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Last edited 23 Apr 2021
- Rivers and the sea.
- Rising groundwater.
- Overwhelmed sewers and drainage systems.
- Reservoirs, canals and lakes.
- Other artificial sources.
 Flood zone and flood risk tables
The Environment Agency has produced a Flood Map for Planning which has three zones of flooding and refers to the probability of the river and sea flooding but does not consider the presence of any defences.
The zones and definitions are as follows:
- Zone 1 Low probability: Land having a less than 1 in 1,000 annual probability of river or sea flooding.
- 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.
- Zone 3a High probability: 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.
- Zone 3b The Functional Floodplain: This zone comprises land where water has to flow or be stored in times of flood.
The National Planning Policy Framework (NPPF) sets out policies to avoid inappropriate development in areas at risk of flooding. This includes requiring new development to be flood resilient and resistant where appropriate. The NPPF defines tests that local planning authorities are expected to abide by in order to protect people and property. If the tests cannot be met, new development should not be permitted.
The steps are:
- Assessment of flood risk: A Strategic Flood Risk Assessment is undertaken by the local planning authority and informs local flood risk. In any areas that are at a high risk of flooding or for development on sites of over 1 hectare, site-specific flood risk assessments are required to accompany planning applications.
- Avoid flood risk: Local planning authorities will aim to avoid site-selection in areas of high flood risk. Development is targeted where the risk of flooding is lowest.
- Manage and mitigate flood risk: If development is needed on land at risk of flooding and no alternative options exist, it is necessary for the development to be flood resilient and resistant for the development’s lifetime.
Approved document C, Site preparation and resistance to contaminants and moisture, suggests that when building in flood prone areas, buildings can be constructed to mitigate some of the effects of flooding:
- Elevated groundwater levels or flow of subsoil water across the site can be alleviated by the provision of adequate sub-soil drainage.
- Sewer flooding due to backflow or surcharging of sewers or drains can be addressed through the use of non-return valves and anti-flooding devices.
- Intrusion of groundwater through floors can be addressed through the use of water resistant construction.
- Where there is a risk of the entry of water into floor voids, provision to inspect and clear out sub-floor voids can be considered.
It references: Improving the flood performance of new buildings – Flood resilient construction, Communities and Local Government, Defra and the Environment Agency, May 2007, as a source of further information.
The Flood and Water Management Act was introduced on 8 April 2010 to implement Sir Michael Pitt’s recommendations following the widespread flooding of 2007 when more than 55,000 homes and businesses were flooded (see Pitt Review).
The Act requires better management of flood risk, it creates safeguards against rises in surface water drainage charges and protects water supplies for consumers. It gives a new responsibility to the Environment Agency for developing a National Flood and Coastal Risk Management Strategy, and gives a new responsibility to local authorities, as Lead Local Flood Authorities (LLFA's), to co-ordinate flood risk management in their area.
 Related articles on Designing Buildings Wiki.
- Applying flood resilience technologies GG 84.
- BS 851188.
- Building flood resilience.
- Catchment flood management plans.
- Coastal defences.
- Flood and Water Management Act.
- Flood defences.
- Flood insurance.
- Flood risk assessment.
- Flood risk management plans.
- Future flood prevention.
- Integrated water management.
- Pitt Review.
- Planning for floods (this article needs more work).
- River engineering.
- Sustainable urban drainage systems.
- Thames barrier.
- Water engineering.
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