Appendix E v 2 - Revised Integrated Water Mangement Plan

0.1 increments for ARI’s from 100 year down to 1 year. Rainfall intensities and temporal patterns were
ARR87 standard, and infiltration rates in both models were specified as zero initial loss and 2 mm/hour
continuing loss, as accepted by a Noosa Council representative in a telephone conversation of 12th
December 2017.


4.2     Comparison To Rational Method Calculations

The peak outflow rates from the Urbs models are compared to the Rational Method calculations in
Table 1, below. The Urbs outflow rates are consistently higher than the Rational Method calculations,
by as much as 18 – 20 percent at the 1 year level, reducing to 2 to 4 percent at the 100 year level.
Overall this is suggested to be an acceptable comparison.

Table 1:     Comparison Of Urbs Outflow Rates To Rational Method Outflow Rates
  ARI             Pre-Development Model         Post-Development Model (No Basin)
 (years)      Rational Method   Urbs    Diff      Rational Method   Urbs Diff
   100              0.43        0.45    4%              0.57        0.58    2%
    10              0.25         0.27   8%              0.33        0.37 12 %
    1               0.11         0.13 18%              0.15        0.18 20 %


4.3     Detention Volumes

With the models deemed to be performing acceptable well, coding for a conceptual detention basin was
added to the post-development model, as can be seen in the catchment file in Appendix C. The coding
was adjusted so as to achieve a reasonably smooth storage-discharge curve while at the same time
ensuring that the outflows from the post-development scenario with the basin in place were reasonably
matched to the outflows from the pre-development scenario, regardless of any difference in the critical
storm durations between the pre- and post-development scenarios. The required 100 year detention
volume was found to be 118 cubic metres, with results across the range of ARI as in Table 2, below.
The 1 year outflow rate is attenuated exactly according to the requirement, while the 100 year outflow
rate is fractionally over-attenuated. Of the intervening ARI’s, there is a fractional shortfall in attenuation
at the 2 year level and a slightly beneficial degree of over-attenuation at the 2 year to 50 year ARI’s.
Overall, this volume is considered to provide an acceptable outcome, notwithstanding that the critical
storm durations were not necessarily the same, between the pre-development and post-development
models.

Table 2: Urbs Design Of Detention Volume, Regardless Of Critical Durations
  ARI (years)   Required Attenuation (m3/sec)    Achieved Attenuation (m3/sec)
       100          0.58     0.45         0.58      0.44
        50          0.50     0.39         0.50      0.36
        20          0.43     0.32         0.43      0.30
        10          0.37     0.27         0.37      0.25
        5           0.31     0.23         0.31      0.22
        2           0.24     0.17         0.24      0.18
        1           0.18     0.13         0.18      0.13

The models were then run for 100 year storm durations up to 3 hours, maintaining the same storm
durations in the pre-development and post-development models for each run. The 100 year 2 hour storm
was found to be the critical duration, resulting in a 100 year detention volume of 140 cubic metres.

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