Green Roofs and Stormwater Management Virginia Stovin Department of Civil and Structural Engineering Pennine Water Group University 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 Interception Evapotranspiration Infiltration Storage Groundwater Infiltration Marsh Surface runoff Groundwater
Urban hydrology Evapotranspiration Interception Reduced Storage Infiltration Groundwater Surface runoff
Urbanisation effects on hydrology Rainfall (mm) Rural (permeable) Urban (impermeable) River flow 3 (m /s) Surface Runoff Time (hours)
UK Sewer System Industrial discharges Surface runoff Combined Sewer System Sanitary sewage Treatment works River Rainfall Sewer flow Time
UK Sewer System increased urbanisation Industrial discharges Surface runoff Combined Sewer System Sanitary sewage Treatment works River Combined sewer overflow (CSO) Rainfall System capacity Sewer flow Time
Combined Sewer Overflows (CSOs)
Conventional solutions, problems and costs
Traditional Engineering Solution Industrial discharges Surface runoff Sanitary sewage Storage tank Treatment works River
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 m 3 (2004 figures) OFWAT (2003) costs for a CSO storage tank: 750 m 3 318,000 3,000 m 3 647,000
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 Conveyance Source control Conveyance Site control Regional control Discharge to watercourse or groundwater Discharge to watercourse or groundwater Discharge to watercourse or groundwater
SUDS Technologies
Soakaways Garden ponds Green roofs Water butts and reuse Source controls
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 quality SuDS amenity Amenity: Landscape; land use; wildlife habitats; recreation
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 greenfield levels for a 100 yr storm event. Ashford, Nov 2000 New 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 m 2 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
14/15 Feb 2006 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 0.00 0.20 Rainfall (mm) 0.10 0.20 0.30 0.40 0.50 0.60 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 0.18 0.16 0.14 0.12 0.10 0.08 Runoff (mm) 0.70 0.06 0.80 0.04 0.90 0.02 1.00 0.00
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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