Green Roofs and Stormwater Management

Virginia Stovin

Department of Civil and Structural EngineeringPennine Water GroupUniversity of Sheffield

Outline

• Urban stormwater management• Conventional solutions, problems and costs• Sustainable (urban) Drainage Systems (SuDS) (BMPs)

– Retrofit barriers and opportunities

• Green roof hydrology– Processes– Potential impacts on storm runoff– Modelling needs and design guidance for engineers

• Conclusions

Urban Stormwater Management

Rural hydrology

InterceptionEvapotranspiration

Marsh

Groundwater

Surface runoff

Storage

Infiltration

Groundwater

Infiltration

Urban hydrology

Interception

Evapotranspiration

Infiltration

Groundwater

ReducedStorage

Surface runoff

Rainfall(mm)

Rural (permeable)

Urban (impermeable)

Surface Runoff

River flow(m /s)3

Time(hours)

Rainfall(mm)

Rural (permeable)

Urban (impermeable)

Surface Runoff

River flow(m /s)3

Time(hours)

Rainfall(mm)

Rural (permeable)

Urban (impermeable)

Surface Runoff

River flow(m /s)3

Time(hours)

Rainfall(mm)

Rural (permeable)

Urban (impermeable)

Surface Runoff

River flow(m /s)3

Time(hours)

Urbanisation – effects on hydrology

UK Sewer System

Sewerflow

Time

Surfacerunoff

Industrialdischarges

Sanitarysewage

Treatmentworks

River

Combined Sewer System

Rainfall

UK Sewer System – increased urbanisation

System capacity

Sewerflow

Time

Surfacerunoff

Industrialdischarges

Sanitarysewage

Treatmentworks

River

Combined Sewer System

Rainfall

Combined sewer overflow (CSO)

Combined Sewer Overflows (CSOs)

Conventional solutions, problems and costs

‘Traditional’ Engineering Solution

Sanitarysewage

Surfacerunoff

Industrialdischarges

Treatmentworks

River

Storage tank

Local Investment in Conventional CSO

Rehabilitation

• YW operates 40,000 miles of water and sewerage mains – enough pipework to circle the earth.

• 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.• £90 million to upgrade and replace Combined Sewer

Overflows across the region.• Around 95 of Sheffield's combined sewer overflows

(CSOs) will be 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

• Proposal to address unsatisfactory CSO discharges• 7.2 m diameter storage and transfer tunnel, new STW• 34.5 km long, £1.5 billion

Costs of CSO Rehabilitation

• OST Foresight Future Flooding report – costs in England for in-sewer storage are between $350 and $1800 per m3 (2004 figures)

• OFWAT (2003) costs for a CSO storage tank:– 750 m3 – £318,000– 3,000 m3 – £647,000

Limitations of conventional approach

• Financial costs• Hard engineering

– Disruptive to infrastructure– Concrete – LCA suggests not a sustainable solution

• 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 or BMPs)

System based solution

• In natural catchments most stormwater infiltrates into the ground at or near to the point where it falls

• Source control technologies attempt to ‘solve’ the problem by mimicking nature– Infiltrate stormwater into ground– Store water for gradual release, evaporation or use

• SuDS = Sustainable (urban) Drainage Systems– “a sequence of management practices and control structures

designed to drain surface water in a ‘more sustainable’ fashion than some conventional techniques” (CIRIA, 2000)

– Developers now ‘strongly encouraged’ to employ SUDS on new developments

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

SUDS Technologies

Source controls• Soakaways • Garden ponds• Green roofs• Water butts and reuse

Porous Pavements

Swales and Infiltration Trenches

Regional scale basins and ponds

Which 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.

The ‘urban drainage triangle’

quantity

amenityAmenity:Landscape; land use; wildlife habitats; recreation …

SuDS

quality

Legislation• PPG25 (Development and

Flood Risk) recommends that SUDS should be considered for new developments and encourages Local Authorities to include them in their development plans.

• The Environment Agency can request LAs put conditions on planning permission such that the developer must restrict runoff from the site to greenfieldlevels for a 100 yr storm event.

Ashford, Nov 2000New housing in SE

Retrofit SuDS• The term retrofit is employed when SUDS-type

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

• Opportunities associated with urban renewal – Planning process treat as new developments

International Examples of SuDS Retrofit• 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.

• Portland Oregon– Community downspout disconnection programmes– Range of other SUDS measures – bioretention areas, green roofs, rain

gardens• Tokyo

– Extensive replacement of storm drainage with infiltration systems• Emscher

– Watercourse rehabilitation (was effectively a combined sewer) and stormwater disconnection programme

Barriers to SUDS retrofitting• Multiple ownership

– Right to connect– Limited formal incentives

• Existing site layouts and infrastructure, particularly in high density urban environments

• (UK) Water utilities – General reluctance to adopt SuDS– Prevent them adding to fixed asset base– Loss of stormwater management income stream– Opportunities associated with new business models linked to

SuDS and/or rainwater harvesting system design/maintenance?

• Single-owner roof space

UK Examples – 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

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?• 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?

Green roof hydrology

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• Peak rainfall intensity 0.4

mm in 5 mins (4.8 mm/hr).

• Peak runoff intensity 0.155 mm in 5 mins (1.86 mm/hr).

• 61% peak reduction• Significant attenuation

14/15 Feb 2006

Monitoring Feasibility Study

• Preliminary results ..• Wet English Spring – Feb to April 2006• Average volume retention of 34%• Average peak reduction of 56.9%• Antecedent Dry Weather Period (ADWP) is significant;

initial losses increase from 2 to 4 mm if ADWP > 2 days

Modelling needs and design guidance• Knowing that a green roof will ‘improve things’ is not the

same as being able to quantify that improvement, or to design systems to meet specific requirements.

• Drainage engineers use drainage modelling tools to design systems (InfoWorks (SWMM), WinDes)– Most appropriate way to represent green roof systems?– Characteristic performance parameters?

• Design guidance for roof systems to meet particular requirements– Greenfield runoff, retention of first 12 mm rainfall ..– CIRIA

• Design of roof drainage• Focus of ongoing research

Conclusions• Urban stormwater management is a major infrastructure issue –

flooding, water quality, amenity• Financial (and other) costs associated with conventional solutions

are significant• SuDS approach now being integrated into new builds; there are

further opportunities linked to retrofit and urban redevelopment• Green roofs are particularly attractive in dense urban areas, as they

‘get SuDS off the ground’, limiting land take• Strong indications that they provide hydrologically-effective

stormwater management, including significant retention and attenuation in a wet UK Spring

• Need to develop/refine modelling tools and design guidance to promote acceptance and understanding by UK urban drainage engineers

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