March 2015 – Issue Version 1 West of England Sustainable Drainage Developer Guide Section 2 Bristol Local Sustainable Drainage Design Guidance
At-Bristol Centre green roof retrofitted onto an existing flat concrete roof in 1999 Southmead Green Streets – concept visualisations © Ove Arup and Partners Ltd March 2015 – Issue Version 1 Contents
Other documents: West of England Guide – Section 1
West of England Guide – Section 2 Bristol Local Sustainable Drainage Design Guidance Variations and details of other consents 4 This guide has been prepared by Variations to Sub-regional Requirements 4 Bristol City Council and is supported Other consents 4 by the Environment Agency, the Lower Our vision 5 Severn Internal Drainage Board and Our vision for sustainable drainage 5 Wessex Water who have all been Area character 6 involved in its preparation. Technical Topography, hydrology and principal drainage features 6 assistance has been provided by Ove Surface Water Flood Risk 9 Geology and Infiltration Potential 10 Arup and Partners Ltd and design by Water Quality 12 Bristol City Council. Bristol Drainage Strategy Drivers 14 Information on flood risk is available at Sustainable Drainage Design Principles and Good Practice 21 www.bristol.gov.uk/page/environment/ Common Considerations 21 flooding-and-drainage General SuDS Design Reference List 21 Guidance Design Drainage Sustainable Local Bristol Minor Development Design Considerations 22 The document was endorsed by Bristol SuDS Features Design Principles and Good Practice 24 City Council in February 2015. SuDS ownership and maintenance 45 This is a living document and will kept Annex 47 under regular review. User feedback is Amenity 47 welcomed through: Biodiversity 49 [email protected] Heritage 53
West of England Guide – Section 2 Bristol Annex - Sustainable Drainage Planning Practice Guidance Note West of England Guide – Section 2 North Somerset Sustainable Drainage Design Guidance
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Page 3 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Variations and details of other consents
Bristol Local Sustainable Drainage Design Guidance Variations to Sub-regional Requirements Other Consents Each authority in the sub-region has The following individual variation from Other consents may be required under reviewed the appropriateness of the the sub-regional requirement for Bristol the Water Resources Act or Land non-statutory technical standards City Council is highlighted. Drainage Act for works in or near a for sustainable drainage systems in Electronic data formats watercourse. The granting of planning conjunction with the National Planning The Council’s preference is for electronic consent does not relieve the applicant of Policy Framework and Planning Practice rather than hardcopy submissions. the need to apply for any such consent. Guidance against their individual Local Electronic CAD drawings should be in For details of consents required contact Plans, Local Flood Risk Management AutoCAD compatible formats, and GIS the Environment Agency (work in or near Strategies and supporting evidence mapping in Arc GIS compatible formats. a main river), internal drainage board bases. (work in or near an ordinary watercourse in a drainage board area) or LLFA (work in or near an ordinary watercourse outside a drainage board area). For No. Non-statutory technical Local Variation further information see: standards for SuDs in to Sub-regional www.bristol.gov.uk/page/environment/ conjunction with NPPF and PPG Requirement carrying-out-work-watercourse S7 The drainage system must be As stated. designed so that, unless an area is designated to hold and/ March 2015 – Issue Version 1 or convey water as part of the West of England Sustainable Drainage Developer Guide Section 1 design, flooding does not occur on any part of the development for a 1 in 30 year rainfall event. Bristol City Council Section two Page 4
March 2015 – Issue Version 1 Our Vision
Our vision for sustainable drainage We consider sustainable drainage (SuDS) to be more than just a number of drainage techniques, systems or devices. Rather it is an approach to drainage that aims to drain a site in a sustainable way with consideration to water quantity and water quality, biodiversity and amenity. Our vision can be summarised through seven guiding principles:
Blue green corridors should be included within Cumulative impacts, both positive and negative, sites to provide physical links and multiple benefits. should be considered. Benefits should be provided Consider the movement of water and its interaction no matter how small so that over time, the with space at the earliest stage of design for cumulative impact becomes significant. efficient sustainable drainage. Identifying and enhancing drainage paths are an essential part of the master-planning stage. Water quality mitigation and improvement are important considerations. Bristol intends to improve Source control – managing runoff at source is the the quality of its watercourses. Discharge from all starting point for SuDS design development sites should include water quality mitigation measures. Bristol Local Sustainable Drainage Design Guidance Design Drainage Sustainable Local Bristol Drainage and urban design should be integrated in site development. Sustainable drainage should Innovation is encouraged. While certain design be integrated into urban realm design. Sustainable standards must be met, innovative approaches to drainage provides an opportunity to create great managing surface water will be encouraged and places to live and work whilst managing water considered on their merits. better.
No space is useless – Sustainable drainage is Manage all risks on site through a proportionate flexible. All areas of the site should be considered risk based approach. for inclusion of surface water management measures - roofs, landscaped areas and public
open space all provide opportunities. two
Page 5 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Area character
Bristol Local Sustainable Drainage Design Guidance Topography, hydrology and principal drainage features Bristol’s city centre is a low lying area Keynsham Lock. High tides often exceed located at the confluence of the Rivers Netham weir in the east of Bristol. Avon and Frome within the tidal extent The River Frome rises on the west side of of the River Avon, originally tidal marsh. the Cotswold scarp slope near Chipping The city now extends north and east Sodbury to the north east and drains onto the adjacent hills rising to typically a predominantly clay and mudstone 90m AOD, and to the south to Dundry catchment, but also includes significant Hill at approximately 200m AOD. The urban areas at Yate, Chipping Sodbury River Avon flows from the central area and the urban expansion of Bristol, towards the Severn Estuary 10km to the particularly Bradley Stoke and Emersons north-west through the Avon Gorge. Green. The catchment area of the River The catchment of the River Avon, Frome to central Bristol is approximately sometimes called the Bristol Avon, is 175km2 and the 1% AEP (1 in 100yr) fluvial some 2300km2. The main geological flows are estimated at approximately features of the catchment are the 70m3/sec at Eastville where the river limestone Mendip Hills, the oolitic enters the urban area2. limestone Cotswolds and the chalk The Floating Harbour is a man-made downs in the east, all of which are major impounded water body stretching from aquifers affecting the hydrology of Netham weir in the east of the city to the catchment. Impermeable clays lie Cumberland Basin in the western central between the west-sloping strata of the area. It was constructed in the 19th limestone and the chalk, while sandstone century by diverting the tidal River Avon and mudstone are exposed in the west of through a man-made tidal canal – the the catchment. The 1 in 100 (1%) annual River Avon New-Cut. The water level chance fluvial flows are estimated at within the harbour is maintained at a 3 1 approximately 540m /s in central Bristol . constant level of 6.2m above ordnance Bristol City Council The flow regime of the River Avon is
Section datum. The Floating Harbour main tidally influenced as far upstream as
1 CAFRA workstream 3 Appendix E
two 2 CAFRA workstream 3 Appendix E Page 6
March 2015 – Issue Version 1 inflow is from the River Avon at Netham Lock, the River Frome discharges to the harbour at St Augustine’s Parade. The flow into the harbour is balanced by culverts and sluices at Underfall Yard which discharge into the River Avon New Cut. The Junction Lock stop gates and Netham lock gates are closed for every tide forecast to reach or exceed the normal harbour operational level. Within the urban area, major drainage works have been undertaken in past years, most notably the Northern Storm Surface water flooding at Whiteladies Road, Water Interceptor (NSWI) from Eastville November 2012 to Black Rocks in the Avon Gorge, and the Malago Interceptor Tunnel in the
southern parts of the city. These schemes Guidance Design Drainage Sustainable Local Bristol are designed to collect and divert flows direct to the tidal section of the River Avon that would otherwise pass through low lying areas in the centre of the city. At present, the Malago Interceptor Tunnel and associated collecting tunnel system is Surcharged surface water sewer in Withywood, considered to have adequate capacity to November 2012 receive direct discharge of the upstream catchments. Ashton, Malago and Brislington Brooks are small tributaries flowing into the River Avon from the south, draining heavily urbanised catchments approximately 17km2, 16km2 and 11km2 respectively.
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Page 7 Section Section Bristol City Council Council City Bristol Map Showing Main & Non-Main Rivers In BMarchristol 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Ashton Brook is culverted in its lower reaches and discharges through flapped outlets into the tidal River Avon. The ¯ Malago has a storm water interceptor
diverting flows from the upper catchment Avonmouth Rhines through culverts into the River Avon. The ‘Airport Road tunnel’ links the upper Brislington Brook catchment to the River Trym Malago interceptor and conveys further flow into the Malago interceptor system. The principal watercourses and drainage features in Bristol are shown in figure 3. River Avon River Frome Watercourses are named on: www.bristol.gov.uk/sites/default/files/as- sets/documents/big-blue-map-of-bristol. pdf
References Ashton and Longmoor Brooks 3 Feeder Canal Bristol Avon CFMP (2012) , Bristol Frome River Avon The Malago FMS (2005)
Colliter's Brook Brislington Brook
Airport Road Tunnel
Pigeonhouse Stream
Surface water high risk areas Tunnels Dundry Springs Floating Harbour Main Rivers 0 0.5 1 1.5 2 Non-Main Rivers Kilometers
Bristol City Council Ordnance Survey data © Crown copyright and database right 2014
Section Figure 3: Plan showing main rivers and principal watercourses in Bristol (source: Bristol City Council), and Surface Water High Risk Areas (area identified as being at high risk to surface water flooding by the Surface Water Management Plan).
3 www.gov.uk/government/uploads/system/uploads/attachment_data/file/294182/Bristol_Avon_Catchment_Flood_Management_Plan.pdf two Page 8
March 2015 – Issue Version 1 Surface Water Flood Risk The predominant flood risk in the A Surface Water Management Plan Relevant Local Plan Policies: city central is from the tidal River (SWMP) has been produced to provide LL ¶ Core Strategy Policy BCS13 (Climate Avon. Rising sea levels expected due an assessment of potential surface water Change) to the effects of climate change will flooding that could arise from very LL ¶ Core Strategy Policy BCS16 (Flood Risk significantly increase this risk. heavy rainfall in the city. It identified the and Water Management) There is also a risk of flooding from a problem of drainage systems transferring combination of tide and fluvial flows in additional runoff into the Ashton Gate either the River Avon or the River Bristol area which exacerbates its surface water Frome. The River Avon’s large catchment flooding problems. Other high risk areas size delays peak flows from being were also identified in other areas of the experienced in Bristol until approximately city. The SWMP hazard maps can be 35 hours after the storm event, although viewed GOV.UK4. the track of the storm can affect this. In References/Links: the smaller River Frome catchment, peak In Bristol, surface water high risk areas flows are generally experienced some 12 (areas identified as being at high risk to hours after the storm event. surface water flooding by the Surface Guidance Design Drainage Sustainable Local Bristol The city is also susceptible to flooding Water Management Plan are available on from severe rainstorm events such as our mapping portal: http://maps.bristol. that experienced in July 1968 when gov.uk/pinpoint/?service=localinfo&lay- many catchments within the urban er=Surface+water+high+risk+areas area were severely affected. This risk of www.bristol.gov.uk/page/planning-and- surface water flooding within the urban building-regulations/flood-risk-evidence- conurbation is likely to increase as climate planning-policy – Strategic Flood Risk change increases storm intensity and Assessment, Surface Water Management frequency in the future. As described Plan above, major interceptor tunnels were constructed in the 1970s to help to www.bristol.gov.uk/page/environment/ reduce the risks from these medium sized local-flood-risk-strategy-investigations- catchments to the lower central areas and-assessment – Local Flood Risk both north and south of the River Avon. Management Strategy, Preliminary Flood Risk Assessment
Surface water flooding in November 2012 in Southmead two 4 watermaps.environment-agency.gov.uk/wiyby/wiyby.aspx?lang=_e&topic=ufmfsw&layer=0&x=360500&y=175500&scale=10&location=Bristol%2c+City+of+Bristol#x- =360503&y=175515&scale=7 Page 9 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Geology and Infiltration Potential The British Geological Survey provide fractured. However, the practicalities detailed published records of the Bristol of excavating through this material area5. should be considered in parallel with These indicate that the solid and drift the drainage design. If the material is materials are very variable within our locally less fractured, the permeability will area. Broadly, the geology comprises decrease accordingly. Carboniferous, Triassic and Jurassic Some low permeability materials contain sedimentary materials, in filled with limestone and sandstone bands which recent Quaternary deposits, particularly could provide limited soakaway capacity around the city centre and out towards if they are present on any particular the Severn Estuary. site. Similarly, the Triassic sandstone Within the SuDS discharge hierarchy, outcrops and (although variable), the infiltration is advocated as the first route Carboniferous coal measures are likely of disposal of surface water runoff to to have some potential for soakaway be considered when developing runoff drainage. management options. Infiltration should Figure 4 shows an outline summary for be used where conditions allow and infiltration SuDS potential7. This map 6 where it is safe . The infiltration potential is not for local assessment and does of a drainage system is governed not provide specific subsurface data or primarily by the permeability of the state the limitations of the subsurface surface geology. Whilst much of our area with respect to infiltration. Site specific is likely to be unsuitable for any form of assessment should be made as required surface level infiltration drainage, due to to determine the infiltration potential. This the generally low permeability of much should be in the form of soakaway tests of the material, up to half may have some conforming to the procedure established opportunity for infiltration, and a quarter in BRE Digest 365 – Soakaway design8, has good opportunities for infiltration. or various other permeability assessment The outcrops of Carboniferous limestone techniques. north-west of the city centre are likely to
Bristol City Council be broadly suitable for infiltration style
Section drainage providing the material is suitably
3 British Geological Survey. (2004). 1:50,000 Geology Series England and Wales Sheet 164 - Bristol. Solid and drift geology.
two Keyworth, Nottingham: British Geological Survey Page 10 6 www.susdrain.org/files/resources/SuDS_manual_output/paper_rp992_19_infiltration_assessment_checklist.pdf 7 www.bgs.ac.uk/products/hydrogeology/infiltrationSuds.html 8 BRE Digest. (2007). Soakaway Design. Bracknell: IHS BRE Press March 2015 – Issue Version 1
Information displayed has been reproduced from the British Gelogical Survey Infiltration SuDS Map: Detailed Licence no. 2012/068SuDS
Legend Bristol Local Sustainable Drainage Design Guidance Design Drainage Sustainable Local Bristol Highly compatible for infiltration SuDS Probably compatible for infiltration SuDS Opportunities for bespoke infiltration SuDS Very significant constraints are indicated
Drainage Summary
Drawn By: MS Date: 10 June 2014
Drawing No. 2014.06.10_BGS_1
0 0.75 1.5 3 km
This map is reproduced from Ordanance Survey material with the permission of Ordanance Survey on behalf of Her Majesty's Stationery Office. 100023406. ¯ Copyright Bristol City Council 2014 CONFIDENTIAL DATA: NOT FOR CIRCULATION
Figure 4: BGS Infiltration SuDS map9 two 9 www.bgs.ac.uk/products/hydrogeology/infiltrationSuds.html
Page 11 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Water Quality The Water Framework Directive (WFD) in Bristol is a mixture of combined and greater faecal contamination than establishes a common approach to separated systems, with most of the city normal. The main source of water supply managing water within the European centre on a combined system. At times of to the Floating Harbour (FH) is the River Community and aims to achieve ‘good’ heavy rainfall, in order to avoid flooding Avon from Bath, together with surface water ecological status for all water of built-up areas, the combined system water systems discharging directly to the bodies and no deterioration. discharges excess flow into watercourses. FH from the River Frome via Castle Ditch Key actions to improve water quality These flows therefore have temporarily and St. Augustine’s Parade. are described in the Severn River Basin ¯ Management Plan10. Other strategies such as Bristol’s Local Flood Risk Management S o u tt h G ll o u c e s tt e rr s h ii rr e Strategy and the Bristol Avon Catchment Flood Management Plan will have multiple benefits that will also help to improve water quality. Since Bristol is at the lower end of the !( Bristol Avon catchment water quality is significantly influenced by the upstream B rr ii s tt o ll water quality of the River Avon and Bristol City Council Water Framework Directive its tributaries. However the urban 2014 classifications !( High Note - Bristol City Council data !( environment of the Bristol area does based on chemical and !( Good ecological status (except River !( !( exert its own particular stresses on the Avon (St Anne's Ferry) and the !( Moderate Floating Harbour (Baltic Wharf !( water quality of watercourses from and BristolBridge) which are N o rr tt h S o m e rr s e tt !( Poor based on chemical status only). !( !( !( sources that include diffuse pollution !( Bad from the runoff from impermeable Environment Agency classification data for 2014 surfaces, storm water overflows and Water bodies in the Bristol City Council area drainage misconnections. The sewerage High systems within these urban areas often Good accept combined flows of foul sewage Moderate and storm water run-off and are under Poor B a tt h a n d N o rr tt h E a s tt S o m e rr s e tt Bad increased pressure during times of heavy Ordnance Survey data © Crown copyright and database right 2014 Bristol City Council
Section rain and flooding. The sewerage system Figure 5: Waterbody classification data (Contains Environment Agency information © Environment Agency and database right). Note Bristol Avon downstream of Hanham Weir is an estuarine watercourse.
10 www.gov.uk/government/publications/river-basin-management-plan-severn-river-basin-district two Page 12
March 2015 – Issue Version 1
Improving water quality in Bristol will WFD, where feasible and viable, either References:- improve the quality of habitats and through measures in the relevant WFD Bristol City Council FH water quality biodiversity and the aesthetics of the plans, local plans and initiatives and as monitoring results - www.bristol.gov. water environment. The importance a minimum recognised good practice uk/page/environment/harbour-and-riv- of mitigating and improving the water such as naturalised river habitats, de- ers-water-quality quality of discharges through SuDS from culverting, buffers and appropriate Relevant Local Plan Policies: development is therefore of primary vegetation management plans. A WFD LL ¶ Core Strategy Policy BCS23 importance both in Bristol and vitally compliance assessment may be required (Pollution), upstream where both flows and sediment by the Environment Agency for such LL ¶ DM33 (Pollution Control, Air Quality loss can be slowed. developments. and Water Quality) Policy DM33 of the Site Allocations and The River Avon is already at Good Development Management Policies ecological status and this should not document (adopted in July 2014) be allowed to deteriorate through states that development adjacent to development. As the FH is a primary underground or surface water bodies amenity and economic asset to the covered by the WFD should contribute city improving water quality is of major importance to the city and the focus on towards those water bodies maintaining Guidance Design Drainage Sustainable Local Bristol or achieving Good Ecological Status. This water quality improvement is required may take the form of on-site measures in the centre around the FH and further or a financial contribution to off-site upstream to the north of the city. measures. Although the FH has not all yet been In terms of water quality the River Frome, assessed and classified under the WFD, Brislington Brook, Malago, River Trym BCC is working with the EA to include it and Colliter’s Brook do not currently for designation in the second cycle and achieve Good Ecological Status due as such it will be BCC, as asset owners, to quality of upstream and local flows responsibility to ensure the water body which are affected by urbanisation, achieves Good Ecological Potential by typically flood protection / land drainage 2027. works. To comply with the WFD water Since the FH is not a designated EU bodies should reach good ecological Bathing Water but has a significant status or potential by 2027. Measures will amount of recreational use BCC therefore be sought from development undertakes routine bacteriological water adjacent to waterways covered by the quality monitoring for harbour users. two
Page 13 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Bristol Drainage Strategy Drivers
Bristol Local Sustainable Drainage Design Guidance The varied topography of the city, the flood peaks from upstream catchments specific information which should be presence of fluvial, impounded and if significant lag was to be introduced sought and, if appropriate, the local tidal rivers passing through the urban through attenuation. A variety of sewer capacity through consultation area and the presence of intercepting objectives have developed in response with Wessex Water. The ‘natural’ water tunnels to divert flood water away to water quality goals and capacity balance within a watercourse also is from the central area of the city results constraints in downstream catchments. a factor. Reducing the peak flows is in a complex potential impact on the For surface water systems discharging generally beneficial, but reducing the hydrology of the fluvial system close to to the FH for example, the enhancement volumes can result in significant changes the tidal limit. of water quality is paramount in view of to overall flow characteristics. In some As a result, an overarching drainage the amenity use of the FH and to satisfy systems, reducing inflows at the top end strategy has evolved over past years for Water Framework Directive requirements. of the system may detrimentally affect different approaches to be applied to In many catchments north of the NSWI the watercourse base flow in summer the discharge of surface water runoff in however, subject to the capacity of months. catchments close to the city centre and the connecting sewerage system, it is new development areas further upstream desirable to discharge runoff as directly within the adjacent contributing as feasible. A similar approach applies catchments, supported by an increasing in catchments draining to the Malago evidence base11. This was used to inform tunnels. our Local Flood Risk Management The overarching drivers outlined in Strategy, adopted in November 2014. Table 2 seek to indicate the likely As Bristol is located low in the River spatially-variable drivers for surface Frome catchment and near the tidal water drainage of developments in limit, discharges from (re-)development Bristol. The boundaries between areas within the urban area sited upstream of shown in Figure 6 are indicative. We the NSWI are discharged as directly as provides case-by-case advice on reasonably possible within the capacity development above minimum risk of the sewer system without introducing thresholds. Developments should test additional lag. This approach is applied the assumptions using site-specific Bristol City Council
Section to avoid the potential for interaction with characteristics as appropriate. Site-
11 Studies including the Bristol Tidal Strategy, the Bristol Frome Flood Management Study, the Level 1 and 2 Strategic Flood Risk Assessments and Central Area Flood Risk
two Assessment studies and the River Avon CFMP and Bristol Frome CFMP Page 14
March 2015 – Issue Version 1
1. Avonmouth and Lawrence Weston 2. Westbury Limestone and River Trym, Clifton 3. Southmead and Henleaze 4. North of Northern Storm Water Interceptor (NSWI) 5. Central area and Floating Harbour 6. Fishponds Brook
7. East Bristol Guidance Design Drainage Sustainable Local Bristol 8. North of River Avon 9. Broom Hill and Brislington 10. Brislington Brook 11. Bedminster 12. Malago and Pigeonhouse tunnels 13. Ashton Gate
Boundary of zones Main Rivers, boundaries of zones
Figure 6: Discharge Strategy Driver Areas (source Bristol City Council, Bing Maps) two
Page 15 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Bristol Local Sustainable Drainage Design Guidance Discharge Zone Overarching drivers Reason 1 Avonmouth and Surface water storage with A largely flat area with some ground levels below spring Lawrence Weston consideration of tide locking. tides that requires storage during times of tide locking and consideration of existing rhyne system. 2 Westbury Limestone Infiltration where possible, or Large areas of limestone with good infiltration and River Trym, Clifton reduction in discharge rate and potential (ref BGS Infiltration for SuDS Map). River water quality improvements. Trym can respond rapidly and has “poor” water quality classification in places12. 3 Southmead and Reduction from existing discharge SWMP identified areas at high risk of surface water Henleaze rate, water quality improvements. flooding. Water quality classification of Trym in Southmead “bad”13. 4 North of Northern Limit discharge to capacity of Majority of this area is drained to the NSWI, which diverts Storm Water Interceptor existing sewer network or existing flow to the tidal Avon. Reducing discharge therefore (NSWI) rate. considered to provide little benefit provided surface water sewer network has capacity. Reduction will be required where localised capacity issues are known; see the SWMP and WaSC. 5 Central area and Water quality mitigation and Testing of the Floating Harbour carried out weekly shows Floating Harbour improvements. Reduction in that water quality deteriorates after heavy rainfall14. existing discharge to combined Reduction in volume and rate of water discharged to the sewers. combined sewer network therefore required. 6 Fishponds Brook Infiltration where possible, or Pockets of ground suitable for infiltration (ref BGS reduction in existing discharge rate Infiltration for SuDS Map).Outside these areas, reductions and water quality improvements. in discharge rate are required because of the potential impact of the Fishponds Brook on the river Frome flows upstream of the NSWI. Water quality classification of Fishponds Brook “poor”14.
Table 2: Discharge Catchment Strategies for SuDS Bristol City Council Section
12 Studies including the Bristol Tidal Strategy, the Bristol Frome Flood Management Study, the Level 1 and 2 Strategic Flood Risk Assessments and Central Area Flood Risk
two Assessment studies and the River Avon CFMP and Bristol Frome CFMP Page 16 13 maps.bristol.gov.uk/pinpoint/?service=localinfo&maptype=js&layer=River+water+quality+2013&sidebar=false&mapopts=legend&theme=none 14 www.bristol.gov.uk/page/environment/harbour-and-rivers-water-quality March 2015 – Issue Version 1
Discharge Zone Overarching drivers Reason 7 East Bristol Reduction in existing discharge rate Historically known to be at high risk of surface water and water quality improvements. flooding. Water quality classification of Coombe Brook “poor”12. 8 North of River Avon Infiltration where possible, or Some areas with good infiltration potential (ref BGS reduction in existing discharge rate Infiltration for SuDS Map). Where infiltration not in places. appropriate, reduction in discharge may be required dependant on location; see the SWMP and WaSC.
9 Broom Hill and Infiltration where possible, or Large areas with good infiltration potential (ref BGS Brislington reduction in existing discharge rate Infiltration for SuDS Map). Brislington Brook is a rapid and water quality improvements. response catchment with history of flooding and has “poor” water quality classification12.
10 Brislington Brook Reduction in existing discharge rate Brislington Brook is a rapid response catchment and water quality improvements. with history of flooding and has “poor” water quality classification12. Guidance Design Drainage Sustainable Local Bristol 11 Bedminster Reduction in existing discharge rate. History of flooding in this area with lower areas vulnerable to the effects of tide locking. 12 Malago and Limit existing discharge to capacity Area benefits from the Malago and Pigeonhouse Pigeonhouse tunnels of existing sewer network or interceptor tunnels. Discharge at capacity of sewer existing rate. network largely appropriate, though reduction will be required where it provides local benefits, particularly at the top of the catchment; see the SWMP and WaSC. 13 Ashton Gate Reduction in existing discharge rate. SWMP identified areas at high risk of surface water flooding, also vulnerable to the effects of tide locking.
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Page 17 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Guidance for achieving discharge zone drivers
Bristol Local Sustainable Drainage Design Guidance The following sections provide guidance on achieving the overarching drivers for each discharge zone. The zones are grouped in to similar overarching drivers to minimise repetition.. Group 1: Westbury Limestone and River Trym, Clifton, Fishponds Brook, North of River Avon, Broom Hill and Brislington The overarching drivers in group 1 are to infiltrate where possible and safe, or provide a reduction in existing discharge rate and water quality improvements where infiltration is not possible. To achieve this, consider: LL ¶ Site specific ground investigation. LL ¶ Discharge rates and attenuation. If Are infiltration rates appropriate for infiltration is limited, what reduction infiltrating SuDS techniques? Does in existing discharge needs to be contaminated land exist and does this accommodated? Speak to the LPA. restrict the potential for infiltration? Can partially infiltrating techniques be What is the ground water level? used? Can storage be provided above LL ¶ Source control. Can surface water ground and can areas be designated runoff be reduced by allowing it to to flood to reduce attenuation infiltrate to the ground at source? Can requirements? Can water re-use be infiltration features be integrated in included? to the landscape and development LL ¶ Water quality. What sources of layout? Are green / blue roofs an pollution are there to the receiving option? Can downpipes be water body? How can these disconnected to reduce the area be mitigated? How can water draining to the system? concentration be minimised? Can water be passed over vegetated areas? Bristol City Council Section two Page 18
March 2015 – Issue Version 1 Group 2: Southmead and Henleaze, East Bristol, Brislington Brook, Bedminster, Ashton Gate The overarching drivers in group 2 are to reduce existing discharge rate and provide water quality improvements. To achieve this, consider: LL ¶ Discharge rate and attenuation. Is the LL ¶ Water quality. What sources of Greenfield rate reasonably practicable? pollution are there to the receiving If not then what is? Speak with the LPA. water body? How can these Can storage be provided above ground be mitigated? How can water and can areas be designated to flood concentration be minimised? Can water to reduce attenuation requirements? be passed over vegetated areas? LL ¶ Site layout. If space is limited providing storage can be a challenge. Can a number of small drainage features be built up to avoid the need for large storage areas? Can impermeable area
be reduced? Guidance Design Drainage Sustainable Local Bristol
Group 3: North of Northern Storm Water Interceptor, Malago and Pigeonhouse tunnels The overarching driver in group 3 is to limit discharge to the capacity of the sewer network. To achieve this, consider: LL ¶ What is the existing drainage regime? LL ¶ Are there any known issues or benefits What is the existing runoff rate? that mean a reduction in discharge Consider a CCTV drainage survey. is required? Speak with the LPA and WaSC. Check the SWMP.
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Page 19 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Group 4: Avonmouth and Lawrence Weston The overarching driver in group 4 is surface water storage with consideration of tide locking. To achieve this, consider: LL ¶ Existing features. A network of LL ¶ Tide levels. Can the site discharge balancing ponds and rhynes exist in during high tides? If not what mitigation this area. Speak with the Lower Severn is required? Consider ground levels and Internal Drainage Board. determine flood levels. Speak with the Environment Agency.
Group 5: Central area and Floating Harbour The overarching driver in group 5 is for water quality mitigation and improvements, and a reduction in discharge to combined sewers. To achieve this, consider: LL ¶ Combined sewers. Does the existing LL ¶ Can source control measures be LL ¶ Location. Is the site located near to site connect to the combined sewer used to reduce the volume of surface the Floating Harbour? How will diffuse system? How can discharge volume to water? pollution be managed? Can discharge the combined network be reduced or rate be unrestricted? Speak with the eliminated? LPA. Bristol City Council Section two Page 20
March 2015 – Issue Version 1 Sustainable Drainage Design Principles and Good Practice
Common Considerations We recognise that a large number and reduced impermeable areas are LL ¶ The Water Performance Directive of developments in Bristol will be on good ways of making the most of 2000/60/EC. previously developed sites that present available space, and more efficient than LL ¶ National Planning Policy Framework specific challenges. traditional approaches. and December 2014 Written Ministerial Statement Some considerations on previously Exceedance – any type of drainage LL ¶ Planning Practice Guidance and related developed sites are common to all system can be exceeded, it is therefore Technical Standards for Sustainable regardless of location. vital that exceedance is managed so Drainage Concentration of flow – concentrating that it does not become uncontrolled LL ¶ Building Regulations Part H, Drainage flow from a large area in to a single flooding. Because exceedance was not and Waste Disposal. discharge point increases flood risk and historically considered, buildings were LL ¶ Biodiversity Action Plans. causes significant pollution problems. occasionally sited in flow routes which LL ¶ Environment Agency Pollution Limit this by discharging flow to multiple resulted in flooding of property. This Prevention Guideline PPG 3. areas across the site. approach is not sustainable and site LL ¶ British Standard BS 7533-13:2009. CIRIA The SuDS Manual, C697. layout must be managed to Existing infrastructure – previously LL ¶ CIRIA Source control using constructed
developed sites may be constrained by pervious surfaces, C582. Guidance Design Drainage Sustainable Local Bristol existing infrastructure. Identify these General SuDS Design LL ¶ CIRIA Designing for exceedance in constraints early and discuss with urban drainage the LPA. Historic systems may not be Reference List LL ¶ a good practice, V635. LL ¶ CIRIA Site handbook for constructing suitable for present day requirements The list below outlines key and emerging SuDS, C698. and therefore may need to be changed legislation and guidance likely to be or replaced. LL ¶ Environment Agency Drainage useful for the design and construction Details. Limited space – inner city sites are often of Sustainable Drainage Systems (SuDS) LL ¶ Highway Agency Drainage and squeezed for space. Innovative solutions elements. Construction Details. will be required to make space for water. This list must not be considered as an LL ¶ BRE 365 Soakaway design guide This may include denser occupancy in exhaustive list as design guidance and LL ¶ For outfalls, inlets, outlets and drainage some parts of the site to make other product development is continually details refer to Bristol City Council 15 parts of the site available. Source control evolving. Standard Engineering Details . two 15 available at www.bristol.gov.uk/page/transport-and-streets/roads
Page 21 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Minor Development Design Considerations Size thresholds Discharge and Storage Requirements These figures have been estimated For residential developments, minor For small scale developments we by restricting discharge from site development is one where the number of recognise that it is not always practicable to the average discharge rate from dwellings to be constructed is between to undertake all of the relevant design a typical greenfield site in Bristol16 1 and 9 inclusive. Where the number of considerations outlined in this guide. with a consideration for a reasonably dwellings to be constructed is not given To help small scale developments, we practicable minimum discharge rate. The in the application, a site area of less have set the following discharge, storage storage requirement figures are based than 0.5 hectares should be used as the and water quality requirements that on the long term storage requirement definition of a minor development. For all will be deemed to satisfy the drainage resulting from a 1:100 year storm event other uses, a minor development is one requirements. However, it may be in the with 6 hour duration, including an where the floor space to be built is less interest of the developer to undertake allowance for climate change. than 1,000 square metres or where the such preliminary design considerations site area is less than 1 hectare. so that storage (and hence cost) Source Control SuDS requirements may be minimised. The use Source control SuDS will allow the of infiltration should always be explored attenuation volume to spread over and utilised, as the values shown in Table different areas and keep water at or near 1 are based on a system which does the surface. Not utilising this will result not allow for infiltration and therefore in a significant attenuation volume in a maximises storage requirements. single location and also make managing the peak discharge rate a potential Total Discharge Storage requirement problem. site (l/s) Another benefit of source control SuDS is area their ability to improve water quality and (ha) boost amenity and biodiversity. Rather 2 0 – 0.5 5 35 litres/m of than using a large attenuation tank, impermeable area allowing runoff to discharge through 0.5 < 1 5 30 litres/m2 of a series of source control SuDS such impermeable area as permeable paving, green roofs, rain gardens, etc. will reduce capital costs and the maintenance liability associated with
Bristol City Council a tank and flow control device. Section
16 This has been calculated using the ICP SuDS method based on data from the Bristol area two Page 22
March 2015 – Issue Version 1
By utilising a series of source control Worked Examples Potential Solution – Permeable Paving SuDS there is the potential to have The following worked example shows By connecting the roof areas into the zero discharge from site; this will result how the above storage requirement and permeable paved parking area for each in a reduced annual water bill for the discharge rate is satisfied for a typical 7 individual house, the total volume is occupier, reduction in downstream flood house urban development. broken down to 1.9m3 per house. A risk and improvement in overall water typical parking space is 12.5m2, therefore quality. Site Size (m2) 872 assuming a voids ratio in the sub-base of Source control measures also provide Impermeable Area 540 (450 for roof 0.3, the total depth of sub-base would be (m2) space, 90 for a treatment stage, for example: using 0.5m. A flow control would be required parking) a section of permeable paving will to satisfy the maximum allowable constitute a treatment stage, which Max. Discharge Rate 5 discharge rate. cannot be achieved when using (l/s) This solution would satisfy both water conventional gullies and asphalt. As Storage Requirement 25l * 540m2 = quality and attenuation requirements gullies are not considered a treatment (m3) 13.5m3 whilst also minimising excavation costs. stage, utilising these will require the This is therefore a very cost effective addition of a treatment stage, such as solution when compared to constructing Guidance Design Drainage Sustainable Local Bristol filter strip or swale, prior to discharging to a single 13.5m3 storage structure and the receiving water body/sewer. associated water quality mitigation device.
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Page 23 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance SuDS Features Design Principles and Good Practice
The below section collates good practice design principles for each SuDS feature.
Swales Description Specific Design Considerations Swales are shallow channels that are used The exact profile of swales will depend to collect and/or convey water and also on specific ground levels, topography, remove pollution. They can be covered ground and site conditions present at the in grass or other vegetation and have site, as well as orientation, aspect and shallow side slopes and a flat base, which proximity to other landscape features means that for most of the time the and buildings. Bed width could be water flows in a thin layer throughout the reduced to zero to maintain suitable bank grass or other vegetation. gradient. Swales are source control element of SuDS. The grass or vegetation slows the water down and traps some allowing it Rounded shoulders Depth to be shallow Vegetation to match to soak into the ground. In addition, the for mowing to suit drainage existing species and suit requirement with an wet/dry design plants help evaporate some water and additional 50mm to filter out pollutants. account for silt deposition Swales can have a wet base, in which Flow Flow case they will behave like a wetland. In Topsoil areas where a wet base is not desirable
a perforated pipe, sand or gravel, can In ltration be installed below the bottom. This can membrane promote water to ground. Optional impermeable Min 1m verge membrane. Material to behind highway suit site conditions (eg (unles otherwise Gently sloping Optional at base Subsoil if sub surface agreed) sides (1:3) max. to encourage contamination is sheet ow run-o
Bristol City Council present) Section two Page 24
March 2015 – Issue Version 1
Where ground conditions do not allow LL ¶ Primary pollution control might be LL ¶ Overland flood routing and swale for infiltration swales can be used as required to suit site conditions. capacity exceedance shall be conveyance structures to pass the runoff LL ¶ Pre-treatment is recommended to considered and safe water routing to the next stage of the treatment train. remove sediment and fine silts prior to identified as part of the design Incorporating an impermeable membrane infiltration (e.g. filter strips). process. beneath the topsoil can help maintain LL ¶ Drop from adjacent surface onto swale LL ¶ The lower section of a 2 part kerb some evaporation and infiltration. to be 50-100mm for direct lateral drain unit can be used to discharge The design should contribute to the flows. from highway to swale/pond and keep depths minimised. A plate will be amenity of the local communities. There LL ¶ Design calculations shall be provided should be an assumption to retain all by the Developer to BCC and include required over the lower unit (refer to existing native trees and vegetation. an allowance for evaporation. detail on Page 43). LL ¶ Where swales are adjacent to highways, Key Design Considerations and best LL ¶ Small piped outlets to swales shall have a minimum 150mm wide concrete refer to the Manual for Contract practise Documents for Highway Works Maximum/minimum velocities should be surround laid flush to the ground profile (refer to BCC standard detail SD-05- (MCHW), edge of pavement drainage considered to determine maintenance details that will provide good practice points within the swale (e.g. areas with 010 ). suggested details. Guidance Design Drainage Sustainable Local Bristol lower velocity will have higher siltation). LL ¶ Larger pipes shall have specific www.dft.gov.uk/ha/standards/mchw/ headwalls and scour protection (refer LL ¶ Infiltration rate to be confirmed by vol3/section1.htm to details BCC standard detail SD-05- Developer and referenced to current LL ¶ 009) Where swales are adjacent to buildings, site investigation. The impact of refer to the Building Regulations introducing water to ground shall be LL ¶ Check dams to be used where Approved Document H to ensure assessed and the risk of mobilising longitudinal slopes are steep to compatibility. potential contaminants both within maximize storage and minimize land the site and on adjacent sites shall be use where not appropriate. Locations assessed and provided to BCC for to be proposed by the developer review and agreement. and a narrative provided to cover maintenance and silt removal. LL ¶ For infiltration swales, the permeability of the topsoil must be greater than the LL ¶ Check dams to be designed, placed permeability of the underlying soils. and modelled to demonstrate conveyance velocity and storage by LL ¶ Design to demonstrate no introduction developer. of water to the lower sections of BCC
adoptable footways and carriageways two
Page 25 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Items to Consider / Practical Issues and Solutions Maintenance of the swale will be required as part of the overall site open space maintenance. If it is incorporated into the general maintenance regime there will only be some additional costs where swale related work needs to be undertaken above and beyond the cost of the general landscaping. Frequency of maintenance to be reviewed and proposed by the Developer. Item Comment Issues Solutions Litter removal Frequency will be site specific. This may be part of general Block movement Laying to be landscaping maintenance. undertaken in accordance with Inspect control Control structures should be clear from debris and litter to reduce manufacturer structures to swale blockage risk and erosion to the structure. requirements. Grass cutting Swale design should aim to have rounded shoulders to assist Block joint opening Cleaning and cutting activities. Grass seed specification shall be confirmed by sweeping to Developer including frequency of growth/cutting regime and manufacture suitability for location (e.g. north facing land/wet/dry/etc.). requirements. Re- Scrub/shrub clearance The developer shall consider planting design, specification and grit where necessary from bankside rate of maintenance. (Consideration for future overhanging after sweeping. branches and encroaching growth undertaken). Clogging Construction debris Remove planting and The Developer shall consider areas in the design to accommodate to be controlled and silt from 25% to 30% of this natural arising from maintenance. appropriate design levels of adjacent base and place in side Carry out maintenance activities between Septembers – piles. landscapes to November if possible to minimise disruption to wildlife. prevent dirt entering Management Plan for The developer shall develop a detailed management plan on how the system. SuDS. the SuDS were designed, any spare capacity and the maintenance regime to be adopted to maintain their design intent. Bristol City Council Section two Page 26
March 2015 – Issue Version 1 Filter Drains / French
Drains Level leading edge Sacri cial single size stone Description where necessary layer of top soil (min. 300mm) with geotextile to trap silt Filter drains – traditionally French drains Flow are shallow excavations filled with gravel or stone, and often have a perforated Grass lter strip to pipe in the bottom. They collect, trap silt where temporarily store water and convey it possible further to downstream SuDS components. Filter drains are Crushed stone for treatment widely used to drain roads and are Over ow perforated pipe and storage with in ltration for in ltration trench often seen along the edge of the main where possible roads. Ideally, filter drains are expected to drain adjacent impermeable areas via Outlet perforated pipe for lter trench lateral inflows, but point flows may be acceptable (to be confirmed with BCC). Impermeable membrane Guidance Design Drainage Sustainable Local Bristol 1m deeo where adjacent to the adotable highway Geotextile lined trench. Permeable for in ltration or impermeable for a sealed trench
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Page 27 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Specific Design Considerations Key Design Considerations and best practise LL ¶ Gently sloping grass verges (filter LL ¶ Filter drains are not suitable for use in strips) a minimum of 1m in width to be trafficked areas. incorporated in the design as means LL ¶ Design to ensure ground water is not of pre-treatment. This is essential to transferred to the public sewers. remove silt and pollutants. LL ¶ Low level outlets to be used when LL ¶ For industrial areas, upstream designed for conveyance, high level treatment must be used before overflows to be used when designed discharge to the filter drains. Refer to for infiltration. the Pollution Prevention Guidelines LL ¶ Where filter drains are adjacent to PPG3. highways it is recommended that a LL ¶ In general to be used where no other verge is utilised between the road and SuDS feature will work. Developers shall the filter drain. Refer to the Manual confirm in the design submission why for Contract Documents for Highway other SuDS features are not suitable. Works (MCHW), edge of pavement LL ¶ Effective upstream pre-treatment to drainage details that will provide good remove sediment and fine silts. practice suggested details. www.dft. gov.uk/ha/standards/mchw/vol3/sec- LL ¶ Filter drains can be used in conjunction with swales to create enhanced swales tion1.htm and act as a pre-treatment system. LL ¶ Filter drains to be used an adequate distance away from any building LL ¶ Size to suit catchment area, infiltration rate and groundwater levels to be or septic tank. Refer to Building provided by the Developer to confirm Regulations Approved Document H. the design. LL ¶ Size of perforated pipe to suit storm events and infiltration rates. LL ¶ A minimum void ratio of 0.3 to be used for the fill material.
Bristol City Council LL ¶ Filter drains to be incorporated into site Section landscaping wherever possible. two Page 28
March 2015 – Issue Version 1
Items to Consider / Practical Issues and Solutions
Item Comment Issues Solutions Sediment removal Special care must be taken for sites with high debris input. Using impermeable Good communication from pretreatment membrane instead between parties system of permeable as during design and pipe lining construction. Pipe lining Vegetation clearance Vegetation closer to the filling material must be cut back should be permeable from sides to promote regularly for best results. and trench membrane lateral runoff inflow can be either permeable Clogging inspection Clogging is closely related to pre-treatment measures; the more or impermeable. effective the pre-treatment, the less clogging. Access to filter system is key for efficient use. Overgrowing of Regular cutting of grass surrounding grass to keep a clear surface Litter removal Quantity and frequency will be site specific. This could form part that causes the and reduce problems of general landscaping maintenance. stone to be invisible associated with mowing at edges and stone. Removal and cleaning Cleaning and replacing the fill materials is more sustainable than or replacing the fill disposing. Strip to be inspected regularly. High siltation during Construction runoff to material construction be managed by other Guidance Design Drainage Sustainable Local Bristol means of drainage Debris in perforated Pipe to be lined with geotextile material to prevent soil and other and filter drains to pipe matter from entering pipe. be protected using Management Plan for The developer shall develop a detailed management plan on geotextile traps. SuDS. how the SuDS were designed, any spare capacity and the Scattering of fill Crushed rock can maintenance regime to be adopted to maintain their design material when be used on top of fill intent. drains are adjacent material to reduce to roads scatter. Rumble strips can also be used. Consideration of a grass verge or a filter strip to separate the adjacent drained hard surface and the filter drain.
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Page 29 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Permeable Paving Description Permeable paving consist of blocks or porous asphalt that act as a source control measure. Water can be discharged to another system or temporarily attenuated before infiltrating into the ground. The paving itself can be of porous material that allows for infiltration across the entire surface area or impervious with voids in between to allow for water infiltration. This is not the preferred option for BCC to adopt. Developers shall fully explore other SuDS techniques. If BCC were to consider adopting permeable paving systems, this will need to be at the head of any drainage system and will require agreeing commuted sums with BCC. As well as ongoing maintenance costs the full replacement value will also be added to any commuted sum. Bristol City Council Section two Page 30
March 2015 – Issue Version 1
Specific Design Considerations LL ¶ Appropriate geotextile membrane BS7533-13. Refer to the manufacturer’s Type of permeable paving for infiltration, to be used to prevent sub-base requirements for the blocks. attenuation or conveyance will depend clogging. LL ¶ Permeability of the surface layer must on specific site requirements and LL ¶ Adequate distance between base of be greater than 5000mm/h. infiltration rates. These will need to infiltration device and ground water LL ¶ Avoid point inflows and consider be confirmed by the developer and table should be considered. Normally inflows in the sub-bases. referenced to current site investigation. at least 1m clearance should be Information to be provided to BCC LL ¶ To be used in areas where traffic speed demonstrated between base of system will not be more than 20mph. Not in prior to construction. Commuted sums and the seasonal variation ground will be required by BCC to cover the public highways by possible in parking water level. Information to be provided areas. maintenance costs for the life of the to BCC. pavement use and development. LL ¶ Permeable pavements must be LL ¶ The maximum designed water depth designed to satisfy the required traffic Key Design Considerations and best must not exceed the top of the sub- loads (axle loads) and frequency. base. practise LL ¶ Refer to Building Regulations Approved LL ¶ Developer must ensure that the LL ¶ Where infiltration is not suitable Document H for further information. design levels are appropriate. (vulnerable groundwater or draining of This is particularly important pollution hotspots), sealed systems can Guidance Design Drainage Sustainable Local Bristol when discharging at certain levels be used. downstream, considering overland LL ¶ Storage capacity should satisfy the flood routing in higher storm events storm requirements, especially for and blockage of the pavement surface steep sites. scenarios. LL ¶ Sub-base storage must be an adequate LL ¶ Base of the paving system to be laid distance from building foundations. flat to maximise storage, or if installed Refer to guidance in the design on a sloping site, baffles should be reference list above. considered to slow flows and promote LL ¶ A preferable void ratio of 0.3 to be maximum infiltration. used for the sub-base. LL ¶ Infiltration systems must not be used LL ¶ Depth of sub-base to be designed in in contaminated site. Sealed systems accordance with BS7533-13 or TRL may be considered subject to suitable Report PPR 482. material specification for the ground LL ¶ Sub-base specification and laying conditions. course to meets the requirements set in two
Page 31 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Items to Consider
Item Comment Issues Solutions Organic matter and silt For blocks and bituminous systems, regular vacuum brushing and Block movement Laying to be removal jet-washing must be considered at the surface. Following cleaning, undertaken in joints to be filled with small stone to maintain their interlocking accordance with design. manufacturer requirements. Weed growth Weed removal and weed killer to be applied when needed. Block joint opening Cleaning and Replacement of blocks Relaying of blocks and impermeable membranes when in poor sweeping to condition. manufacture Outfalls For sealed systems, inspection of outfalls should be undertaken requirements. Re- regularly. grit where necessary after sweeping. Warranty Terms & Manufacturer’s warranty to be obtained by developer and Conditions provided to BCC. Clogging Construction debris to be controlled and Stockpiling No stockpiling of materials to be held by BCC. The developer shall appropriate design agree the modular block dimensions with BCC. levels of adjacent landscapes to Management Plan for The developer shall develop a detailed management plan on how prevent dirt entering SuDS. the SuDS were designed, any spare capacity and the maintenance the system. regime to be adopted to maintain their design intent. Bristol City Council Section two Page 32
March 2015 – Issue Version 1 Soakaways Description Soakaways are underground excavations used to store surface water and allow it to infiltrate when conditions permit. The soakaways structure can either be lined with perforated brickwork or precast concrete rings. Depending on how much storage is needed and land space available, either can be used. The type of soil is an important factor for the soakaways to work as they are highly dependable on infiltration. A series of soakaways can be used to increase the storage capacity.
Specific Design Considerations Bristol Local Sustainable Drainage Design Guidance Design Drainage Sustainable Local Bristol Soakaways can be used wherever conditions are suitable for infiltration. Developers should provide ground investigation data before they can proceed with the soakaway design.
Key Design Considerations and best practise LL ¶ Soakaway design must allow for half capacity infiltration within 24 hours. Source: Bristol City Council , Drawing SD05-006 (detail subject ot change) LL ¶ Developers to provide infiltration rates and groundwater table information. LL ¶ Size to accommodate expected runoff in accordance with BRE guidelines. LL ¶ Soakaways not to be used in contaminated sites.
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Page 33 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Bristol Local Sustainable Drainage Design Guidance LL ¶ Outfalls from soakaways not be downstream propriety silt removal units LL ¶ Soakaways must not be constructed connected to public sewers, unless to be used as means pre-treatment to to influence vulnerable ground agreed with BCC. However, discharge reduce velocities and catch coarser groundwater, drinking water wells, can be released to another SuDS sediments. septic tanks, buildings or highways. element downstream. LL ¶ External wall of chambers to be lined LL ¶ Areas upstream of the trench should be LL ¶ Sediment control measures to be with geotextile layer to prevent silt from stabilised for health and safety reasons considered when discharging to other entering the system. Observation wells/ and to prevent collapsing of soil systems downstream. inspection tubes must be provided for LL ¶ Refer to Building Regulations Approved LL ¶ Filter strips and/or swales and /or considerably large soakaways. Document H and BRE Digest 365.
Items to Consider
Item Comment Issues Solutions Sedimentation Debris to be removed from the pre-treatment device to prevent Finding soakaway Inspection access clogging. Catchment areas to be cleaned regularly. locations for to be provided. This maintenance will help locate the System failure Soakaways to be inspected in accordance with BCC drainage soakaways. requirements for the area and on regular intervals throughout the design life. Polluted run-off Pre-treatment may be required for Management Plan for The developer shall develop a detailed management plan on how polluted sites (e.g. SuDS. the SuDS were designed, any spare capacity and the maintenance petrol interceptors regime to be adopted to maintain their design intent. for car parks and car wash-down areas). Failure of structure Soakaway design must ensure that the structure is stable and has sufficient strength. Soil stabilisation around the soakaway chamber must be certified. Bristol City Council Section two Page 34
March 2015 – Issue Version 1 Detention/Infiltration Basins Description Detention and infiltration basins are two types of vegetated depressions in the ground designed to store surface water runoff on the surface. Both types can be dry most of the time except in periods of heavy rain. Infiltration basins allow for water to gradually soak into the ground, while detention basins store water and can be permanently wet, hence are usually at the end of the treatment train as they allow for extra treatment. Extract from SD-005-010 – general requirements for outfalls through a bank for pipes up to 150mm in diameter Specific Design Considerations (detail subject to change) Basins should be designed as landscape Guidance Design Drainage Sustainable Local Bristol features that act as visual enhancement and habitat creation. When dry, they can be used for social space, and habitat creation. Therefore, source control measure are advisable to control pollution and siltation. In Bristol, detention basins may be more appropriate due to the clay nature of the soil, and hence the lower infiltration rates. However, either of the basins should be used where ever the ground conditions allow.
Extract from SD-005-009 – general requirements for Extract from SD-005-012 – alternative arrangement for outfalls through a bank for pipes 200mm to 1750mm outfalls through a bank for pipes 200mm to 1750mm (detail subject to change) diameter where space is not restricted. two
Page 35 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Bristol Local Sustainable Drainage Design Guidance Basins to be incorporated into large areas LL ¶ Within permanently wet basin design, infiltration to be provided to BCC. or open space. Where next to housing, consideration must be provided to LL ¶ Vehicle access to be limited for they should be designed to overlook the BCC assessing the safety risk especially structures and areas for maintenance basins rather than hiding them away. to children. Child safety must be only. considered in pond and wetland Key Design Considerations and best design. Detention Basins practise LL ¶ Basins to have a 2:1 to 5:1 length to LL ¶ Side slopes to include densely General vegetated areas with native species width ratio to further encourage settlement and filtration of run-off. LL ¶ Basins must have pre-treatment and be to provide slope stability and assist in part of a cascading SuDS. They should sediment removal. LL ¶ When detention basins are off-line, flow diverters will be required to store larger not just be connected to the end of a LL ¶ Where there is potential for seepage piped system. of pollutants to groundwater, volumes of water. LL ¶ Erosion control measures to be installed impermeable liners should be LL ¶ Retained water should not cause at inflows to the pond. Inlet flow incorporated. clogging to trees. spreaders can be used to reduce the LL ¶ Gradual sediment accumulation should water velocity. be accounted for when specifying the LL ¶ A gentle fall (1 in 100 to 1 in 300) should size. be used to encourage surface sheet LL ¶ Lower parts of the basins can be flow by gravity. installed at different levels to allow for LL ¶ Pre outfall forebays (over deepened some recreation. areas) can be used prior to outfalls to LL ¶ All existing vegetation and tree species enhance treatment and provide extra are expected to be retained. Tree roots sediment control. must not be compromised. LL ¶ Side slopes should be 1 in 3 maximum, with clear access for maintenance. Infiltration Basins LL ¶ Geotechnical tests must be carried out LL ¶ An overflow should be included in the to confirm suitability for infiltration. design to account for extreme events and prevent water from over topping LL ¶ Ground stability must be confirmed and eroding the embankment. and risks to surrounding features must be eliminated. LL ¶ Treatment height (grass length) to be LL ¶ Depth of water table to be confirmed
Bristol City Council 75-100mm minimum. by developer and suitability for Section two Page 36
March 2015 – Issue Version 1
Items to Consider / Practical Issues and Solutions
Item Comment Issues Solutions Litter and debris Can be part of the general Reduced infiltration Ground conditions to be tested after a removal landscaping maintenance. rate period of time. Occasional silt removal is also Frequency will be site specific and recommended. Hollow tine and scarifying of after large storms. ground will increase infiltration. Grass cutting Basins to have rounded shoulders Silt build up during Construction runoff prevention and to assist cutting activities. Grass construction management systems to be utilised. seed specification shall be Geotextile traps or straw bales can be used confirmed by Developer including to control construction runoff and avoid silt frequency of growth/cutting from running into the feature. regime and suitability for location. Erosion before Allow existing vegetation to be established Inlets and Inlets and outlets to be positioned planting is quickly by reusing the topsoil without outlets to be visible for ease of access and established application of weed killer. Alternatively, inspection maintenance. Regular inspections biodegradable erosion control mats or turfs are required to prevent blockages. to be used. Bristol Local Sustainable Drainage Design Guidance Design Drainage Sustainable Local Bristol Sediment Source controls and pre- Erosion after Correct design and construction levels to removal treatment to be used to reduce silt planting is manage flow velocity. High water velocities to accumulation. established be controlled and vegetation to be checked after larger storm events. Management The developer shall develop a Poor maintenance Retention basins to be located in visible Plan for SuDS. detailed management plan on spaces and less remote areas. Visible features how the SuDS were designed, receive better maintenance. Encourage any spare capacity and the community engagement. maintenance regime to be adopted to maintain their design Debris accumulation Forebays to be inspected and cleaned intent. in forebays and regularly. This will enhance the pre-treatment pretreatment of water before entering the pond. structures.
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Page 37 Section Section Bristol City Council Council City Bristol A1 A B C D E F G H I J K L M N March 2015 – Issue Version 1
1 Bristol Local Sustainable Drainage Design Guidance Example highway bio-retention planter Kerb detail as per BCC drawing SD 02-002 with Footpath and kerb construction custom haunching B as per BCC drawings SD Gravel and planting t Gully overflow set to TWL. HB2 KERB 01-004 and SD 02-002 HB2 KERB
Carriageway
2 SCOUR PROTECTION SCOUR PROTECTION SEE...... SEE......
HAUNCHING OR HAUNCHING OR CONSERVATION KERB (SEE CONSERVATION KERB (SEE DETAIL SHEET ....) DETAIL SHEET ....) Filter Media/Topsoil
1A 1A 2A 2A 3
Granular Drainage Material HB2 QUADRANT Perforated collectorHB2 QUADRANT drain 500mm MAX WIDTH 1500mm 500mm MAX WIDTH 1500mm Section A-A PERMEABLE A SUBSOIL DRAIN
4 TOP OF BANK LOCALISED REGRADE TO ENCOURAGEOPTION 1 BREAK OPTION 2 DROP
1 IN 4 LOCAL SURFACE WATER TO RUNOFF TO Kerb detail as per BCC 4 IN 1 SWALE FEATURE KERB - PLAN TWL drawing SD 02-002 with KERB - PLAN
Scale: 1:20 Scale: 1:20 2400 CARRIAGEWAY custom haunching
1200 EXISTING FOOTPATH 1500 A Carriageway 500 A 1000 B Carriageway 5 B
1500 2400 CONNECTION TO EXISTING EXISTING HB2 QUADRANTS (ANGLES CAN EXISTING TRANSITION KERB DOWNSTREAM HB2 KERB OVERFLOW BE USED IF PREFERRED) HB2 KERB (BULLNOSE TO HB2) 450dia custom NETWORKEXISTING PAVEMENT SEE DWG 4003 1 EXISTING PAVEMENT HB2 QUADRANTS overflow gully D EXISTING ROADKERB WITH CUSTOM EXISTING ROAD A 2000 HAUNCHING 6 Rodding point with 135deg elbow joints 2 2 RODDING POINT SEE DWG 4003 C2 C2 500 900 FLUSH KERB 1
(0mm UPSTAND) EXISTING FOOTPATH FLUSH BULLNOSE KERB C1 C1 (0mm UPSTAND) Perforated collector drain Carrier drain connection to downstream network
7 D SECTION 1A-1A SECTION 2A-2A Scale: 1:20 Scale: 1:20 Section B-B B KERB INLET DETAIL T00 10/02/15 AEJ SK RB SEE DWG 4004 POTENTIAL REGRADE ROCKY WEIR/DAMS OR KERB INLET DETAILT Plan TO USE FLUSH INLET STEPPED APPROACH TO FOR DISCUSSION SEE DWG 4004 ATTENUATE FLOWS IN Issue Date By Chkd Appd STEEP FEATURES SEE DWG 4002
8
A SCOUR PROTECTION EXISTING COBBLES SET IN M1 HB2 KERB MORTAR EXISTING PAVEMENT EXISTING PAVEMENT 200-250mm 50mm HIGH, 500mm WIDE EXISTING ROAD WEIR/FLOW SPREADER SET Client IN M1 MORTAR Footpath and kerb construction Footpath and kerbB construction B Kerbs detail as per BCC as per BCC drawings SD as per BCC drawings SD drawing SD 02-002 with 01-004 and SD 02-002 Rocky weir and flow spreader 01-004 and SD 02-002 custom haunching 9 25mm STEP DOWN FROM 50mm HIGH, 500mm WIDE 50mm HIGH, 500mm WIDE INCOMING ROAD LEVEL WEIR/FLOW SPREADER SET IN WEIR/FLOW SPREADER SET IN Carriageway M1 MORTAR M1 MORTAR
Job Title WEIR AT 25mm
A BELOW INLET LEVEL SECTION A-A SECTION B-B SOUTHMEAD GREEN (ROAD LEVEL) Scale: 1:20 Scale: 1:20 Inverted flush kerb (0mm upstand) STREETS Flush kerb (0mm upstand) as per Scour Protection as per BCC drawings SD 02-002 Bristol City Council BCC drawings SD 02-002 laid at 1 laid at 1 in 20 10 in 20 Section D-D
Section Drawing Title Section C2-C2 Section C1-C1 BIORETENTIONBIORETENTIONBioretentionBioretention SWALE PodsPOD Pod INLETLAYOUT OPTIONS OPTION 1 BREAK On-Street Buildout
KERB - PLAN Scale at A1 Scale: 1:20 1:20 Discipline two
11 Drawing Status Page 38 FOR DISCUSSION
Job No Drawing No Issue 240411-00 40012004 T00
Do not scale © Arup Created using CADplot http://www.oasys-software.com/cadplot/ March 2015 – Issue Version 1 Geocellular Storage Systems Description Specific Design Considerations Key Design Considerations and best Geocellular systems are modular plastic It is not permitted to install these under a practise structures with high void ration used BCC adoptable highway or area subject LL ¶ Correct design levels must be achieved, to store storm water, to be gradually to full highway loading. It is unlikely that especially in conveyance systems, to conveyed or infiltrated into the ground. BCC will adopt these systems in other achieve adequate drainage. Geocellular systems are often used in areas but with early consultation with LL ¶ Adequate protection measures must be conjunction with other SuDS features. BCC to agree use, type and location, used for trafficked areas. Underground storage systems attenuate there may be opportunity to develop LL ¶ Ground conditions should be confirmed agreed volume with control structures an adoptable system. There are many by developer prior to installing to limit the discharge rate. Treatment different types of proprietary geo cellular infiltration systems. is limited with these type of storage storage systems on the market. They LL ¶ Pipework in the system should be structures. Structural design is key for the should be selected to suit the specific site designed in accordance with the longevity of these structures, especially requirements. Information on systems current guidance and legislation. that they are used under all types of proposed shall be provided to BCC. Silt traffic loadings. traps must be used upstream of the LL ¶ Vertical loading to be confirmed by
storage structures so that there is a developer and provided to BCC. Guidance Design Drainage Sustainable Local Bristol manageable and minimised maintenance LL ¶ Lateral loading must be confirmed by requirement in long term. developer for systems prone to high lateral loading from the sides. LL ¶ Developers to obtain creep test data and results before installing any geocellular systems. LL ¶ Installation to manufacturer’s requirements. LL ¶ Sealed systems to be used in contaminated land. LL ¶ Inspection chambers must be provided for regular checks. LL ¶ When storage is used as part of an online storage, flow channels must be wholly accessible for maintenance.
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Page 39 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Items to Consider / Practical Issues and Solutions
Item Comment Issues Solutions Inspection Regular inspections to ensure that Access difficulties Access chambers to be located adjacent to of treatment any blockages and silts are cleared areas a vehicle can gain access train (silt traps, and the system is functioning. Silt accumulation in High pressure water jetting. manholes and geocellular systems Vortex flow control upstream. pipe work) Silt traps upstream. Removal of Systems must be inspected and Management Plan The developer shall develop a detailed sediment and cleaned as prescribed by the for SuDS. management plan on how the SuDS were debris supplier. designed, any spare capacity and the Warranty Terms Developer to obtain products maintenance regime to be adopted to & Conditions warranty. maintain their design intent. Bristol City Council Section two Page 40
March 2015 – Issue Version 1 Other SuDS Components There are numerous drainage and SuDS components that can be used in conjunction the main features outlined above. This section will identify each component and identify the key guidance for their design and construction. These component should be designed in accordance with the relevant legislation in the design reference list above.
SuDS Comments Items to Consider Component Planters and Tree pits are very useful pre-treatments and features Tree pit design is critical to ensure the root ball has free draining Tree Pits that can sit within or on the margins of basins, ponds material. Refer to BCC SD-04-019 and swales. The tree species fits with the condition it will endure. These are promoted for use by BCC The design location allows for the tree to grow without detriment to conveyance of other features such as paved areas. Linear Drain Linear drains are trenches filled with permeable Maintenance regime should be considered by the developer to (Channel) material with a perforated pipe at the bottom to assist ensure longevity of the system. in drainage. This form of sustainable drainage can be Where linear drains are used independently as a form of SuDS, used in conjunction with the above systems (e.g. base design has to be justified and agreed with BCC.
of swale or pond) or independently, where space is Guidance Design Drainage Sustainable Local Bristol prohibitive for a surface conveyance feature. Manholes and Design, placing, and spacing of manholes and Refer to current guidance and legislation and BCC standard Catchpits catchpits must be approved by BCC. details
Gullies Gullies must be trapped to provide one level of Where gullies are used as outlets in landscaped areas (e.g. swales/ treatment by trapping silt. Where in highways, gullies ponds), consideration to their location for maintenance should be must be designed to BCC and Highway Agency (HA) given by the developer. adoption standards. Gullies in these locations must have a minimum of a 150mm wide concrete (or similar) mowing strip/surround.
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Page 41 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1 Bristol Local Sustainable Drainage Design Guidance SuDS Comments Items to Consider Component Kerb Drains Kerbs can be used as inlets to swales and other It is recommended that a concrete bed/slab to be used adjacent systems. Design should be in accordance with BCC to the dropped kerbs before discharge to the vegetated surfaces and HA adoption standards. to protect against erosion. Consideration to loads, location and maintenance regime should be given by the developer. Where kerbs are adjacent to roads and pavements, refer to the HA Construction Details. Flow Controls Flow controls are key elements of a well-designed Flow controls should be integrated with the urban surroundings SuDS. They must be designed to ensure longevity of wherever possible. the system. Flow controls to be designed with silt management in mind. Ensure these units are accessible for maintenance. Inlets and Inlets and outlets should be designed to allow for It is preferable the inlets and outlets are placed above ground Outlets erosion protection and maximum infiltration in the to allow for ease of maintenance. A typical single dropped kerb relevant systems. details is shown below. If draining form a highway 2 part kerb drain unit the lower section of the unit can be used to convey surface water flows to the pond basin of swale, keeping connections shallow. A typical detail is shown below. Silt control measurements to be considered to allow for maximum use of SuDS features. Bristol City Council Section two Page 42
March 2015 – Issue Version 1 SuDS Typical details Linking sustainable drainage elements to one another can be challenging especially to keep depths shallow whilst delivering adoptable infrastructure. Below are possible details for draining from Highway to ponds or swales
Kerb outlet detail: Carriageway Grass verge with 450 x 450 x 50mm Flush kerb as per formation as per BCC rounded shoulder Paving slabs BCC drawing SD drawing SD 01-001 Kerbs to suit Bristol City 02-002 Council specification and construction details 450x450x50mm slab erosion control where necessary at 1:20 fall
Erosion control 25mm Bed matting if required
Minimum 1m Impermeable membrane with a minimum 1m depth to Normal Flush mounted protect highway formation kerb Drop kerb inverted bullnose kerb 1 in 40 (impermeable Kerb outlet detail Kerb outlet surface) detail to to swale/pond. swale/pond. Section A-A. Bristol Local Sustainable Drainage Design Guidance Design Drainage Sustainable Local Bristol Kerb outlet detail to swale:
Footpath construction as per Grass verge with smooth Highway BCC drawing SD 01 -004 shoulder Two part kerb Edging detail as Kerb and drain unit per BCC drawing SD 02-005 Verge Carriageway
Steel plate cut to suit Recess in kerb drain unit Telecomms Lower section from LV Street Kerb drain or pipe outlet zone 2 part kerb drain Lighting unit HV Gas Kerb drain outlet or piped outlet to BCC standard detail SD 05-009 Water or SD 05-010 Swale/pond bank
Utilities within kerb drain or 2 part Kerb outlet detail to Min 150mm concrete pipe outlet zone to be locally swale/pond. Typical section surround/mowing strip Utility depths as per 8CC drawing lowered to suit through Footway. 2 Part Kerb SD 07-004 Drainage Outlet to Pond/Swale two
Page 43 Section Section Bristol City Council Council City Bristol A1 A B C D E F G H I J K L M N
1 March 2015 – Issue Version 1 t HB2 KERB HB2 KERB Bristol Local Sustainable Drainage Design Guidance
2 SCOUR PROTECTION SCOUR PROTECTION SEE...... SEE......
HAUNCHING GullyOR construction to BCC Rodding access point HAUNCHING OR CONSERVATIONdrawing KERB SD (SEE 05-007 to tie in with CONSERVATION KERB (SEE DETAIL SHEETkerb ....) and channel drains DETAIL SHEET ....)
A A 1A 1A 2A 2A 3 Shallow channel with non-removable cover, or steel Scour protection plate below footpath HB2 QUADRANT construction. HB2 QUADRANT 500mm MAX WIDTH 1500mm 500mm MAX WIDTH 1500mm PERMEABLE A SUBSOIL DRAIN
4 TOP OF BANK Two part kerb and drain units LOCALISED REGRADE TO ENCOURAGEOPTION 1 BREAK OPTION 2 DROP
1 IN 4 LOCAL SURFACE WATER TO RUNOFF TO 4 IN 1 SWALE FEATURE KERB - PLAN TWL Footpath construction as per KERB - PLAN Scale: 1:20 BCC drawing SD 01-004 Scale: 1:20 2400
1200 1500 500 1000 B B 5 Edging detail as per BCC drawing SD 02-005 1500 Rodding access point 2400 CONNECTION TO EXISTING EXISTING HB2 QUADRANTS (ANGLES CAN EXISTING TRANSITION KERB DOWNSTREAM HB2 KERB OVERFLOW BE USED IF PREFERRED) HB2 KERB (BULLNOSE TO HB2) NETWORKEXISTING PAVEMENT SEE DWG 4003 1 EXISTING PAVEMENT HB2 QUADRANTS EXISTING ROAD EXISTING ROAD A 2000
6
2 2 RODDING POINT SEE DWG 4003 500 900 FLUSH KERB 1 (0mm UPSTAND) FLUSH BULLNOSE KERB (0mm UPSTAND)
7 SECTION 1A-1A SECTION 2A-2A Scale: 1:20 Scale: 1:20
KERB INLET DETAIL T00 10/02/15 AEJ SK RB SEE DWG 4004 POTENTIAL REGRADE ROCKY WEIR/DAMS OR Edging detail as per BCC TO USE FLUSH INLET STEPPED APPROACH TO FOR DISCUSSION drawing SD 02-005 Grass verge with smooth SEE DWG 4004 ATTENUATE FLOWS IN Issue Date By Chkd Appd shoulder Gully outlet to channel STEEP FEATURES SEE DWG 4002
8
A SCOUR PROTECTION EXISTING COBBLES SET IN M1 HB2 KERB MORTAR EXISTING PAVEMENT EXISTING PAVEMENT 200-250mm 50mm HIGH, 500mm WIDE EXISTING ROAD Kerb drain outlet to swale (1 WEIR/FLOW SPREADER SET in 4 side slopes minimum) Client IN M1 MORTAR B B Gully construction to BCC drawing SD 05-007 to tie in with 9 kerb and channel drains 25mm STEP DOWN FROM 50mm HIGH, 500mm WIDE 50mm HIGH, 500mm WIDE INCOMING ROAD LEVEL Section A-A WEIR/FLOW SPREADER SET IN WEIR/FLOW SPREADER SET IN M1 MORTAR M1 MORTAR
Job Title WEIR AT 25mm
A BELOW INLET LEVEL SECTION A-A SECTION B-B SOUTHMEAD GREEN (ROAD LEVEL) Scale: 1:20 Scale: 1:20 STREETS
10 Drawing Title SuDS Feature Inlet BIORETENTIONBIORETENTION SWALE POD INLETLAYOUTThrough OPTIONS Footpath OPTION 1 BREAK (Option 2)
Bristol City Council KERB - PLAN Scale at A1 Scale: 1:20 SuDS Feature Inlet Through Footpath 1:20 Section (Option 2) Discipline 11 Drawing Status FOR DISCUSSION
Job No Drawing No Issue 240411-00 40012004 T00
Do not scale © Arup two
Created using CADplot http://www.oasys-software.com/cadplot/ Page 44
March 2015 – Issue Version 1 SuDS ownership and maintenance
When planning a sustainable drainage in Section 1, 3.7 Adoption of SuDS and They are anticipated to offer to adopt system, developers need to ensure their consideration should be on a site-specific certain types of SuDS components design takes account of the construction, basis. The list below is not exhaustive and serving more than one property, again operation and maintenance requirements is subject to change. subject to a number of safeguards. of both surface and subsurface LL ¶ Where a SuDS is within the private LL ¶ SuDS serving the public highway may components, allowing for any personnel, curtilage of a property it is reasonable also be adopted as part of a publicly vehicle or machinery access required to to expect the owners/occupiers of maintainable highway constructed undertake this work. properties drained by SuDS that do not in line with guidelines, following Management and Maintenance plans also drain other properties to maintain agreement between developer and will need to allow the Local Planning their own SuDS. The developer should Bristol City Council using a model Authority to satisfy themselves that provide the owner(s) with clear agreement and commuted sum, the proposed minimum standards of instructions on the maintenance of the under a Section 38 Agreement of the operation area appropriate and that SuDS including repair and replacement Highways Act 1980. there are clear arrangements in plan for requirements. LL ¶ SuDS serving more than one property ongoing maintenance over the life time of LL ¶ In the Lower Severn Internal Drainage could be maintained by a maintenance the development. Board areas of Avonmouth and company. Management company
It is critical that the most appropriate Severnside, subject to IDB consent, would be funded by annual payment Guidance Design Drainage Sustainable Local Bristol party own and maintain the feature. by agreement and following either by way of service charge in a sum to There are a number of factors to consider payment of a commuted sum or be agreed which may be reviewed when determining the most appropriate ongoing infrastructure charge, a each year. Collected by or on behalf party: developer may build (or contribute to) of the management company from SuDS that the IDB subsequently owns each occupier. Developers will need LL ¶ Location of feature and/or maintains. to ensure that any requirement to pay LL ¶ Complexity of system fees is binding. Service charge will LL ¶ Wessex Water supports the use of LL ¶ Level of operation and maintenance SuDS to manage surface water flood need to be secured by a rent change (see Section 1, 3.6 Maintenance) risk, sewer flooding and improve water together with an obligation within a LL ¶ Primary function of feature quality. Wessex Water are, at the time sale contract/transfer for new owner to pay the relevant charge. LL ¶ Benefiting parties of SuDS features of writing, reviewing their policy on the Potential parties include those listed maintenance and adoption of SuDS. two
Page 45 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance For further information: Bristol City Council www.bristol.gov.uk/page/environment/ flood-risk-drainage-and-development Email: [email protected] Lower Severn Internal Drainage Board www.lowersevernidb.org.uk Wessex Water www.wessexwater.co.uk/developers Bristol City Council Section two Page 46
March 2015 – Issue Version 1 Annex
Amenity Bristol has a strong network of public area and at the same time complementing Area Plan parks and other green infrastructure the proposed SuDS elements. Relevant Local Plan Policies: elements which any future development The amenity uses of the Floating Harbour DM15 (Green Infrastructure Policies), should acknowledge and consider, and improvements in water quality strengthening and making more resilient DM16 (Open Space for Recreation), DM17 standards being driven through the (Important Open Space), DM19 (SNCIs) the existing citywide Green Infrastructure Water Framework Directive have resulted (GI) network. and DM31 (Local Historic Parks and in a particular emphasis on drainage Gardens) In general terms, development that systems discharging directly to the harms the amenity of an area is not seen Floating Harbour to include robust SuDS Core Strategy Policies BCS11 favourably. Therefore, any development to improve quality. (Development Principles), BCS9 proposals should be sited and designed (Green Infrastructure), BCS13 (Climate in a way as to avoid adverse impacts Plans & GIS Layers: BCC Boundary, Change), BCS15 (Sustainable Design and on amenity by reason of smell, water Roads, Rail, Water Courses, Local Construction), BCS20 (Effective and pollution or others. The inclusion of well- Sites (SINCs, SNCIs, County Wildlife Efficient Use of Land), BCS21 (Quality designed SuDS, as important assets of Sites), World Heritage Sites, Country Urban Design), BCS22 (Conservation and the wider GI network, has the potential Parks, AONB, Ancient Woodland, the Historic Environment), 1997 Saved Guidance Design Drainage Sustainable Local Bristol to increase the profile and profitability of Registered Parks and Gardens, National Policies NE01 (Open Space), B05 (Layout new developments as well as to cater for Trails, National Cycle Network, Local and Form), B15 (Conservation Areas: amenity, recreation, well-being and other Cycle Network, Public Rights of Way, Streets and Open Spaces), Core Strategy social and environmental benefits. Even Conservation Areas, NE9 Parks and Policies BCS23 (Pollution), 1997 Saved if an area is deemed to be affected by Gardens, Common Land and Village Policies NE01 (Open Space), B05 (Layout temporary flooding, is still possible to make Greens, DM25 Greenways, Open Access and Form), B15 (Conservation Areas: it multifunctional through conscious design Land, Woodland Trust Land, Tree Streets and Open Spaces) solutions regarding access, materials, Preservation Orders, Living Environment Deprivation Index. Core Strategy Policy BCS9 contains the structures and planting. Other solutions requirement ‘new Development should should also be considered like tree and References: Magic Map Application, incorporate, or contribute towards an hedgerow planting and green roofs as Bristol Pinpoint map, Bristol Parks and appropriate level and quality of open means to improve the amenity value of an Green Space Strategy, Bristol Central space’. two
Page 47 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Policies DM15 and DM16 of the development creates substantial need, Site Allocations and Development new open space for recreation may Management Policies document (adopted need to be delivered as part of the in July 2014) set out the requirements development. This would be dependent for provision of new public open spaces; on the size of the development, need for including a requirement that the spaces open space for recreation in the locality are provided to be as multifunctional as and practicability of provision. DM16 sets practicable. out the requirements for new open space The first policy provision of DM15 sets provision. out that any green infrastructure assets DM16 and Appendix 1 embed in planning provided should maximise multifunctional policy the open space standards within benefits and role, wherever practicable the Parks and Green Spaces Strategy and viable. Open spaces, landscape (PGSS). The PGSS is an evidence base features, or any wildlife habitats provided document for the purposes of local or enhanced (covered by DM19) that are plan preparation. It was adopted by designed to also serve a SuDS function the Council in 2008 and remains the will be considered to respond positively Council’s adopted strategy for Parks and to this aspect of the policy. Green Spaces. Paragraph 2.16.8 of DM16 is explicit in The Planning Obligations SPD was its support for open space which is adopted on September 2012 (and also designed to have a SuDS function. replaced SPD4). This details Landscaping DM16 requires development to meet the Scheme and Tree obligations. Provision need it creates for public open space. of, or improvements to, open space as DM16 sets out standards for a quantity identified in the PGSS will be covered (per person), quality and distance to via Community Infrastructure Levy public open space for recreation. These contributions. The justification in respect standards are shown in Appendix 1 of of the provision of Landscaping Schemes the Site Allocations and Development is set out in Policies BCS9 and BCS11 of Management Policies document. the Core Strategy. Supporting text of DM16, 2.16.5 and 2.16.6, provide further guidance as to
Bristol City Council how this policy requirement is practicably Section applied. In certain circumstances, where two Page 48
March 2015 – Issue Version 1 Biodiversity Bristol contains a wide range of important nature conservation sites that contribute to a varied stock of natural habitats and species. The use of SuDS to contribute towards maintaining and enhancing the biodiversity of Bristol is encouraged. Plans: SSSIs, SAC/SPA/Ramsar etc. designated areas from GIS data References: Bristol Biodiversity Action Plan (BBAP) www.bristol.gov.uk/sites/de- fault/files/assets/documents/BBAP.pdf Relevant Local Plan Policies: Core Strategy policy BCS9 provides details on the requirement to maintain Guidance Design Drainage Sustainable Local Bristol the integrity of the wildlife network, along with development management policy DM19. This provides further detailed criteria for the consideration of proposals affecting nature conservation sites and features of value in Bristol. The use of SuDS can contribute towards complying with these requirements.
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Page 49 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance The city has two sites of international importance: The Severn Estuary, which is a Special Protection Area (SPA), Special Area of Conservation (SAC) and Ramsar site, and; the Avon Gorge SAC. The findings of the Severnside & Avonmouth Wetland Habitat Project, October 2010 and December 2011 (the Cresswell Study Stages 1 and 2) should also be taken into account in determining any proposals which affect the international designations of the Severn Estuary. There are also currently five Sites of Special Scientific Interest (SSSI) in Bristol, which are of national importance for habitat conservation value. The SPA, SAC and Ramsar international sites receive the highest level of protection and no significant negative effects upon the habitats, species and special features of the sites will be permitted. Development on land within or outside SSSIs will be expected to meet the requirements for SSSIs set out in the National Planning Policy Framework. As well as these statutory designated sites, Local nature conservation sites help to ensure the habitats, species and features of value are adequately protected and allow for appropriate public access to nature. Local nature
Bristol City Council conservation sites in Bristol include Sites
Section of Nature Conservation Interest (SNCIs), two Page 50
March 2015 – Issue Version 1
Regionally Important Geological Sites and local nature conservation sites are the Wildlife Network. Where loss of (RIGS) and Wildlife Corridors . These shown on the Bristol Policies Map. nature conservation value would arise sites provide a refuge for flora and fauna; Policy DM19: Development and Nature development will be expected to contribute to national biodiversity and Conservation of the Bristol Local Plan provide mitigation on-site and where geodiversity targets; add to the local - Site Allocations and Development this is not possible provide mitigation character and distinctiveness of an area; Management Policies (Adopted July off-site. contribute to quality of life; enhance 2014) builds on the adopted Core Development on or adjacent to sites the natural processes that support Strategy policy BCS9. This emerging of nature conservation value will be quality of life by maintaining air, soil policy provides further detailed criteria expected to enhance the site’s nature and water quality; and can also reduce for the consideration of proposals conservation value through the the effects of flooding and pollution. affecting nature conservation sites and design and placement of any green Together the SNCIs and connected features of value in Bristol. This policy has infrastructure provided. sites in Wildlife Corridors form the been used to provide a summary of the Development which would have Bristol Wildlife Network. This network ecological resource and value of Bristol a harmful impact on the nature strengthens the resilience of species and how that sits within any type of conservation value of a Site of Nature and habitats to changes in the built and development, including changes and new Conservation Interest will not be natural environment, including rising drainage strategies. Within Policy DM19 permitted. Guidance Design Drainage Sustainable Local Bristol temperatures and flood risk associated it is stated ‘Development which would be Development which would have a with climate change. In order to maintain likely to have any impact upon habitat, harmful impact on the connectivity and the integrity of the Wildlife Network, as species or features, which contribute function of sites in Wildlife Corridors required by Core Strategy policy BCS9, to nature conservation in Bristol will be will only be permitted where the loss in development proposals will be expected expected to: to understand the role and route of any connectivity, or function, of an existing i. Be informed by an appropriate survey Wildlife Corridors on a development Wildlife Corridor is mitigated in line with and assessment of impacts; and site and any habitats and features the following hierarchy: which contribute to a Wildlife Corridors ii. Be designed and sited, in so far as a. Creation of a new wildlife corridor function, along with current and potential practicably and viably possible, to within the development site; species which might utilise the wildlife avoid any harm to identified habitats, corridor. The Bristol Nature Conservation species and features of importance; a. Enhancement of an existing corridor Map displays the location of Wildlife and or creation of a new corridor off-site to Corridors within Bristol and also gives an iii. Take opportunities to connect any maintain the connectivity of the Bristol overview of valuable habitats and species identified on-site habitats, species Wildlife Network.’
on each site. Statutory designated sites or features to nearby corridors in two
Page 51 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Protected Species within Bristol are and Wildlife Corridors). This can be subject to separate legislation which used to initially identify where these determines appropriate development nature conservation assets exist on a and approaches to mitigation. Protected development site. Species legislation will need to be met An appropriate survey and assessment before planning permission can be of impacts will also be needed to granted. To assist in determining the type determine developments likely to impact of development and locations where upon Species, or Habitats of Principal Protected Species might be present, Importance. To assist in determining guidance is contained within a BCC the type of development and locations report ‘Bristol survey and assessment of where Species or Habitats of Principal impacts upon Nature Conservation’ (In Importance might be present, guidance preparation and which may be discussed is contained within the ‘Bristol survey with BCC officers). and assessment of impacts upon Nature Species or Habitats of Principal Conservation’. The guidance note Importance15 are determined under assists with understanding the type Section 41 of the Natural Environment of survey and assessment that should and Rural Communities Act 2006. It sets be undertaken to inform assessment out the habitats or species of Principal of impact, potential harm, suitable importance for the conservation of development and potential need for biodiversity in England. Principal Species mitigation or compensation. of Importance in Bristol include otters, water voles, hedgehogs and house sparrows. Often Species or Habitats of Principal Importance will be contained with local nature conservation sites such as SNCIs and Wildlife Corridors. The Bristol Nature Conservation Map (available through the Bristol Regional Environmental Records Centre16) sets out the known Species and Habitats
Bristol City Council of Principal Importance on sites in
Section the Bristol Wildlife Network (SNCIs
15 www.naturalengland.org.uk
two 16 www.brerc.org.uk Page 52
March 2015 – Issue Version 1 Heritage Approach to SuDS for designated heritage assets Designated heritage assets are those Where the assessment identifies that that are afforded statutory protection by direct impacts may occur, the design means of Scheduled Monument status of the proposed development should for archaeological sites and monuments, be amended to remove these direct or Listed Building status for buildings impacts; direct impact upon designated of national significance (an initial guide assets would likely be considered as to relevant assets within Bristol is substantial harm, which under NPPF available17). When planning works in should only be permitted under the vicinity of designated assets there exceptional circumstances18. For such a are 2 primary concerns: direct impacts development to proceed it would have (changes to the fabric of the asset) and to be demonstrated that there in an impacts upon setting (visual or noise overwhelming need for the development effects that reduce the significance of the and that the design selected is the only asset). practical solution. Guidance Design Drainage Sustainable Local Bristol At the initial planning stage, English Consultees for developments affecting Heritage GIS datasets should be designated assets will be: consulted to identify whether designated LL ¶ Bristol City Archaeological Officer assets are present in the vicinity of LL ¶ English Heritage (Scheduled the proposed works; for an urban Monuments) environment the search radius should be 500m. This process would usually form LL ¶ BCC Conservation Officer (Listed part of an archaeological desk based Buildings) assessment, required by NPPF as the most basic level of assessment required to consider the impacts of a development upon heritage assets.
two 17 www.bristol.gov.uk/knowyourplace 18 planningguidance.planningportal.gov.uk/blog/policy/achieving-sustainable-development/delivering-sustainable-development/12-conserving-and-enhancing-the-histor- ic-environment/ Page 53 Section Section Bristol City Council Council City Bristol March 2015 – Issue Version 1
Bristol Local Sustainable Drainage Design Guidance Approach to SuDS for non-designated Landscape & Townscape Character heritage assets We seek to preserve and enhance the The approach where non-designated character of neighbourhoods as set heritage assets are concerned is similar to out in the adopted Core Strategy and that for designated assets; however there Development Management Policies. is substantially greater leeway with regard Emerging highway protocols also seek to impacts than for designated assets. An to reinforce this aspiration by seeking to archaeological desk based assessment ensure that historic street materials are would be required as the first stage of retained, whether in a conservation area assessment, drawing on data held by the or not, to enhance local character and city Historic Environment Record. Where distinctiveness. potential direct impacts are identified, Planning Policy requires developers to discussions should be held with the City carefully consider the context and setting Archaeological Officer to determine of future development and to make whether archaeological surveys are use of documentation which helps to required to investigate the nature of inform the proposed layouts and detailed the archaeological remains present designs. Detailed assessments will be and to design mitigation proposals as required whenever a development is appropriate. proposed within or near a designated site For impacts upon locally listed buildings or area. the BCC Conservation Officer should The layout of the built environment be consulted with regard to appropriate makes a key contribution to creating mitigation for the loss of the building, quality urban design. Policy DM27: Layout although there may be circumstances and Form includes the expectation that where a locally listed building is slightly the landscape design and planting of below the threshold for full Listed development will integration SuDS. Building status, where demolition would not be deemed acceptable. BCC area sits within two National Character Areas; the Severn and Avon Vales, to the Northwest of the M5 and the Bristol, Avon Valleys and
Bristol City Council Ridges for the remaining area. It lacks a
Section Townscape Character Assessment but two Page 54
March 2015 – Issue Version 1 a Citywide Urban Context Analysis will Character Areas, Local Sites (SINCs, be undertaken as set out under policy SNCIs, County Wildlife Sites), World BCS21 and several Character Appraisals Heritage Sites, Country Parks, AONB, have been adopted for a large number of Agricultural Land Classification, Ancient Conservation Areas. These appraisals also Woodland, Registered Parks and set a series of management proposals Gardens, National Trails, National Cycle for each area. Bristol City character is Network, Local Cycle Network, Public predominately urban with Clifton Down, Rights of Way, Conservation Areas, NE9 Blaise Castle Estate and Stoke Park Parks and Gardens, Common Land and being the main large scale areas of open Village Greens, DM25 (Greenways), DM27 green space. The River Avon limits the (Layout and Form), Open Access Land, south western boundary of the urban Woodland Trust Land. area, which stretches from Avonmouth References: National Character Area to the Floating Harbour, for then to cut 106, National Character Area 118, Magic through the urban fabric on a West- Map Application, Conservation Areas East direction. The Floating Harbour, the Character Appraisals, Bristol Pinpoint Feeder Canal and the Avon New Cut are
map, Bristol Central Area Plan Policies Guidance Design Drainage Sustainable Local Bristol central features which have been and will DM17 (Important Open Space), DM19 be shaping the development around this of the Port, its supporting infrastructure (SNCIs) and DM31 (Local Historic Parks central area of Bristol. and activities. and Gardens), Core Strategy Policies Several smaller water courses run from BCS11 (Development Principles), BCS9 Future development should aim to the north and south towards the Avon (Green Infrastructure), BCS13 (Climate protect and enhance the character, originating numerous valleys and hills Change), BCS15 (Sustainable Design and distinctiveness, diversity and quality which give Bristol an undulating character Construction), BCS20 (Effective and of the landscape and townscape. The and provide a series of green corridors. Efficient Use of Land), BCS21 (Quality references below should be considered It comes natural that some of the Urban Design), BCS22 (Conservation and throughout the development process aforementioned open green spaces are the Historic Environment), 1997 Saved in order to create a landscape which located within incised valleys, like Blaise Policies NE01 (Open Space), B02 (Local integrates SuDS in the most sympathetic Castle Estate and Stoke Park, which run Context), B05 (Layout and Form), B15 way into the existing landscape and along Hazel Brook and the River Frome (Conservation Areas: Streets and Open townscape of the area. respectively. The Avonmouth area is Spaces), severed by the M5 and has a markedly Plans & GIS Layers: BCC Boundary, industrial character due to the presence Roads, Rail, Water Courses, National two
Page 55 Section Section Bristol City Council Council City Bristol Image © Ove Arup and Partners Ltd BD6791 Designed Bristol by Design, Bristol City Council, April 2015 Southmead Green Streets – concept visualisations © Ove Arup and Partners Ltd