RFI - Stormwater Mgm Plan 20169 - SBSMP - Rev 4 (2)

                                                                                 20169 - SBSMP/4
                                                                                          Page 12




5.4.3 On-site Detention
In order for the post-development stormwater discharge conditions to match the pre-development
conditions, it is proposed to incorporate an open detention basin with a maximum storage volume
of 200kL for the southern catchment and an open detention basin with a maximum storage volume
of 160kL for the northern catchment. The detention basins are to be located above ground within
the southeast corner of the site and the northeast corner of the site, respectively.

The proposed OSD basin for the southern catchment will have the following properties:

       Specified base area = 170m2;

       Specified internal depth = 1.15m;

       Low flow orifice diameter = 0.15m;

       Overflow weir pit elevation from base of tank = 0.85m;

       Required basin volume = 179kL; and

       Provided basin volume = 200kL.

The proposed OSD basin for the northern catchment will have the following properties:

       Specified base area = 160m2;

       Specified internal depth = 1.00m;

       Low flow orifice diameter = 0.22m;

       Overflow weir pit elevation from base of tank = 0.70m;

       Required basin volume = 136kL; and

       Provided basin volume = 160kL.

The detention basin will be designed to contain all flows up to a Q100 storm event and mitigated to
the capacity of the existing stormwater infrastructure at a Q20 storm event.

Refer to Appendix F for the Concept Site Levels and Drainage Plan which details the proposed on-
site detention system.

5.4.4 Comparison of Peak Flows at Lawful Point of Discharge
The pre-development and post-development mitigated peak mean/median flow rates were
compared for the catchment and are shown in Table 5-5 below.




Site Based Stormwater Management Plan
Proposed Townhouse Development at 99, 101 & 105 Eumundi Noosa Road, Noosaville