| Proposed Development – Surface Water Management | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 7 Sensitivity: General Table 4. Proposed channel drain to be installed in Catchment 4. Hydraulics 10-year 100-year Flow Depth (mm) 295 356 Flow (L/s) 1940 3440 Velocity (m/s) 2.3 3.2 Channel Width with 300mm freeboard 5.2m 5.5m 4.3 Swales Swales are proposed to convey and provide treatment to runoff from developed areas. Swales at developed areas such as the SFA and private vehicle car park are designed to provide a treatment residence time of nine minutes or longer (in accordance with WRC TR2020/07), and flow velocity less than 0.8m/s for the water quality event. For storms up to a 10-year event, swale velocities will seek to be less than 1.5m/s to prevent scour where practical. The SFA detention pond will provide additional treatment for the runoff from the SFA which is expected to be the area of site with a higher proportion of contaminants (excluding the rock stack). Elements of the SFA which are expected to be higher generators of contaminants will be assessed to provide additional and specific treatment methods where required i.e. oily water separators at workshops, wheel wash catchpits. Shown in Figure 3, the swale sizing adopts a typical profile of a trapezoid with 1V:4H side slopes, 1m wide base, and desirable longitudinal slope of 1.5-3% where practical. Where topography constrains desirable slopes, erosion protection will be installed. The depth of the swale is dependant of the expected 10-year flow and is noted in the detailed sub-catchment calculations, refer Appendix D. The swale along the eastern border of the SFA is to be dependent on the 100-year flow so that conveyance to the detention pond is provided. Figure 3: Typical Swale Profile 4.4 Culverts Culverts will be provided where proposed infrastructure conflicts with overland flow paths. Using the most conservative method between the Rational Method or SCS method, culverts will typically be sized to accommodate the 20-yr storm flow. However, key culverts (see Table 5) will be sized to the 100-year return event, where build-up of headwater and overland flow would have detrimental effects, such as inundation of the SFA, access roads and adjacent property. Where required, erosion protections will be installed. A typical inlet and outlet detail for culverts is shown in Figure 1. Culverts will either be concrete pipe or box culverts, subject to detailed design. Key culverts are listed in Table 5. Culverts relating to the WRS have not been included under this stormwater management plan.
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