Att 1 - RFI Amended SWMP

                                                                                        23098 - SBSMP/3
                                                                                              Page 10




5.3.2 Initial and Continuing Losses
Initial and continuing losses were approximated based on calibrating the pre-development flows
within XPSTORM to be comparable with Rational Method results. Refer to Table 5-4 below for the
adopted initial and continuing losses.

                           Table 5-4 – Adopted Initial and Continuing Losses

                             Designation               Initial Loss   Continuing Loss

                           Impervious Areas                0.0             0.0
                            Pervious Areas                 10.0            2.6

5.3.3 On-site Detention
In order for the post-development stormwater discharge conditions to match the pre-development
conditions, it is proposed to incorporate a series of on-site above ground detention tanks with a
maximum storage volume of 27.5kL. The detention tanks will be located adjacent to each roof
catchment within the site.

The proposed OSD tanks will typically have the following properties:

    •   Available detention depth above reuse component = 1.420 m;

    •   Low flow orifice diameter = 0.070 m;

    •   High flow pipe diameter = 0.150 m;

    •   High flow pipe elevation above reuse component = 1.135 m;

    •   Total required detention volume = 23.5 kL; and

    •   Total provided detention volume = 27.5 kL.

The detention tanks will be designed to contain all flows up to a Q20 storm event from the roof
catchment and will mitigate flows at or less than pre-development for all flows up to a Q100 storm
event.

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

5.3.4 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 5 Pearl Street, Cooroy