Reference Documents - MCU23-0142

                                         STATE          EXISTING PROPOSED

                                  Catchment Area                0.2          0.2
                                       (Ha)

                                         (Tc)                   10            6

                                      Fraction                  0            0.88
                                   Impervious (Fi)


Peak flow rates for the median design storm at year 2100 for the existing site are given in Table 6.3


                                    Table 6.3 –Peak Flow Rates (L/s) (At year 2100)

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

    Existing          33            44             64                 77            92         109        123
      PFR

  Developed           62            78             96                 110          126         147        163
     PFR

(PFR – Peak Flow Rate)


As shown in Table 6.3 Stormwater peak discharge will increase as a result of the proposed development
due to the increase in impervious area. Therefore, it is proposed to utilise an OSD system to mitigate
the peak stormwater discharge rates from the proposed development prior to discharging to the lawful
point of discharge as discussed in section 4.2.



6.1.3       Peak Flow Mitigation (OSD)
To mitigate the peak flow increase from the developed site, it is proposed to include a below ground
44kL detention tank for all roof water runoff from the covered areas within the site.

The detention tank is to be fitted with a low flow orifice plate to detain a portion of the runoff from the
site and limit the peak flow rates of pre-development levels for all storms up to the Q100 storm event.
The proposed on-site detention tank will have the following properties: -
        -
              Roof Catchment                                =           1172 m2(100% Impervious)
        -
              Tank Base Area                                =           66.5 m2
        -
              Internal Depth                                =            0.6 m
        -
              Low Flow Orifice                              =            150 mm diameter
        -     High Flow Orifice                             =            175 mm diameter
        -
              Required Volume                               =         39.765 m3
        -
              Provided Volume                               =         39.900 m3.




 SCG Consulting engineers                                  13                            SCG Ref: 33094(SWM).Rev A