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
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Retrofitting SuDS
Virginia Stovin
Department of Civil and Structural EngineeringPennine Water GroupUniversity 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
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Urban Stormwater Management
UK Sewer System – increased urbanisation
System capacity
Sewerflow
Time
Surfacerunoff
Industrialdischarges
Sanitarysewage
Treatmentworks
River
Combined Sewer System
Rainfall
Combined sewer overflow (CSO)
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Combined Sewer Overflows (CSOs)
‘Traditional’ Engineering Solution
Sanitarysewage
Surfacerunoff
Industrialdischarges
Treatmentworks
River
Storage tank
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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 CSOsupgraded 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
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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
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
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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.
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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 anoverall scheme.
• Water quality – The use of a range of structures, forming a treatment train, has significant advantages for water quality.
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Surface Water Management Train
Source control
Regional control
Site control
Conveyance
Conveyance
Discharge to watercourse or groundwater
Discharge to watercourse or groundwater
Discharge to watercourse or groundwater
UK Examples – Gipton, Leeds
To land drains
Soakaway/Infiltration
Swales
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Cost/Performance comparison
0
20000
40000
60000
80000
100000
120000
0 500 1000 1500 2000 2500 3000
Construction costs (£1,000s)
Pred
icte
d an
nual
CSO
spi
ll Vo
lum
e (m
3 )
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?
Potential swale and off-site infiltration basin network0.375 ha residential roof area to off-site infiltration
in preference to source-based storage
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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
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SNIFFER Project – Caw BurnCulvert drains to Burn,Adverse impacts on water quality
SNIFFER – Phase I: Feasibility Assessment
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SNIFFER – Phase II: Detailed Design
Large roofs*
Car parks
Residential roofs
Privately-owned
Source control
Conveyance and offsite control
Large roofs*
Car parks
Highways*
Publicly-owned
Retention at source: green roofs and porous car parks
Infiltration
Disposal
Storage
Reuse
Site/regional controls
Separately seweredsystem or branch
Decreasingpracticality of implementation
Mode of operation
Surface water management
train
Urban Surface Type
*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?
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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
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Green Roofs
Green Roofs
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Green roof hydrology
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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 m2 – largest green roof retrofit in Europe
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Impact of Green Roof Legislation in Linz
International Indicators of Performance
• 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?
• 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?
Modelling/design questions
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Quantifying Performance
• Roof configuration variables– Slope– Drainage layer characteristics– Substrate type and depth– Plant type