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Hillingdon SuDS Design & Evaluation Guide

Concept

Concept Design 7.4.8 Method of discharge – how rainfall leaves the site Rainfall should not discharge into the foul sewer. The way that rainfall leaves a development should follow the preferred hierarchy: 1. 2. 3. re-use on site infiltration into the ground a natural watercourse 4. surface water sewer 5. combined sewer. The final swale at Bewdley School is a colourful outfall into the existing watercourse. 39 London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates

7.4.9 Preliminary flow and volume calculations It is convenient to consider flow and volume comparison of storage values between similar requirements at this stage in the design sites. process to ensure that natural losses are Expressing storage as ‘volume per m2’ allows replicated and sufficient volumes of runoff the designer to allocate storage throughout a can be temporarily accommodated to allow site in discrete sub-catchments, and provides for discharge from site via a flow control a straightforward way for the evaluation team and/or infiltration. to check that calculated storage volumes are In some circumstances, for example where acceptable. development is speculative, it may be Ideally each sub-catchment will manage its acceptable for the Concept Stage to omit own runoff up to the 1 in 100 year return flow and volume calculations, but a Modified period rainfall event. Where this is not viable, Flow Route analysis will be required to show part of the storage volume will be provided that runoff can be effectively conveyed to a depending upon the opportunities for discharge location. storage within the subcatchment, with all Storage volumes are usually presented as a residual flows cascaded into an adjacent single volume. sub-catchment or ‘site control’. This form of expression encourages the ‘pipe This approach maximises the opportunity for to pond’ practice and prevents simple storage throughout the development. Concept Design Flow control with 40 C3 C1 1 in 2 C2 1 in 10 controlled discharge from one catchment to the next Residual flows 1 in 30 C4 1 in 100 yr (+CCA) + residual flows from C1, C2 & C3 upto 1 in 100 yr (+CCA) Each catchment may only control and attenuate runoff up to lesser rainfall events (eg. 1 in 2 years, 1 in 10 year, 1 in 30 years) with residual flows passing into the next subcatchment. In this example the first three catchments (C1, C2 & C3) only partially attenuate their own runoff, with residual flows passing into catchment C4 where these residual flows must be attenuated, along with C4’s own runoff, to the maixmum design storm (eg. 1 in 100 + CCA). London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates

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