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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

Sustainable (urban) Drainage Systems(SuDS)

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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

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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?

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Conceptual Basis

Urban surface type

Surface water management train

Mode of operation

Cost

Institutional roofsCar parksResidential roofsHighways

Source controlConveyance and offsite control

InfiltrationDisposalStorageRe-use

CheapestMost expensive

Increasing complexity (in terms of detailed design work required)

Decreasing order of preference

Are INSTITUTIONAL ROOFS connected to combined system?

1. Suitable soil percolation rate (>4.63 x 10-6 m/s)

2. Groundwater contamination risk3. Water table level4. Space for construction5. Building regulations6. Responsibility/maintenance/safety

1. Adjacent watercourse2. Discharge consents

1. Space for construction2. Water table level3. Overflow

Denotes £0-500 per device, based on a 200m2 contributory surface

Details of relevant design guidance

BasinsSoakawaysPonds +Infilt. TrenchesPorous Pavements

+

Redirect to watercourse BasinsPonds +Porous Pavements

+

Reuse +

Infiltration SuDS Disposal SuDS Storage SuDS Reuse SuDS

Do these measures resolve the catchment’s hydraulic problems?

STOPyes Continue through framework:

(Conveyance, Site/Regional controls,Car-parks, residential roofs, roads)

no

Explore viability of SOURCE CONTROL SuDS

yes

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The Meanwood Catchment

• 4 km NW of Leeds City Centre• 55.8 ha

Parkside Road

Tongue Lane

West Lea

King Alfred’s

Parklands

Meanwood Road

Stonegate Road

Trunk sewerLocation of flooding

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Application of the framework

Urban surface type

Surface water management train

Mode of operation

Cost

Increasing complexity (in terms of detailed design work required)

Decreasing order of preference

Institutional roofsCar parksResidential roofsHighways

Source controlConveyance and offsite control

InfiltrationDisposalStorageRe-use

CheapestMost expensive

Viable region for infiltration-based retrofit source control SuDS(3.022 ha of residential roofs)

Region initially allocated to storage-based retrofit source control SuDS

(4.348 ha of residential roofs)

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Application of the framework

Urban surface type

Surface water management train

Mode of operation

Cost

Increasing complexity (in terms of detailed design work required)

Decreasing order of preference

Institutional roofsCar parksResidential roofsHighways

Source controlConveyance and offsite control

InfiltrationDisposalStorageRe-use

CheapestMost expensive

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

• 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

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• 9.2 mm rainfall• 3.55 mm runoff• 61% retention• 61% peak reduction• Significant attenuation

Monitored data for Spring 2006

• Average volume retention 34%

• Average peak reduction 56.9%

Scientific data

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