Last edited 03 Nov 2016

Flood risk

Contents

[edit] Introduction

The National Planning Policy Framework considers flood risk to be the combination of the probability and the potential consequences of flooding from all sources, including:

  • Rivers and the sea.
  • Rainfall.
  • Rising groundwater.
  • Overwhelmed sewers and drainage systems.
  • Reservoirs, canals and lakes.
  • Other artificial sources.

[edit] 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.

[edit] Planning and flood risk

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 outlined below.

  • 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.

[edit] Building Regulations

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.

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