Retrofitting SuDS Virginia Stovin Department of Civil and - PDF document

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

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

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

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

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

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

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

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

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

Cost/Performance comparison Existing 120000 Predicted annual CSO spill Volume New CSO 100000 SUDS 100/100 SUDS 100/50 80000 SUDS 80/100 (m 3 ) SUDS 80/50 60000 SUDS 60/100 40000 SUDS 60/50 SUDS 40/100 20000 SUDS 40/50 0 SUDS 20/100 0 500 1000 1500 2000 2500 3000 SUDS 20/50 Construction costs (£1,000s) 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

Conceptual Basis Increasing complexity (in terms of detailed design work required) Urban surface Surface water Mode of Cost type management train operation Decreasing Institutional Source control Infiltration Cheapest order of roofs Conveyance and Disposal Most preference Car parks offsite control Storage expensive Residential Re-use roofs Highways Are INSTITUTIONAL ROOFS connected to combined system? yes Explore viability of SOURCE CONTROL SuDS Infiltration SuDS Disposal SuDS Storage SuDS Reuse SuDS 1. Adjacent watercourse 1. Space for construction 1. Suitable soil percolation rate (>4.63 x 10 -6 m/s) 2. Discharge consents 2. Water table level 3. Overflow 2. Groundwater contamination risk 3. Water table level 4. Space for construction 5. Building regulations 6. Responsibility/maintenance/safety � � ���� + Basins Redirect to watercourse Basins Reuse �� ��� + Soakaways Ponds ��� + Ponds Porous Pavements ���� �������� + Infilt. Trenches Porous Pavements �������� + Details of relevant design guidance Do these measures resolve the catchment’s hydraulic problems? no Continue through framework: yes (Conveyance, Site/Regional controls, STOP � Denotes £0-500 per device, based on a 200m 2 contributory surface Car-parks, residential roofs, roads) 11

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 Trunk sewer Location of flooding Meanwood Road 12

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

Application of the framework Increasing complexity (in terms of detailed design work required) Urban surface Surface water Mode of Cost type management train operation Decreasing Institutional Source control Infiltration Cheapest order of roofs Disposal Most Conveyance and preference Car parks Storage expensive offsite control Re-use Residential roofs Highways Potential swale and off-site infiltration basin network 0.375 ha residential roof area to off-site infiltration in preference to source-based storage 14

Proposal • Disconnect 3.022 ha of residential roofs using soakaways • Disconnect 0.375 ha of residential roofs and 2.886 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 3.973 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

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

SNIFFER – Phase II: Detailed Design Urban Surface Surface water Mode of Type management operation train Decreasing Separately sewered Site/regional Retention at source: practicality of system or branch controls green roofs and porous car parks implementation Publicly- Large roofs* Source control Infiltration owned Car parks Conveyance and Disposal offsite control Highways* Storage Privately Large roofs* -owned Reuse Car parks 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