Retrofitting SuDS Virginia Stovin Department of Civil and - - PDF document

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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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Urban Stormwater Management

UK Sewer System – increased urbanisation

System capacity

Sewer flow Time Surface runoff Industrial discharges Sanitary sewage Treatment works River

Combined Sewer System

Rainfall

Combined sewer overflow (CSO)

3 Combined Sewer Overflows (CSOs)

‘Traditional’ Engineering Solution

Sanitary sewage Surface runoff Industrial discharges Treatment works River Storage tank

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

• rder 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
• f 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

• verall 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

• r groundwater

Discharge to watercourse

• r groundwater

Discharge to watercourse

• r groundwater

UK Examples – Gipton, Leeds

To land drains Soakaway/Infiltrati

• n

Swales

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Cost/Performance comparison

20000 40000 60000 80000 100000 120000 500 1000 1500 2000 2500 3000 Construction costs (£1,000s) Predicted annual CSO spill Volume (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

• peration

Cost Institutional roofs Car parks Residential roofs Highways Source control Conveyance and

• ffsite control

Infiltration Disposal Storage Re-use Cheapest Most expensive Increasing complexity (in terms of detailed design work required) Decreasing

• rder of

preference

Are INSTITUTIONAL ROOFS connected to combined system?

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

Denotes £0-500 per device, based on a 200m2 contributory surface Details of relevant design guidance Basins

• Soakaways
• Ponds

+

• Infilt. Trenches
• Porous Pavements

+ Redirect to watercourse Basins

• Ponds

+ Porous Pavements + Reuse +

Infiltration SuDS Disposal SuDS Storage SuDS Reuse SuDS

Do these measures resolve the catchment’s hydraulic problems? STOP yes 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 sewer Location of flooding

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Application of the framework

Urban surface type Surface water management train Mode of

• peration

Cost Increasing complexity (in terms of detailed design work required) Decreasing

• rder of

preference Institutional roofs Car parks Residential roofs Highways Source control Conveyance and

• ffsite 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 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

• peration

Cost Increasing complexity (in terms of detailed design work required) Decreasing

• rder of

preference Institutional roofs Car parks Residential roofs Highways Source control Conveyance and

• ffsite control

Infiltration Disposal Storage Re-use Cheapest Most expensive Potential swale and off-site infiltration basin network 0.375 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
• pportunities
• Not supported by

current water industry funding structures or legislation

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SNIFFER Project – Caw Burn

Culvert 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

• ffsite control

Large roofs* Car parks Highways* Publicly-

• wned

Retention at source: green roofs and porous car parks Infiltration Disposal Storage Reuse Site/regional controls Separately sewered system or branch Decreasing practicality of implementation

Mode of

• peration

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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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 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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0.00 0.10 0.20 0.30 0.40 0.50 0.60 0.70 0.80 0.90 1.00 14-02-06 22-00 14-02-06 23-00 15-02-06 00-00 15-02-06 01-00 15-02-06 02-00 15-02-06 03-00 15-02-06 04-00 15-02-06 05-00 15-02-06 06-00 15-02-06 07-00 15-02-06 08-00 15-02-06 09-00 15-02-06 10-00 Rainfall (mm) 0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14 0.16 0.18 0.20 Runoff (mm)

• 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