Geological Indicators of Flooding

• Introduction
• Details

Introduction

Geological maps show where all the floodplains and coastal plains in Britain are located and therefore the main areas at greatest risk of flooding; from this information BGS has produced the Geological indicators of flooding dataset.

The map shows areas vulnerable to two main types of flooding:

• Inland (river floodplains)
• Coastal/Estuarine

Inland flood plains are the flattish areas near to the river where mud, sand and gravel were deposited by previous floods On geological maps these materials are commonly known as 'alluvium', but they can also include lake deposits, and river terraces.

For coastal plains a range of marine deposits, for example tidal flats, define areas where the sea has formerly occupied the land.

The map is based on observation of the types of geological deposit present and does not take into account any man-made influences such as house building or flood protection schemes. It also doesn’t take into account low-lying areas where flooding could occur but where there are no materials indicating flooding in the geological past.

The BGS Geological indicators of flooding data should be regarded as complementary to, and not a replacement for, existing flood risk maps such as those provided by the Environment Agency.

Details

This data is available as a national dataset in Shapefile format. It is a very large file and may take some time to download.

The data is based on version 6 of the 1:50,000 geology data; the scale means that it is appropriate to use for regional studies rather than site specific research.

The Geological Indicators of Flooding dataset characterises superficial deposits on DiGMapGB-50 in terms of their likely susceptibility to flooding, either from coastal inundation or fluvial (inland) water flow.

• Flooding from rivers where the capacity of the river channel is exceeded and water overflows. This is identified as “fluvial” in the GIF, and is subdivided into higher (zone 1) and lower (zone 2) susceptibility categories.
• Flooding from the sea as a result of high tides and storm surges is identified as “coastal” flooding in the GIF. This is similarly subdivided into higher (zone 1) and lower (zone 2) susceptibility categories.

Both classifications are also subject to an element of pluvial flooding from land as a result of and episode of heavy intense rainfall. We do not have a specific category to identify this type of event. Typically, this dataset does not currently identify the presence of permanent standing bodies of water such as lakes or canals.

The data is divided into Classes: this is the ‘zone’ and ‘mode’ combined giving 4 possible types of flooding-

• Coastal – flood deposits from the sea
• • zone 1 – areas susceptible to the first influx of flood waters
  • zone 2 – areas that are susceptible in extreme flood events (e.g. storms surges and exceptionally high tides)
• Fluvial – inland fluvial flood deposits
• • zone 1 – areas susceptible to the first influx of flood waters
  • zone 2 – areas that are susceptible in extreme flood events