| Proposed Development – Surface Water Management | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 9 Sensitivity: General 4.5.2 SFA Detention Pond The proposed management philosophy for the SFA and Willows Road extension is by way of a dry extended detention pond, where runoff will be managed and discharged to the receiving environment at predevelopment flow rates for the 2- and 10-year storm events. The pond is designed to release the extended detention volume over a 24-hour period and the water quality volume will remain in the pond as semi-permanent storage and will drain via soakage. Water quality treatment will be provided by swales up-stream of the pond as detailed in Section 4.3 with additional treatment provided through extended detention. The pond profile will be optimised at detailed design to achieve the desirable flow path for runoff. The pond will be designed to attenuate the post-developed flow rate for the 10-year, 24-hour storm, with discharge occurring via the primary outlet. Events greater than this storm will discharge via the emergency spillway as outlined in Section 4.5.4. A cross-section of the proposed pond is shown in Figure 4. The SCS method has been used to determine pre- and post-development flow rates for the SFA. Figure 5: Typical Section of Pond 2 Outlet 4.5.3 SFA Detention Pond Service Spillway A service outlet (manhole riser with scruffy dome and orifice) will be installed at the SFA detention pond to discharge the extended detention volume, and 2 and 10-year pre-development flows. Events less than a 2year storm will discharge via an orifice at the extended detention level, while events greater than the 2-year storm will overtop the manhole and flow to the primary outlet pipe. 4.5.4 Emergency Spillways Emergency spillways will be provided for both ponds with at least 300mm of freeboard (TR2020/07, Section 8.5.6.5). Due to the location of the ponds and topographical constraints, these spillways are likely to be on fill material. In these cases, armouring will be provided to prevent scouring of the embankment. From the spillway, discharge will follow the overland flow path to the Mataura Stream.
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