| Proposed Development – Surface Water Management | Stormwater Management Plan | 2210835-1190907474-109 | 21/06/2022 | 6 Sensitivity: General 4 Proposed Development – Surface Water Management 4.1 Summary This section outlines the proposed stormwater management works in line with Waikato Regional Council guidance. Calculations/concept design sizing is provided for main stormwater management features i.e, SFA detention pond sizing and service outlets. The clean water (from undeveloped sub-catchments) is diverted around developed areas and discharged to their natural receiving environment, required minimum sizing has been calculated for formalised overland flow paths, however the natural topography will be used where possible. This will be developed further in the detailed design stage. 4.2 Clean Water [1] Management 4.2.1 Clean Water Diversions Contour Drains Upstream of proposed developed areas, diversion drains in the form of earthen bunds and swale channels will be installed to divert clean runoff away from downstream developed areas. Clean water, which will flow toward developed areas, typically over steep topography, will be slowed prior to reaching the diversion such that erosion is mitigated and velocities within the drains are managed. Diversions are designed to accommodate the 10-year storm flow with 300 freeboard. The 100-yr flow can also be conveyed by the diversion however with reduced freeboard which may result in localised overtopping in isolated locations. Due to the topography in which diversions will be installed, side wall and longitudinal slopes vary and will not be maintained by conventional mowing if grassed. Where necessary erosion protections will be installed. Table 3. Clean water diversion drains to be installed at the SFA. Drain ID Area (m2) Length (m) Approx. Slope (%) Side Slope Design Storm (year) Curve Number Width (m) Bund height/ channel depth CW-C3A 3130 86 5% 1in2 10 65 2 0.50 CW-C3B 4425 100 5% 1in2 10 65 2.1 0.52 CW-C3C 1890 105 5% 1in3 10 80 2.8 0.46 CW-C3D 2080 115 5% 1in3 10 80 2.8 0.47 CW-C6 21250 220 10% 1in2 10 70 2.42 0.57 4.2.2 Overland Flow Path Channelisation An overland flow path will be formalised in Catchment 4, south of the proposed SFA, where currently water flows from a steep gully and across flatter farmland. A channel will be formed across the flatter portion of the catchment with new culverts installed beneath the private vehicle path and Willows Road and will convey the 100-yr flow with 300mm freeboard. Intervention may be required at the base of the gully to reduce the velocity of water entering the upstream end of the proposed channel. It is proposed that the channel will have a slope of approximately 4-7%, side slopes of 1V:2H, and erosion protection to be installed as required. At the further design stage, the design will consider flow velocity control measures to reduce the risk of erosion. 1 Clean water refers to surface water runoff from undeveloped areas located upstream of developed areas.
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