Att 1 - Engineering - RFI Reponse · MCU21/0117.01

                                                                                23098 - SBSMP/4
                                                                                       Page 12




In order for the post-development stormwater discharge conditions to match the pre-development
conditions, it is proposed to incorporate 10kL above-ground rainwater tanks with a maximum
detention storage volume of 37.5kL. The combined detention/rainwater reuse tanks are to be
located adjacent to the proposed buildings.

The proposed OSD rainwater tanks will have the following properties:

Building 1, 2, & 3:

    •   Specified internal depth = 1.74m;

    •   Low flow orifice diameter = 0.028m;

    •   High flow orifice diameter = 0.140mm;

    •   Detention volume = 7.5kL; and

    •   Tank volume = 2x10kL (7.5kL detention + 2.5kL rainwater reuse) = 20kL

Building 2:

    •   Specified internal depth = 1.74m;

    •   Low flow orifice diameter = 0.028m;

    •   High flow orifice diameter = 0.140mm;

    •   Detention volume = 7.5kL; and

    •   Tank volume = 10kL (7.5kL detention + 2.5kL rainwater reuse)

Building 3:

    •   Specified internal depth = 1.74m;

    •   Low flow orifice diameter = 0.028m;

    •   High flow orifice diameter = 0.140mm;

    •   Detention volume = 7.5kL; and

    •   Tank volume = 2x10kL (7.5kL detention + 2.5kL rainwater reuse) = 20kL

The tanks will be designed to contain all roofwater flows up to a Q100 storm event and mitigate the
outflow at or less than pre-development flows. Stormwater runoff not conveyed to the detention
tanks will sheet flow across the development boundaries in conjunction with the mitigated flows
from the detention tanks at or less than pre-development.

Refer to Appendix F for the Concept Site Levels and Drainage Plan which details the proposed on-
site detention system and Figure 5.3 below for a screenshot of the post-development mitigated
model.




Site Based Stormwater Management Plan
Proposed Townhouse Development at 5 Pearl Street, Cooroy