Using green infrastructure to alleviate flood risk
Green infrastructure networks can be used to reduce surface water run-off and store flood water.
By developing and restoring green infrastructure with a presumption in favour of open water courses through channel restoration and de-culverting, a more natural and slower response to heavy rainfall can be achieved.
In this sense, conserving river corridors can help to absorb fluctuating water volumes as well as increasing their aesthetic quality and public enjoyment.
Research by the University of Manchester has shown that:
- increasing the green space cover in urban areas by 10 per cent reduces surface run-off by almost 5 per cent
- increasing tree cover in urban areas by 10 per cent reduces surface water run-off by almost 6 per cent
- adding green roofs to all the buildings in town centres can reduce surface water run-off by almost 20 per cent
New catchment strategies and drainage design should seek to mimic natural drainage processes wherever possible. Green infrastructure strategies should incorporate the management of water quality, flood risk and water resources into wider green infrastructure networks. Information emerging from local (PPG17) green space audits should also inform these as a useful evidence base for strategies.
Land uses across regions and cities should be planned to maximise the potential beneficial impacts on catchment management and run-off. Using landscape character assessment to assess the suitability of land uses can be helpful in achieving this. Factors such as land cover, habitat, soil type, topography and groundwater will combine to influence such character-based decisions.
The regional scale is the starting point for environmental assessment and landscape character work and is therefore an appropriate scale for thinking jointly about green infrastructure and water management. This joint approach should be advocated and investigated as part of strategic environmental assessments and the regional and smaller scales.
At the city scale, some cities have found immense value in producing green and blue grid strategies that encompass both green infrastructure and water management objectives. The East London Green Grid is one example that has been particularly successful in influencing wider planning and design strategies.
By establishing a clear strategy for water management and identifying its functions (existing and potential) in accordance with the principles of water sensitive urban design these documents have steered planned developments towards more sustainable solutions. As a rule, open space planning should integrate multi-functional use of public open space with flood water attenuation in green space, parks and sports facilities. This includes protecting wetlands as natural sites for surface water management and treatment and incorporating wetland creation in planning processes.
At a site scale, public spaces should be designed to incorporate a balance of hard and soft landscape elements to meet surface water challenges. Both planting and surfacing should minimise surface water run-off in addition to meeting other design requirements. Opportunities to maximise permeable paving, retrofit and maintain existing catch basins have a part to play in hard landscaping. Examples include the Green Streets project in Portland and the Green Alleys programme in Chicago, USA. Ensuring creative design of new and refurbished water features such as lakes, ponds and wetlands network can add distinctive character to a site and increase its value for people and wildlife.
Sequential flood risk assessments should be undertaken in conjunction with open space strategies so that there is full consideration of the multi-functional value of green space, wetland networks and river corridors rather than isolated consideration of buildings in relation to flood risk.
Some types of development and green infrastructure are more ‘water compatible’ – and therefore suitable to be located in the floodplain. These include amenity open space, nature conservation and biodiversity, outdoor sports and recreation and essential facilities such as changing rooms, flood control infrastructure, water transmission infrastructure and pumping stations and sewage transmission infrastructure and pumping stations.
Milton Keynes floodplain forest
The floodplain forest is an example of one approach at this scale. The restoration of a site adjacent to the River Nene to forest floodplain required significant extraction of minerals to leave the site an average of 1.27 metres lower.
Once fully completed, it is envisaged that the project will create a total flood storage capacity of 460,000 cubic metres. Work has started and will happen in a number of phases, some requiring five to 10 years to adapt and mature to a natural state.
In addition to the removal of the gravel layer and restoration of the worked areas, the scheme also encourages improved public access though the creation of benches, paths, boardwalks and bridges. The new habitat will include a range of topographically diverse features including braided channels, reeds and ponds and species associated with reedbed, wet grassland, tall herb ‘fen’ vegetation and woodland ecosystems.
A key factor in delivery was the enabling mineral extraction works. The lease and royalities from selling the gravel are passed to the Parks Trust with approximately 35 per cent held in perpetuity to fund long-term management with the remaining balance for capital expenditure.
CABE and Urban Practitioners
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