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Ten Ways to Manage Flood Risk by Designing for Exceedance

Climate change is bringing extremes of weather, amongst the most damaging of which are extreme downpours of precipitation which overwhelm the ability of urban drainage systems to take the water away, and cause flooding.

Surplus water flow pathway in Upton

A swale designed to accommodate surplus surface water at times of high rainfall can also be a pleasant feature that makes space for nature in an urban environment.

CIRIA, the nonprofit member organisation, the Construction Industry Research and Information Association, has for many years been promoting sustainable urban drainage and conducting research as to its effectiveness. Now it has issued a set of guidelines and recommendations for local authorities, developers, governments, policymakers and the like on how to manage the risk of flooding in urban areas.

Flooding can be managed by designing safe and resilient flood routes, temporary storage areas, and making use of other measures, in a set of techniques CIRIA calls "designing for exceedance".

Designing for exceedance can make the most of shared (public or private) spaces and create multifunctional infrastructure. Examples include the use of highways to channel water, raising or dropping curbs to redirect water and using car parks and green open space to store it.

Water may be prevented from entering properties using appropriately levelled from protection and walls, both retrofitting them in exist in areas and making sure that developers include such features in new developments.

A variety of exceedance design strategies

A variety of exceedance design strategies.

The challenges require different disciplines, people and organisations to work together in partnership.

  • For new developments, spatial planners, urban designers, landscape architects and architects are the key players.
  • For retrofitting, drainage engineers may be the first to be involved, before a wider range of disciplines help develop solutions.

Designing for exceedance is still a very new discipline and the CIRIA guidance is packed with ideas about how to manage it. CIRIA says that we must accept that infrequently we may have water on the surface in specified and designed places in order to reduce the impact of flooding.

Considering drainage at the start of the scheme helps to incorporate designing for exceedance into a master plan and drainage concept.

Diagram showing how to design for exceedance at the side of a road

Diagram showing a method of designing for exceedance at the side of a road.

A regeneration scheme in Camborne, Poole and Redruth in Cornwall did this and produced projects that allowed exceedance to occur safely in a controlled manner for rainfall events with a 1 in 200 chance of occurring in any given year.

Another scheme near Cambridge had a drainage system designed for rainfall with a 1 in 100 chance of happening in any year. Computer modelling is often used to predict what may happen with different levels of precipitation.

Engaging the community successfully supports an improved understanding of flood risk and how to deal with it. This is easier in areas where flooding has already occurred. The same applies to designing for exceedance. It's possible for the community to become part of the solution by taking an active role in managing flooding on the surface when it happens.

A flood management plan in Bodmin, Cornwall, England.

A flood management plan in Bodmin, Cornwall, England. (Click on the image for a larger version.)

The 10 recommendations

CIRIA makes the following recommendations to local authorities, highway authorities and flood authorities:

  1. promote collaboration within and between organisations to increase awareness and share expertise;
  2. make the most of multi-functional infrastructure and shared space, such as playing fields and highways to temporarily store water;
  3. use exceedance approaches in the design of solutions to manage local flooding;
  4. make the most of opportunities when regenerating or building new urban places, drainage and roads to include designing for exceedance even in areas with no previous history of surface flooding;
  5. set planning conditions and follow frameworks and design codes;
  6. use ISO 31000:2009 to assess risk levels for different stakeholders as part of the design process;
  7. manage exceedance as part of ongoing regeneration and redevelopment;
  8. use demonstration projects, pilots and case studies to provide confidence and inspire practitioners;
  9. ensure that developers demonstrate that exceedance is designed for and managed in planning applications;
  10. ensure designing for exceedance approaches are incorporated into local flooding risk management strategies.

 More information is available on the CIRIA website.