Swmp

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 On Site Detention (OSD)
systems 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.

6.1.4       Peak Flow Mitigation (OSD)
To mitigate the peak flow increase from the developed site, it is proposed to include a mixture of
inground detention tanks, located under the proposed driveway and an inground open bio
retention/detention basins.

The site has been divided into sub catchments to divert flows through the two Bio-Retention Beds,
namely Bio-Retention Bed B (BR-B) located in the south east corner of the site and Bio-Retention Bed
C (BR-C), located in the north east corner of the site.

These bio-retention beds will also include storage capabilities to provide OSD. An additional inground
OSD tank has been included to further detain flows with sub-catchment B upstream from BR-B.

We propose for the detention systems to cater for flows up to the Q100 storm event and a limit the peak
flow rates of pre-development levels for all storms up to the Q100 storm event.

The proposed BR-B will have the following properties: -

        -
                Base Area                                 =   35m2
        -
                Internal Depth                            =   0.9m
        -
                Low Flow Orifice                          =   225mm diameter
        -       High Flow Orifice                         =   Bio Bed grated inlet
        -
                Required Volume                           =   33m3
        -
                Provided Volume                           =   35m3



The proposed BR-C will have the following properties: -

            •     Base Area                               =   89m2
            •     Internal Depth                          =   0.75m
            •     Low Flow Orifice                        =   275mm diameter
            •     High Flow Orifice                       =   Bio Bed grated inlet
            •     Required Volume                         =   54m3
            •     Provided Volume                         =   71m3




SCG Consulting Engineers                   Page 17                                   SCG Ref: 33132(SWMP)