Swmp

Peak flow rates determined for each 3 cases are outlined in Table 6.6.

                                 Table 6.6 – Comparison of Peak Flows (L/s)

   AEP Storm          Q1        Q2          Q5           Q10           Q20        Q50          Q100
                    (1EY)    (0.5EY)     (0.2EY)     (10%AEP)       (5%AEP)     (2%AEP)     (1%AEP)

     Existing         79       119         160           186           218        259          287
      PFR A

     Existing         27        39          52           59             69         83           91
      PFR B

    Mitigated         80       102         147           185           209        230          287

      PFR A          (+1)     (-17)       (-13)          (-1)          (-9)       (-19)        (0)

    Mitigated         30        39          47           52             59         72           87

      PFR B          (+3)       (0)        (-5)          (-7)          (-10)      (-11)        (-4)

  (PFR – Peak Flow Rate)

As shown in Table 6.6 the proposed OSD facilities have demonstrated the proposed mitigated peak
discharge flow rates will not exceed the total pre-development flows for all design storms above 2 year
ARI, up to and including the 1% AEP Design Storm. We note that there is a marginal increase in the
mitigated developed peak flow rate for the Q1 (1EY) storm event of 3L/s. However, as the increase in
stormwater discharge is considered very minor and below the design ARI for the minor stormwater
drainage system, the effect on the downstream network is considered to be insignificant and negligible.



Further, all stormwater discharging site in the developed case will discharge to the bio-retention basins
which both have an extended detention depth of 200mm prior to the allocated detention zone. This
equates to approximately 6kL of additional detention storage which has not been considered in the
calculation of peak stormwater discharge rates for the developed site. The peak stormwater discharge
values are therefore considered to be conservative and are expected to be less than the existing site. We
consider the developed site will not have a non-worsening effect on surrounding properties and
downstream network, with regard to stormwater discharge.



The proposed stormwater detention system is therefore considered to be adequate for the development.




SCG Consulting Engineers                   Page 21                             SCG Ref: 33132(SWMP).RevB