RFI - Appendix D- Stormwater Management Plan · MCU22/0140

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The WBNM pre-developed model was calibrated to achieve an acceptable alignment with the existing site
Rational Method estimates for whole of site discharge.

As the development site is only 1012m2 there is not a practical way to produce better alignment than that
achieved in this analysis as;
    • Increasing initial losses further would result in the 1% AEP storm estimates becoming unconservative,
    • Adjusting the Lag Parameter primarily affects the timing of the peak, so only serves to reduce
        discharge rates by extending the critical duration, and has limited effect on smaller catchments.

On larger catchments Lag Parameter adjustments can bring the frequent events into good agreement with
the Rational Method.

As the appreciable magnitude of the difference in discharge between the Rational Method and WBNM
estimates peak at 6L/s it is considered that the calibration undertaken is acceptable. The calibration between
the WBNM and Rational Method existing flow estimations are presented in Table 1.

The adopted parameters for the WBNM analysis are presented in Table 2.


 WBNM ANALYSIS PARAMETERS
      Parameter                     Value                                       Source / Justification
                                                     ARRDH Suggested 36mm Initial Loss, However this was deemed
      Initial Loss                  15mm             excessive, and the loss rate was calibrated to get a close alignment
                                                     with the rational method
   Imp. Initial Loss                 1mm             Adopted as per industry practice
   Continuing Loss                2.6mm/hr           ARRDH, this has been adopted as it is deemed acceptable
                                                     This has been decreased from the recommended value of 1.7 in the
                                                     WBNM documentation as part of the calibration, and deemed to give
   Lag Parameter                      1.6
                                                     a great fit if the shorter duration events are compared to the rational
                                                     method.
                                                 Table 2: WBNM analysis parameters

6.1     Detention Analysis

The WBNM parameters in Table 2 have been used with the proposed development areas to obtain an
estimation of how the site will perform with peak flow mitigation measures implemented.

For this analysis it has been adopted that 100% of the roof areas is to be connected to the detention and
retention tanks, with the eaves gutters and connecting pipework sized for a 1% AEP storm event.


6.2     Detention Sizing and Details

The required detention volume that was determined to maintain or improve the pre-development discharge
rates for the site was 6,000L. This is proposed to be comprised of two 3000L slimline tanks (half-half split for
detention/retention – 1,500L detention/retention per tank) connected to the Units discharging to Opal Lane,
and one 3,000L tank for the Units discharging to Ruby Street with properties as listed in Table 3 and Table 4
respectively.




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