LATE RFI - SCG 33114 (SWM) Rev A

6.1.3        Peak Flow Comparison
Table 6.5 compares the peak flow rates for the existing and developed cases.
                            Table 6.5 – Comparison of Existing Vs Developed Peak Flows

             AEP       Q1     Q2      Q5      Q10      Q20     Q50    Q100
            Storm    (1EY) (0.5EY) (0.2EY) (10%AEP) (5%AEP) (2%AEP) (1%AEP)
        Existing       24        33          45           56            69               83            95
         (L/s)
        Developed      40        50          62           69            83           102               118
          (L/s)
        Increase       16        17          17           13            14               19            23
          (L/s)


As shown in Table 6.5 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 an inground
detention tank for all roof water runoff from units 3-10, located under the proposed driveway. The
detention tank is to be fitted with a low flow and high 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                             =        705 m2 (100% Impervious)
        -
               Base Area                                  =         36 m2
        -
               Internal Depth                             =          1 m
        -
               Low Flow Orifice                           =         70 mm diameter
        -      High Flow Orifice                          =         95 mm diameter
        -
               Overflow Pit Elevation from base           =        0.5 m
        -
               Required Volume                            =         31 m3
        -
               Provided Volume                            =         36 m3
Figure 6.1 illustrates the DRAINS Model layout for the detention tank.




SCG Consulting Engineers                      Page 13                                         SCG Ref: 33114(SWMP)