Retrofitting SuDS. Virginia Stovin. Department of Civil and Structural Engineering Pennine Water Group University of Sheffield.

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1 Retrofitting SuDS Virginia Stovin Department of Civil and Structural Engineering Pennine Water Group University of Sheffield Outline Urban stormwater management Conventional approach, problems and costs Sustainable (urban) Drainage Systems (SuDS) Retrofit SuDS theory and practice Green roofs an underutilised source control Conclusions 1

2 Urban Stormwater Management UK Sewer System increased urbanisation Industrial discharges Surface runoff Combined Sewer System Sanitary sewage Treatment works River Combined sewer overflow (CSO) Rainfall System capacity Sewer flow Time 2

3 Combined Sewer Overflows (CSOs) Traditional Engineering Solution Sanitary sewage Industrial discharges Surface runoff Storage tank Treatment works River 3

4 Indicative Investment in Conventional CSO Rehabilitation 5 year investment programme worth nearly 1.5 billion. 39 million to resolve sewer flooding at 386 properties and to resolve outdoor flooding at 88 locations. Around 95 of Sheffield's CSOs upgraded at a cost of 30 million. Concrete storage chambers in four of Sheffield s public parks, each probably costing in the order of 1 million. Thames Tideway Strategic Study 7.2 m diameter storage and transfer tunnel, new STW 34.5 km long, 1.5 billion 4

5 Limitations of conventional approach Financial costs Hard engineering Increased volumes of (diluted) sewage passed on to treatment works waste of resources treating rainwater Storage tanks and screens require maintenance Treats stormwater as a nuisance rather than a resource Not future proof Sustainable (urban) Drainage Systems (SuDS) 5

6 SuDS = Sustainable (urban) Drainage Systems SuDS (or source control) technologies attempt to solve the problem by mimicking nature Infiltrate stormwater into ground Store water for gradual release, evaporation or use Toolbox of technologies Quantity, quality, amenity Developers strongly encouraged to use SuDS on new developments Retrofit SuDS Retrofit when SuDS approaches are intended to replace and/or augment an existing drainage system in a developed catchment. Examples of retrofit SuDS: the diversion of roof drainage from a combined sewer system into a garden soakaway the conveyance of road runoff via roadside swales into a pond sited in an area of open space Installation of green roofs 6

7 Augustenborg, Malmö, Sweden Inner-city suburb in Malmö, CSO and flooding problems In 2001 Augustenborg was disconnected from the existing combined sewer and drained by means of an open stormwater system. Stormwater is now led through a complex arrangement of green roofs, swales, channels, ponds and small wetlands. 7

8 Gipton, Leeds (1 of 4 sub-catchments) Contributory surface area: 80 ha Residential area (largely semi-detached housing and some institutional buildings) North of catchment underlain by millstone grit (high permeability) South of catchment underlain by mudstone (low permeability) CSO discharges in very accessible public area Which (retrofit) SuDS technology? Infiltration-based components are designed primarily to dispose of the water into the ground complete removal from the stormwater drainage system require permeable substrate (not clay) Storage-based components retain a portion of the flow, but have a finite capacity; once capacity is reached they will pass flows into the stormwater drainage system Some SuDS components (e.g. swales incorporating checkdams) may provide both; many SuDS systems offer a combination of both by integrating a range of structures into an overall scheme. Water quality The use of a range of structures, forming a treatment train, has significant advantages for water quality. 8

9 Surface Water Management Train Conveyance Source control Conveyance Site control Discharge to watercourse or groundwater Discharge to watercourse or groundwater Regional control Discharge to watercourse or groundwater UK Examples Gipton, Leeds Swales Soakaway/Infiltrati on To land drains 9

10 Cost/Performance comparison Predicted annual CSO spill Volume (m 3 ) Construction costs ( 1,000s) Existing New CSO SUDS 100/100 SUDS 100/50 SUDS 80/100 SUDS 80/50 SUDS 60/100 SUDS 60/50 SUDS 40/100 SUDS 40/50 SUDS 20/100 SUDS 20/50 Designing retrofit SuDS What should I disconnect (houses, roads, hospitals)? How will that affect my system hydraulics? Should I infiltrate or dispose or store or re-use? Which technology best suits my situation? What catchment data should I collect? Shall I develop a regional scheme with conveyance or is it always best to deal with rainfall at source? Will property owners accept my suggestions? Who will maintain the scheme (adoption issues)? How much will is cost? 10

11 Conceptual Basis Increasing complexity (in terms of detailed design work required) Urban surface type Surface water management train Mode of operation Cost Decreasing order of preference Institutional roofs Car parks Residential roofs Highways Source control Conveyance and offsite control Infiltration Disposal Storage Re-use Cheapest Most expensive Are INSTITUTIONAL ROOFS connected to combined system? yes Explore viability of SOURCE CONTROL SuDS Infiltration SuDS Disposal SuDS Storage SuDS Reuse SuDS 1. Suitable soil percolation rate (>4.63 x 10-6 m/s) 2. Groundwater contamination risk 3. Water table level 4. Space for construction 5. Building regulations 6. Responsibility/maintenance/safety 1. Adjacent watercourse 2. Discharge consents 1. Space for construction 2. Water table level 3. Overflow Basins Soakaways Ponds + Infilt. Trenches Porous Pavements + Redirect to watercourse Basins Ponds + Porous Pavements + Details of relevant design guidance Reuse + Do these measures resolve the catchment s hydraulic problems? yes no STOP Denotes per device, based on a 200m 2 contributory surface Continue through framework: (Conveyance, Site/Regional controls, Car-parks, residential roofs, roads) 11

12 The Meanwood Catchment 4 km NW of Leeds City Centre 55.8 ha Tongue Lane Parkside Road West Lea King Alfred s Parklands Stonegate Road Location of flooding Trunk sewer Meanwood Road 12

13 Application of the framework Increasing complexity (in terms of detailed design work required) Urban surface type Surface water management train Mode of operation Cost Decreasing order of preference Institutional roofs Car parks Residential roofs Highways Source control Conveyance and offsite control Infiltration Disposal Storage Re-use Cheapest Most expensive Viable region for infiltration-based retrofit source control SuDS (3.022 ha of residential roofs) Region initially allocated to storagebased retrofit source control SuDS (4.348 ha of residential roofs) 13

14 Application of the framework Increasing complexity (in terms of detailed design work required) Urban surface type Surface water management train Mode of operation Cost Decreasing order of preference Institutional roofs Car parks Residential roofs Highways Source control Conveyance and offsite control Infiltration Disposal Storage Re-use Cheapest Most expensive Potential swale and off-site infiltration basin network ha residential roof area to off-site infiltration in preference to source-based storage 14

15 Proposal Disconnect ha of residential roofs using soakaways Disconnect ha of residential roofs and ha of paved area using swales-based off-site controls (infiltration basins) (46% of roofed area; 31% of paved area) Retrofit water butts to remaining ha roofed area 68% reduction in the ten year design storm flood volume; need to be coupled with reduced level of conventional sewer rehabilitation (hybrid solution) UK Retrofit SuDS Implementation Case Studies Cromer, North Norfolk Storm sewer flooding Water-stressed area Good infiltration characteristics Obvious retrofit opportunities Not supported by current water industry funding structures or legislation 15

16 SNIFFER Project Caw Burn Culvert drains to Burn, Adverse impacts on water quality SNIFFER Phase I: Feasibility Assessment 16

17 SNIFFER Phase II: Detailed Design Urban Surface Type Surface water management train Mode of operation Decreasing practicality of implementation Separately sewered system or branch Publiclyowned Privately -owned Large roofs* Car parks Highways* Large roofs* Car parks Site/regional controls Source control Conveyance and offsite control Retention at source: green roofs and porous car parks Infiltration Disposal Storage Reuse Residential roofs *Water quality improvements may be maximised by disconnecting industrial/commercial roofs and/or highways; however adequate protection against local contamination needs to be ensured in the design of SUDS options But how would this type of retrofit be funded? 17

18 Barriers to SuDS retrofitting Practical problems Existing site layouts and infrastructure, particularly in high density urban environments Multiple ownership Legislation and the way the water industry is structured in the UK acts against water utilities, environmental regulators and local authorities working collaboratively with this type of approach Driver/incentive/funding mechanism DEFRA pilot projects starting to tackle this Salford/Lower Irwell IUD Pilot Single-owner roof space 18

19 Green Roofs Green Roofs 19

20 Green roof hydrology 20

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23 UK Example of Retrofit Green Roof Ethelred Housing Estate, Lambeth Estate considered for demolition in the early 1990s Tenant Management Organisation opposed demolition Various refurbishment works required including roofing repairs Tenants proposed green roof 6000 m 2 largest green roof retrofit in Europe 23

24 Impact of Green Roof Legislation in Linz Modelling/design questions How should I design my green roof to retain the first 12 mm of a 1 in 10 year rainfall event? What costs saving in sewer storage implementation would be achieved if 50% of office buildings in Sheffield were retrofitted with green roofs? International Indicators of Performance Test facilities roof configuration variables and planting Instrumented full scale roofs Annual retention of 45-70% rainfall volume Peak runoff reduction of up to 100% Variations between storm events and between locations How relevant are these indicators in a UK climatic context? 24

25 Quantifying Performance Roof configuration variables Slope Drainage layer characteristics Substrate type and depth Plant type Climatic variation Annual rainfall Predominant rainfall characteristics Links between the two plant growth and health Need for local data and for appropriate engineering modelling and design tools Green Roof test rig 25

26 Monitored data for Spring 2006 Rainfall (mm) Runoff (mm) Average volume retention 34% Average peak reduction 56.9% Scientific data mm rainfall 3.55 mm runoff 61% retention 61% peak reduction Significant attenuation Engineering models and design tools Implementation? Conclusions Retrofit SuDS may offer a practical option for addressing current problems associated with stormwater quantity and quality in urban areas Range of design options potentially there to be matched to constraints of existing land uses and layouts Decision-support framework assists with identifying most appropriate options Green Roofs merit further consideration and research Usefulness of modelling tools integrated models required for urban flooding problems Implementation/adoption issues 26