SuDS Design Guidance for Hertfordshire March 2015 V2. – Publication date 1st April 2015 Contents 1. Introduction ............................................................................................... 1 1.1 SuDS design guidance for Hertfordshire ............................................. 2 1.2 Guidance structure ............................................................................. 2 2. Context ..................................................................................................... 4 2.1 Legislation & policy ............................................................................. 4 2.1.1 The Flood & Water Management Act ........................................... 4 2.1.2 National Planning Policy Framework4 2.1.3 Local Policy .................................................................................. 4 2.2 Sustainable Drainage Systems ........................................................... 6 2.3 SuDS design process ......................................................................... 8 2.3.1 SuDS Masterplanning .................................................................. 9 2.3.2 SuDS concept .............................................................................. 9 2.3.3 Outline SuDS design .................................................................. 11 2.3.4 Detailed SuDS design ................................................................ 11 3. Technical & Spatial Framework .............................................................. 12 3.1 The Hertfordshire environment ......................................................... 12 3.1.1 Geology & soils .......................................................................... 12 3.1.2 Hydrology ................................................................................... 13 3.1.3 Landscape .................................................................................. 15 3.1.4 Land contamination & instability ................................................. 16 3.1.5 Biodiversity ................................................................................. 16 3.1.6 Historic environment ................................................................... 17 3.2 Quantity and quality criteria .............................................................. 18 3.2.1 Runoff destination ...................................................................... 18 3.2.2 Runoff rates ................................................................................ 19 3.2.3 Storage volumes ........................................................................ 19 3.2.4 Flood risk .................................................................................... 20 3.3 Site constraints & opportunities ........................................................ 21 3.3.1 Topography ................................................................................ 21 3.3.2 Land availability .......................................................................... 22 3.3.3 Land remediation........................................................................ 23 3.3.4 Groundwater .............................................................................. 23 3.3.5 Ground permeability ................................................................... 24 i 3.3.6 Flood risk areas .......................................................................... 25 4. Local Design Principles........................................................................... 26 4.1 SuDS network ................................................................................... 26 4.1.1 The management train ............................................................... 26 4.2 Water quality ..................................................................................... 27 4.2.1 The treatment train ..................................................................... 27 4.2.2 Treatment principles ................................................................... 29 4.3 Local distinctiveness ......................................................................... 29 4.3.1 Detailed design & materials ........................................................ 30 4.4 Biodiversity ....................................................................................... 30 4.5 Happy & healthy communities .......................................................... 31 4.5.1 Multifunctional spaces ................................................................ 31 4.5.2 Natural security .......................................................................... 32 5. SuDS Features ....................................................................................... 34 6. DELIVERY .............................................................................................. 38 6.1 Construction ...................................................................................... 38 6.1.1 Construction method .................................................................. 38 6.1.2 Phasing ...................................................................................... 38 6.1.3 Affordability ................................................................................ 38 6.2 Operation & maintenance ................................................................. 38 6.2.1 Management plan....................................................................... 39 6.2.2 Safe access ................................................................................ 39 6.2.3 Waste management ................................................................... 40 7. APPENDIX .............................................................................................. 41 7.1 Signposts .......................................................................................... 41 7.2 Quantity & quality criteria .................................................................. 44 7.2.1 Peak flow & runoff rates ............................................................. 44 7.2.2 Greenfield runoff rates ................................................................ 44 7.2.3 Previously developed land runoff rates ...................................... 46 7.2.4 Flow controls .............................................................................. 46 7.2.5 Storage volumes ........................................................................ 47 7.2.6 Quantity data template ............................................................... 48 ii 7.3 SuDS features guidance sheets ....................................................... 54 7.3.1 Green roofs ................................................................................ 55 7.3.2 Permeable pavements ............................................................... 57 7.3.3 Rainwater harvesting .................................................................. 59 7.3.4 Ponds & wetlands....................................................................... 61 7.3.5 Retention & infiltration basins ..................................................... 64 7.3.6 Bio-retention ............................................................................... 67 7.3.7 Swales & filter strips ................................................................... 69 7.3.8 Filter drains ................................................................................ 74 7.4 Sample maintenance schedule ......................................................... 76 7.5 Glossary ............................................................................................ 80 This Guidance was originally produced by Hertfordshire County Council in April 2014. In March 2015, it was updated to reflect changes in the planning system regarding surface water management. iii iv 1. Introduction The effects of climate change, population growth and urbanisation are placing significant pressure on the effective management of surface water. Changing patterns of rainfall and a growing area of impermeable development are giving rise to high rates and volumes of surface water runoff. Conventional drainage systems constructed from constrained networks of underground pipes and storage tanks are struggling to cope with overwhelming quantities of surface water runoff, resulting in problems such as flooding and pollution. Sustainable Drainage Systems (SuDS) manage surface water based on an understanding of the ‘hydrologic cycle’ – the movement of water within the natural environment. They are an environmentally-friendly and cost-effective approach to drainage that avoid the problems associated with conventional drainage systems by managing water at the surface, replicating the flow routes and stopping places that water finds in the natural landscape. This integrated approach to SuDS and the landscape is referred to as the SuDS philosophy, and provides an opportunity for the delivery of multiple environmental, economic and social benefits. The SuDS philosophy is to replicate, as closely as possible, the natural drainage from a site before development.’1 This should be achieved in line with the delivery of three objectives, Quantity, Quality and Amenity & Biodiversity. Each objective should be considered in equal measure, however their delivery will vary according to the constraints and opportunities presented by a site and the local community. Southern Country Park, St Michaels Meadow 1 CIRIA SuDS Manual (C697) 1 1.1 SuDS design guidance for Hertfordshire This guidance is for developers involved in the design and development