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Hillingdon SuDS Design & Evaluation Guide

Detailed

Detailed Design 91 9.5.10 Calculation checklist Key calculation inputs and outputs should be presented in the ‘Flows and Volumes checklist’ (see appendix). The following checklist identifies useful calculation checks: Parameter Guidance on design/calculation input Rainfall FEH 2013 rainfall data preferred. Where FSR rainfall data. data is used, conversion factors should be applied to bring in line with FEH rainfall data. Areas All area of contributing runoff should be represented generating within the storage calculation. runoff The designer must justify where a Cv of less than 0.9 for impermeable area is used for calculations. Maximum Statutory authorities e.g. LLFA, sewerage undertaker, flow control IDB or EA, might place restrictions on the outfall flow rate rates based on the available capacity of receiving infrastructure. Climate CCA has been applied within calculations based on change design life of development and any applied sensitivity allowance assessment. Urban creep Urban creep allowance applied to non-adoptable impermeable areas on developments where permitted development is likely to occur. Information for technical assessment Confirm the rainfall source and any conversions applied to data. Provide a drawing clearly identifying the areas of surface runoff contribution within each subcatchment. Designer to state Cvs used and justify use of Cv less than 0.9. The flow control rate should be identified along with the method for defining the rate. Designer to justify selection of CCA based on development type and design life. Designer to justify selection of Urban Creep percentage Initial interception losses Critical duration Control of runoff volume As a rule of thumb, where the area of development is no greater than 4 times the SuDS wetted area, a 5mm allowance may be made for interception losses for each m 2 of development. A range of rainfall durations must be considered when calculating attenuation storage. Where the designer demonstrates that water can be ‘lost’ or stored separately Approach 1 can be applied for the control of flow being discharge from the site. Designer to confirm whether 5mm interception losses have been applied in calculation. Designer to demonstrate that sufficient rainfall durations have been considered to achieve worst case scenario. Designer to confirm how volume of runoff has been controlled. London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates

Parameter Modelling of the SuDS layout. Outfall Guidance on design/calculation input It is not anticipated that SuDS design will require modelling of extensive piped systems. In some instances where the scheme is relatively small and not hydraulically complex standard calculations will be accepted in lieu of a hydraulic model. Layout drawings should be clearly labelled with the numbering convention used by models. Outfalls into receiving sewers or watercourses can be Information for technical assessment The designer is to justify where no hydraulic modelling is undertaken. Calculations/model outputs should be provided to support the Flows and Volumes proforma Designer is to indicate whether Detailed Design design at risk of surcharge and lack of free discharge due to SuDS storage calculation is elevated water levels. This can result in additional likely to be influenced by high storage being required. Free discharge should not be water levels at the point of assumed. The risk of surcharge should be assessed discharge. and accounted for within calculations as appropriate. Long Long sections will allow detailed consideration of Long section showing peak section levels across the site. water levels. Erosion Flows along swales (or other vegetated surfaces) are Designer to demonstrate that check at risk from erosion. Peak flow velocities should be they have considered risk of less than 1 - 2 l/s. erosion and taken measures to Concentrated inlet points are also prone to erosion. safeguard scheme. Peak flow velocity calculations to be provided as appropriate. Designing for exceedance The design should incorporate overflows at each SuDS component. Hydraulic calculations should demonstrate that overflows have sufficient capacity to deal with anticipated flow rates. SuDS layout drawing Locations of overflows should be identified on the layout drawing along with proposed exceedance flow route. 92 should identify the anticipated flow route for exceedance events. Managing The FRA should identify the potential for flows from The designer should flows from offsite. These flows can be unpredictable and difficult demonstrate how anticipated off site. to quantify. Management of flows through the site should not increase flood risk elsewhere. flows from off site will be managed through the site using Detailed modelling to establish the rates of flow anticipated would not be considered compulsory (but the layout drawing and design statement. may be required on a case by case basis). Consistency Detailed design of SuDS components should reflect Drawings should clearly identify of hydraulic calculations / hydraulic models, taking into site levels, storage locations and calculations and design. account slopes and low lying levels. The LLFA will consider design drawings to ensure that flow control sizing and storage provision is as per calculations. flow controls with cross sections and long sections. The design statement should confirm that drawings deliver calculated volumes. London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates

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