Callaghan and Toth IWMP Report revised

4.2   Comparison To Rational Method Calculations

The peak outflow rates from the Urbs models for the pre- and post-development scenarios are compared
to the Rational Method flow rates in Table 1, below. The Urbs outflow rates are mostly fractionally higher
than the Rational Method flow rates, and 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               Rational Method    Urbs
   100                0.26        0.27                    0.35         0.34
    10                0.15        0.16                    0.20         0.21
     1                0.07       0.07                     0.09         0.10


4.3   Detention Volumes

With the models deemed to be performing acceptably 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 80 cubic metres, with results across the range of ARI as in Table 2, below. The
required attenuation is exactly achieved, apart from a fractional shortfall in attenuation at the 1 and 2 year
level. 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           0.34    0.27          0.34     0.27
        50           0.30    0.24          0.30     0.24
        20           0.25    0.19          0.25     0.19
        10           0.21    0.16          0.21     0.16
         5           0.18    0.14          0.18     0.14
         2           0.14    0.10          0.14     0.11
         1           0.10    0.07          0.10     0.08

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 89 cubic metres.

In summary, the modelling has suggested a 100 year detention volume of 80 cubic metres, or 89 cubic
metres if like-for-like storm durations are considered to be a vital requirement. Both of those volumes are
consistent with the broad-brush estimate of 85 cubic metres at the bottom of page 4 of this report. A
nominal volume of 82 cubic metres has been adopted, on the basis that the use of the latest infiltration
allowances and temporal patterns at the detailed design stage, together with an appropriate allowance for
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