20                                       2.20                  1.83
                   50                                       2.61                  2.32
                   100                                      3.07                  2.64
                                            * - Refer to Table 6 of JF&P (2008) report

The results indicate that the peak flows from the WBNM in general are slightly higher than those previously calculated
by JF&P. As the slightly higher peak flows will result in more conservative results (both in post-development peak
flows and sizing of detention storages) the existing scenario results are considered to be appropriate.

3.3               Developed Scenario Model
Under the developed conditions sub-catchments B, C1 and C2 are to be developed with residential lots and road with
open space reserved along the environmental corridor. Works requird to Sea Eagle Drive will also divert minor design
storm flows from a small external catchment into the site.
The WBNM was therefore configured and run with the developed scenario sub-catchment areas and fraction
impervious vales as presented in Table 3.2. A diversion was incorporated into the model to direct flows greater than
the minor design storm (5 year ARI) around the downstream site sub-catchments. No other changes to the model
were made from the existing scenario configuration. The results from the developed scenario are presented in Table
3.8 below.
                                      Table 3.8: Developed Scenario Peak Flows
                                                                Peak Flow (m3/s)
                   ARI
                                                     Developed WBNM            Increase (%)
                   2                                       1.81                    103
                   5                                       2.53                     78
                   10                                      2.96                     69
                   20                                      3.55                     61
                   50                                      4.09                     57
                   100                                     4.67                     52




B09182.W-02A                                                                                                     Page 6