2576 - SBSMP-01 (Feb 2026)

4.4    WBNM Analysis Pre-Development Scenario


A WBNM was approximated for the existing catchments according to existing site parameters.
The model estimated that the existing 1% AEP peak flow discharge from the site was
0.8m3/s overall. Pre-development peak flow rates for all design storms are provided in Table
4-4.


                                        EXISTING CASE - COMBINED
                                               Peak Flow Rates - Q (m3/s)
                         AEP             WBNM             RM           Difference
                        63.2%            0.197           0.209             6.1%
                         50%             0.242           0.277            14.5%
                         20%             0.457           0.384           -16.0%
                         10%             0.485           0.465            -4.1%
                         5%              0.578           0.552            -4.5%
                         2%              0.725           0.704            -2.9%
                         1%              0.801           0.807             0.7%
                        Table 4-4 WBNM discharge flow rates for pre-development catchments




The Rational Method was used to provide an estimate of a range of flows for the total pre-
development catchment for comparison and verification of the WBNM model. See Appendix
D for calculations. The Rational Method results are within 20% of the results obtained from
the WBNM analysis for the pre-development case across all storm events. From the
comparison of results, it is determined that the Rational Method verification corresponds well
with the WBNM analysis results.




2576-SBSMP-01_Version 1.0 (February 2026)                                                    Page 17 of 23
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