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Minimising the impact of future flooding

In 2007, flooding caused devastation to both urban and rural areas of the UK. NBS Technical Author Mike O’Brien explains how NBS work sections can help construction professionals to alleviate future flooding.

We have just experienced the wettest July in England since records began 240 years ago.  Due to the flooding, thousands of properties were damaged, hectares of crops were ruined and sadly several people lost their lives as a consequence.

However, we must remember that many other parts of the world have been devastated by flooding.  Hundreds of people have been killed and millions driven from their homes across Bangladesh and parts of India following weeks of torrential rain in the foothills of the Himalayas.

The potential for increased flooding due to the effects of climate change will pose complex challenges in the future, especially for developing counties. In the UK we have the infrastructure and resources to minimise the impact of this flooding.

The Department for Environmental Food and Rural Affairs (Defra) is responsible for government policy and The Environment Agency has overall responsibility for the delivery of flood management. The agency states that flooding cannot be prevented but can be managed, in other words minimising the impact when flooding occurs. For guidance on government policy and objectives relating to flooding, see PPS25 Development and Flood risk and Making Space for Water available from the Defra website.

NBS specification products cover a wide range of construction topics and there are several sections that allow you to specify systems and products that conserve water and safeguard against future flooding including:

• D20 Excavating and filling - this section covers erosion control measures and can be used to specify swales and bunding of earthworks to contain and direct surface water. It also covers the installation of compacted clay liners to ponds and water courses.
• J10 Cementitious mortar tanking/ damp proofing; J20 Mastic asphalt tanking/ damp proofing; J30 Liquid applied tanking/ damp proofing; J40 Flexible sheet tanking/ damp proofing - these sections are primarily used to specify tanking to structures to exclude ground water and dampness and also to line structures containing water.
• J44 Sheet linings for pools/ lakes/ waterways - flexible sheet linings laid to retain water in balancing or attenuation ponds within a sustainable drainage system. When planted with suitable vegetation, these ponds can improve water quality and provide a valuable wetland environment for local animals.
• N13 Sanitary appliances and fittings - low volume flushing WCs are a requirement of the Defra ‘WC suite performance specification’. Other water conserving equipment such as composting toilets, waterless urinals and the use of grey water (see section S17) help to reduce water wastage.
• Q2 Sections - impermeable pavements channel storm water directly into the surface water drainage system.  Permeable pavings of gravel, asphalt or paving block allow percolation of water through the surface, regenerating groundwater and reducing surface water run-off.
• Q30 Seeding/ turfing & Q31 external planting - vegetation on flood plains and riverbanks helps to reinforce surface soil layers, reducing erosion.
• Q35 Landscape maintenance - proper maintenance of the edges of natural and man made water bodies, including dredging, will ensure that water flows are not obstructed by the unnecessary accumulation of silt, vegetation, fallen trees, shopping trolleys and old bikes.
• Q37 Green roofs - Green roofs are recognised by the Environment Agency as an appropriate source control for surface water run-off.  Green roofs contribute to storm water management by absorbing and retaining rainwater, delaying its entry into the storm water system.
• R10 Rainwater drainage systems; R12 Below ground drainage systems; R13 Land drainage and R17 Soakaways; septic tank and sewage treatment units - these sections deal with both foul/rain drainage and include measures for rainwater collection, storage and reuse, soakaways and stormwater attenuation units.  Such measures can be used to reduce, in the short term, the impact of stormwater on drainage systems.   The use of water conserving features such as green roofs and waterless WCs will impact upon the sizing of pipes. The inclusion of groundwater pressure relief drainage systems around new structures will help to direct excess water away. Conversely, soakaways can be used to dispose of excess surface water and to recharge groundwater levels.
• S17 Water reclamation systems - the use of grey water systems within new building developments will reduce the pressure on existing sewers allowing them to cope better with sudden storm flows.

For further information about Sustainable Urban Drainage Systems (SUDS) visit the CIRIA website.

Access to reference documents covering water conservation, water management and flood alleviation, as well as other construction related topics, are available from The Construction Information Service website.

Related NBS information:

Articles:

• Sustainable (Urban) Drainage Systems (SUDS): an introduction
• Sustainable Urban Drainage Systems (SUDS)
• Part H: Drainage and waste disposal

Selected links:

• Drainage, sewerage

Written August 2007

 

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