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Technical Blog: Planning - Flood Risk Assessments and flood design

— 11 Dec 2018 by Rick Gilroy


Historic Mill and linked extension on stilts within Flood Zone 3


Mill House, Lee Bay – A current challenging project of ours on the wild North Devon coast


The Farmer’s Arms Woolsery incorporated a permeable landscape along with underground attenuation tanks to take the rainwater runoff from the roofs


We integrated a rainwater harvesting system with recent re-roofing works at St. Peter’s, Ilfracombe

Flood technical thumbnail Large

Flood risk to existing and new buildings has become an increasing concern from the impacts that intense and prolonged rainfall has on the UK’s rivers, sea and surface water drainage.

Continued climate change is likely to increase the risks of flooding, threats to life and damage to property.

National Planning Policy seeks to avoid new development in flood risk areas however there is a vast quantity of existing buildings at risk and the need for increased housing puts pressure on Local Authorities to consider areas within flood risk zones.

Flood zones

Any proposed development will fall within either flood zones 1, 2 or 3 as described below: 

  • Flood zone 1 – Low Probability - Land having a less than 1 in 1,000 annual probability of river or sea flooding.
  • Flood zone 2 – Medium Probability - Land having between a 1 in 100 and 1 in 1,000 annual probability of river flooding; or land having between a 1 in 200 and 1 in 1,000 annual probability of sea flooding.
  • Flood zone 3a – Land having a 1 in 100 or greater annual probability of river flooding; or land having a 1 in 200 or greater annual probability of sea flooding.
  • Flood zone 3b – This zone comprises land where water has to flow or be stored in times of flood.

Flood risk is considered at the start of any project we work on as part of our holistic design approach. We consider impact upon the property we are working on but also the impact any proposals could have upon the wider environment. We often look for sustainable drainage solutions that help slow the flow of storm water from roofs and landscape to the nearest watercourse helping relieve the burden on existing drainage infrastructure which is often at full capacity. This can be designed into a project in a number of creative ways such as using permeable surfaces, integrating landscape features such as ponds or reed beds or by introducing soakaways and rainwater harvesting systems.

Our design approach can factor in both flood resistance and flood resilience measures.

Flood resistance measures aim to protect a building or structure by preventing water entering whereas flood resilience measures allow for flood water to enter and then manage how the water gets out, impacts on finishes, fit out, services etc.

Our conversion of a historic mill with a contemporary extension in North Devon is a good example of flood resistant design. This site was in a Flood Zone 3 however we gained planning permission and Listed Building Consent with an innovative design raised on stilts allowing flood water to pass below the building.

We have also incorporated many flood resilient measures to existing buildings such as non-return valves to pipework, door barriers, flood resistant materials, sump and pump systems etc.

For particularly challenging sites we often work alongside specialist consultants and are able to provide the necessary documentation required by the Local Authority such as Flood Risk Assessments, Sequential testing and Flood Management Plans.

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