16-001014-01A (Swmp)

3.2.2     INFILTRATION LOSS & LAG PARAMETERS

The same Infiltration Loss and Lag Parameter values were adopted as per the Existing Scenario (refer to Section 3.1.2),
which are consistent with Water Technology’s Cooroy Flood Study.

3.2.3     CHANNEL ROUTING

Channel Routing (XP-RAFTS Link Routing) was modelled as per Water Technology’s Cooroy Flood Study (2013), by
determining ‘k’ lag values based on flowpath lengths and average channel velocities, with a constant ‘x’ coefficient of 0.2.
Refer to Section 3.1.3 for details.

3.2.4     RAINFALL DATA

Rainfall data was generated as per the Existing Scenario in Section 3.1.4.

3.2.5     PROPOSED MITIGATION STRATEGY

The following detention basins (incorporated with stormwater quality treatment) are proposed as part of the stormwater
mitigation strategy to reduce peak flows back to or below existing levels. This is consistent with the Concept SWMP
Report (No. B14158.W-03A dated November 2015) and other past approved documentation.

   Northern Basin consists of a combined bioretention and detention basin to mitigate peak flows from the northern
    portion of the site (i.e. Sub-Catchments A1 and A2). This basin is consistent with the previously proposed basin in
    the past DA approval and only been slightly updated to account for the new development layout and the use of a Bio-
    Retention system rather than a water quality control pond for treatment.

   Central Basin is a water quality control pond (WQCP) which incorporates both detention storage for peak flow
    mitigation and water quality components. The WQCP will mitigate peak flows from the central and southern portion of
    the site (i.e. Sub-Catchments B1, B2, B3, B4, C1, C2 and C3). This concept is different from the previously approved
    Central Basin as the wetland has been replaced with a WQCP and the basin has been upsized to mitigate runoff
    from the southern portion of the site.

   Southern Catchment Diversion – Minor flows from the southern portion of the site are to be directed from the
    proposed South Bioretention Basin towards the Central Basin for flow attenuation. This flow diversion will be sized for
    the minor design storm (63% AEP), which is a flow of 1.25m3/s. Runoff that exceeds this capacity will overtop the
    bioretention basin and be conveyed to the eastern corner of the site. The southern bioretention basin has been
    modelled in XP-RAFTS to ensure that the outlet configuration is adequate to convey runoff as proposed.

Details of the proposed detention basins are presented in Table 3-6. For details of the proposed South Bioretention Basin
outlet configuration (sized for diversion of 63% AEP peak flows), refer to Drawing No. B14158.W-SK105 in
Appendix G.




16-001014-01 | STORMWATER MANAGEMENT PLAN (SWMP)                 COOROY RETIREMENT COMMUNITY, TRADING POST
| PALM LAKE RESORTS                                                                          ROAD, COOROY                | 10