Designing for Exceedance in Urban Drainage: Good Practice

Table of Contents:
1 Introduction to the guidance; 1.1 Aims and objectives of the guidance; 1.2 Limitations of this guidance; 1.3 Structure of the guide; 1.4 Sources of information; 1.5 Associated publications; 1.6 Background to drainage exceedance; PART A OVERVIEW; 2 The process of exceedance and definitions; 3. Stakeholder roles and drainage performance; 3.1 Drainage stakeholders; 3.2 Managing extreme events in existing urban areas; 3.3 The role of the planner and developer in new developments; 3.4 Drainage design and performance standards; 3.5 Key stakeholder lessons; 4. Effective management of exceedance 25; 4.1 Identifying above ground flood pathways 25; 4.2 The capacity of surface pathways; 4.3 Providing surface storage; 4.4 The effect of building layout; 4.5 Impact on downstream systems; 4.6 Post event clean up; PART B. DETAILED DESIGN; 5 Managing stakeholder interaction; 5.1 The planning process; 5.2 Stakeholder responsibilities; 5.2.1 Local authorities; 5.2.2 Sewerage undertakers; 5.2.3 Environmental regulators; 5.3 Stakeholder consultation process; 5.3.1 Initial stakeholder consultation phase; 5.3.2 Stakeholder consultation phase; 5.4 Good practice in stakeholder interaction; 5.4.1 Glasgow East urban flooding; 5.4.2 Yorkshire property flooding solutions; 5.4.3 Flooding of residential area in Birmingham; 5.5 Ownership and legal rights; 5.6 Education - the public as stakeholders; 5.7 Flood warning; 5.8 Stakeholder collaboration; 6 Runoff from natural catchments; 6.1 Introduction; 6.2 Natural drainage processes; 6.3 Rainfall; 6.3.1 Spatial rainfall; 6.3.2 Seasonal rainfall; 6.4 Rural runoff; 6.4.1 Characteristics of rainfall and rural runoff; 6.5 Models for estimating rural runoff; 7 Hydrological processes and the effects of urbanisation; 7.1 Hydrological processes; 7.1.1 Introduction; 7.1.2 Interception; 7.1.3 Depression storage; 7.1.4 Infiltration; 7.1.5 Surface flow; 7.1.6 Evaporation and evapo-transpiration; 7.2 Runoff; 7.3 Stream network and channel morphology; 7.4 Floods in natural catchments; 7.5 The effects of urbanisation; 7.6 Urban runoff behaviour; 7.7 Urbanisation and Flooding; 8 Runoff from urban catchments; 8.1 Urban Runoff models; 8.1.2 The constant (Old UK) runoff model; 8.1.2 The variable (New UK) runoff model; 8.1.3 The fixed percentage runoff model; 8.2 Estimation of the difference between greenfield and development runoff; 9 Interaction between major and minor systems; 9.1 Principles of interaction; 9.1.1 Flooding from manholes and other drainage connections; 9.1.2 Limitation of inlet capacity; 9.1.3 Surface run-off from pervious area; 9.2 Calculating exceedance flow; 9.3 Calculating flows in surface flood pathways; 9.3.1 Surface run-off; 9.3.2 Adding run-off from permeable areas; 9.3.3 Surface conveyance; 9.4 Calculating drainage inlet capacity and exceedance; 9.4.1 Highway gullies; 9.4.2 Roof drains; 9.4.3 Yards and other paved area drainage gullies; 9.4.4 Applying limiting inlet capacity to calculate exceedance flows; 9.5 Inlet capacity of SUDS systems; 10 Developing a risk assessment; 10.1 An introduction to Exceedance Flood Risk Assessment; 10.2 Components of the EFRA; 10.3 Determining the risk value. 10.3.1 EFRA Process; 10.3.2 Selection of the appropriate EFRA level; 10.3.3 Level 1 EFRA - Simple small areas 10.3.4 Level 2 EFRA - Large or complex areas; 10.3.5 Level 3 EFRA - Large and complex areas; 10.4 Assessing the probability; 10.5 Assessing the consequence; 10.5.1 Consequence hierarchy for building types or land use as a result of flooding; 10.5.2 Damage to property; 10.5.3 Damage due to depth; 10.5.4 Damage due to depth and velocity; 10.5.5 Health and Safety; 10.5.6 Loss of facility / business; 10.5.7 Emergency services; 10.5.8 Social implications; 10.6 Calculation of risk; 11 Designing for surface conveyance; 11.1 Principles of design; 11.2 Identifying flood pathways; 11.3 Designing flood channels; 11.3.1 Channel conveyance; 11.3.2 Velocity and depth of flow; 11.3.3 Cross section details; 11.4 Channel transitions; 11.4.1 General principles; 11.4.2 Transition between single channel reaches; 11.4.3 Road junctions; 11.4.4 Inlets; 11.4.5 Outlets; 12 Designing for surface storage; 12.1 Principles of design; 12.2 Storage area design process; 12.2.1 Size; 12.2.2 Health and safety; 12.2.3 Maintenance; 12.2.4 Outfall design; 12.2.5 Diversion control design; 12.3 Types of storage areas; 12.3.1 Storage options hierarchy; 12.3.2 Additional storage in SUDS; 12.3.3 Car parks; 12.3.4 Minor roads; 12.3.5 Playing fields, recreational areas and parkland; 13 Building layout and detail; 13.1 Design principles; 13.2 Building type and layout; 13.2.1 Layout and flood pathways; 13.2.2 Utilising existing features of the site; 13.3 Building detail; 13.3.1 Building in protection measures; 13.3.2 Property elevation / threshold levels; 13.3.3 Selection of the building materials; 13.3.4 Venting; 13.3.5 Entrance details; 13.3.6 Driveways and cartilage; 10.3.7 Siting of services; 13.3.8 Inadvertent modifications to existing flood pathways; 13.3.9 Under building flood paths; 14 Downstream impact assessment; 14.1 Conveyance and storage; 14.1.1 Flood conveyance impacts; 14.1.2 Conveyance with storage; 14.2 Procedure for assessing and mitigating impacts; 14.3 Assessing the impact on downstream systems; 14.4 Mitigating the effects of downstream impacts; 15 Case study 1: Bishopbriggs South; 15.1 Introduction; 15.2 Stakeholder Involvement; 15.3 Calculating exceedance flow; 15.3.1 Collecting data; 15.3.2 Using models to assess system performance; 15.3.3 Verifying against historic flooding; 15.3.4 Upgrading to a Level 3 study; 15.4 Exceedance risk assessment; 15.5 Solution development; 15.6 Impact on downstream systems; 16 Case Study 2: Upton - Northampton; 16.1 introduction; 16.2 Stakeholder Involvement; 16.3 Drainage of Developed Areas; 16.4 Interaction between the minor and major systems; 16.5 Risk Assessment; 16.5.1 Collecting data and building a hydraulic model; 16.5.2 Assessing system performance (1 in 30 year return period - 0.033 annual probability); 16.5.3 Assessing system performance (1 in 100 year return period - 0.01 annual probability); 16.5.4 Assessment of risk outside school; 16.6 Building layout and detail; 16.6.1 Amending building layout and threshold levels; 16.7 Impact on downstream system; 16.7 Conclusions; Appendix A: Modelling exceedance; A.1 Surface flood pathways; A.2 Surface flooding; A.3 Modelling inlet capacity; A.4 Modelling flood risk; A.5 Further guidance; Appendix B: Exceedance flow at highway gully inlets; Appendix C: Conveyance in surface flood pathways; C.1 Introduction; C.2 Flood pathway channels; Appendix D Assessment approach to determine flood volumes and rates from SUDS; D.1 Assessment approach; D.2 Hydrology; D.3 Pervious pavement performance; Results; Application of results and conclusions; D.4 Swale performance; Contributing area; Gradient of the swale; Length of the swale; Outflow control from the swale; Application of results and conclusions; D.5 Infiltration system performance; Application of results and conclusions; Appendix E Generic guidance on assessing flood volumes and rates from, SUDS; E.1 Assumptions, Inflow, Outflow, Storage; E.2 The principles of the flood estimation method; E.3 Method of application; E.4 Check against modelling results, Pervious pavement, Infiltration trench, Swale; Appendix F Design example of a permeable pavement; G.1 Rational method; G.2 The TRRL method (Young and Prudhoe1973), G.3 Flood Studies Report (FSR) - (NERC, 1975); G.4 FSSR 6 - Flood prediction for small catchments; G.5 Poots & Cochrane method, 1979; G.6 The ADAS method (Agricultural Development and Advisory Service, 1980); G.7 The SCS method (Soil Conservation Service, 1985 - 1993); G.8 Institute of Hydrology Report No. 124 (1994); G.9 Flood Estimation Handbook, FEH, (1999); G.10 Statistical procedures for flood estimation; G.11 Rainfall-runoff method for flood estimation; G.12 Advantages and disadvantages of the flood estimation handbook techniques; G.13 FSR and FSSR 5 and 16 percentage runoff estimation; G.14 FEH runoff model - variable percentage. runoff; References; British and international standards; UK Legislation and regulations; Legal rulings; Glossary; Abbreviations.