Hillingdon SuDS Design & Evaluation Guide

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Concept Design 45 7.4.13 Flow rate calculations The aim of controlling flow from a development, whether it has been previously developed or not, is to restrict outflow rates to pre-existing ‘greenfield runoff rates’. There are two approaches to controlling outflow rates: Approach 1, as set out in the NSTS (non-statutory technical standards) requiring additional volume management, and Approach 2, the current practice commonly called the Qbar method. Approach 1 – (NSTS S2 and S4), where the volume of runoff is managed to Greenfield volume, the allowable discharge rate is permitted to vary between the 1 in 1 year and 1 in 100 year Greenfield runoff rates for the respective rainfall return periods. Approach 2 – (NSTS S6), where additional runoff volumes cannot be managed on site, runoff rates must be further restricted to ensure that there is no increase in flood risk elsewhere. The general approach that is adopted is to limit the maximum outflow rate to Qbar (approximately equivalent to 1 in 2 year greenfield rate) for all rainfall return periods up to the 1 in 100 year rainfall event depending on the local soil type. Approach 2 is simpler but usually results in larger storage volumes than Approach 1. An allowance for climate change, and in certain situations urban creep, should be included in hydraulic calculations. An online tool for estimating Greenfield runoff rates can be found at www.uksuds. com or calculated using the methodology in the SuDS Manual 2015. The uksuds.com calculator is based on regional geological mapping which can be unrepresentative of actual site conditions. Inputs to the Greenfield runoff calculation should rely upon actual soil types for the site rather than regional geological maps. In Approach 1 the ‘greenfield runoff rate’ will increase with increasing storm return periods. The flow control mechanism will need to account for this increase in flow rate. In Approach 2 the Qbar value for a site will only be achieved for the site or subcatchment when the storage feature is full. Most of the time the flow rate is less until a full storage head is generated. See Climate Change Allowance (CCA) Section 9.5.4.6 and Urban Creep Section 9.5.4.7 London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
approach 1 approach 2 inflow rainfall x area inflow rainfall x area Concept Design interception losses interception losses attenuation storage other long term losses attenuation storage variable outflow from 1in1 to 1in100yr greenfield runoff rates 2L /sec /ha outflow for 1in100 yr rainfall event limited to 2yr greenfield runoff rate 46 Approach 1 and Approach 2 - Discharge Requirements 1 in 1 year rainfall (maximum outflow rate) 1 in 100 year rainfall (maximum outflow rate) Long term storagevolume control Approach 1 1 in 1 year 1 in 100 year Yes greenfield rate greenfield rate Approach 2 Qbar/ Qmed Qbar/ Qmed No London Borough of Hillingdon SuDS D & E Guide © 2018 McCloy Consulting & Robert Bray Associates
Page 1 and 2: Sustainable Drainage Design & Evalu
Page 3 and 4: Contents 1.0 Introduction 2.0 Under
Page 5 and 6: This development guide has the supp
Page 7 and 8: 1.2 SuDS today There have been a nu
Page 9 and 10: 2.0 Understanding Rainfall It is im
Page 11 and 12: Where the ground is impermeable, wa
Page 14 and 15: Overview 3.0 The Impact of Developm
Page 16 and 17: Overview 3.3 Consequences of piped
Page 18 and 19: Overview 4.0 The Role of SuDS Susta
Page 20 and 21: Overview SuDS schemes offer diverse
Page 22 and 23: Overview 5.0 The SuDS Design & Eval
Page 24 and 25: Overview 5.3 The objectives of the
Page 26 and 27: Local SuDS requirements 21 6.0 Land
Page 28: Local SuDS requirements 23 Local Su
Page 31 and 32: Concept Design for Holyoakes School
Page 33 and 34: 7.4 Concept Design process 7.4.1 Fl
Page 35 and 36: Modifi Step 2 - Modified Flow Route
Page 37 and 38: 7.4.3 Collection of runoff from har
Page 39 and 40: 7.4.5 Conveyance of runoff between
Page 41 and 42: A flow control generally defines th
Page 43 and 44: Additional considerations for infil
Page 45 and 46: 7.4.9 Preliminary flow and volume c
Page 47 and 48: 7.4.11 Managing runoff from site If
Page 52 and 53: Concept Design Long Term Storage Su
Page 54 and 55: Concept Design 7.5 Concept informat
Page 56 and 57: Concept Design 51 7.5.3 Preliminary
Page 58: Concept Design 7.5.6 Management and
Page 61 and 62: 5. Quality Confirm ‘treatment sta
Page 63 and 64: 8.2 Objectives of SuDS Outline Desi
Page 65 and 66: 8.4 Design criteria considerations
Page 67 and 68: 8.5.2 Aspects of Health and Safety
Page 69 and 70: 8.8 Outline information required fo
Page 72 and 73: Detailed Design 9.0 Design and Eval
Page 74 and 75: Detailed Design 9.3 Typical Detaile
Page 76 and 77: Detailed Design 71 Plan excerpt: pr
Page 78 and 79: Detailed Design 73 9.3.4 Detailed D
Page 81 and 82: 9.4 Critical levels Levels are impo
Page 83 and 84: 9.5.3 Calculation processes Calcula
Page 85 and 86: 9.5.4.3 Making allowances for inter
Page 87 and 88: 9.5.4.6 Accounting for Climate Chan
Page 89 and 90: 9.5.5 Calculating storage requireme
Page 91 and 92: Approach 2 Where volumes cannot be
Page 93 and 94: Detailed Design 88 Designers should
Page 95 and 96: Detailed Design Lamb Drove, Cambour
Page 97 and 98: Parameter Modelling of the SuDS lay
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9.6.1 Design flexibility A framewor
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9.6.4 Single, tiered storage compon
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9.6.6 The importance of protected o
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the 1 in 100 flow rate - for exampl
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9.7 Water quality Rainfall picks up
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For low to medium risk sites, the i
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9.7.5 Water quality design checklis
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9.8.2 Accessibility All parts of th
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Firstly the collection and conveyan
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9.9.3 Key design criteria for biodi
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9.10.2 The Principles of SuDS plant
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Amenity grass An everyday grass sur
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Meadow vegetation Meadow vegetation
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Bioretention features are defined b
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9.10.3.4 A place for trees and shru
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Source Controls providing collectio
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Raingardens The raingarden concept
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Permeable surfaces Permeable surfac
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Filter drains Filter drains, someti
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Filter strips The hard edge from a
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Detailed Design 135 The safety cons
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Detailed Design 9.11.5 Storage stru
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Detailed Design 9.12 Management of
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Detailed Design 9.12.3 Example of S
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Sheffield Grey to Green : an excell
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Hillingdon SuDS Design & Evaluation Guide

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