More information about Geological Indicators of flooding

Geological indicators of flooding map

Geological maps show where all the floodplains and coastal plains in Britain are located and therefore the main areas at greatest vulnerability from flooding; from this information BGS has produced a map dataset called Geological Indicators of Flooding.

The Geological Indicators of Flooding (GIF) dataset is a digital map based on the BGS Digital Geological Map of Great Britain at 1:50 000 scale (DiGMapGB-50). Coverage includes England, Wales and Scotland. It characterises superficial deposits on DiGMapGB-50 in terms of their likely susceptibility to flooding, either from coastal inundation or fluvial (inland) water flow.

The majority of these coastal and fluvial superficial deposits are considered ‘young' in geological terms, most having been formed within the last few tens of thousands of years. Typically they have been laid down by processes of erosion and deposition and they have produced subtle topographical features, resulting in low-lying landforms we call floodplains and coastal plains.

Landform mapping

The mapping of floodplains and coastal plain landforms, in conjunction with characterisation of deposits that underlie them, allows us to determine the extent of the coastal and inland flooding that created them. Observations made during recent major inland and coastal flooding events have demonstrated that the current floodplains and coastal plains continue to play a role in controlling where and how flooding occurs.

On this basis, the floodplains and coastal plains constituting areas of greatest susceptibility from flooding can be both visualised and defined by superficial deposits as depicted on geological maps.

Deposition zones of the Geological Indicators of Flooding dataset

Zone 1 deposits: include river alluvium and lacustrine (lake) alluvium.

Zone 2 deposits: younger river terraces or 'floodplain terrace' (raised level areas adjacent to or within floodplains, which represent the level of the floodplain prior to the most recent episode of floodplain formation).

Zone 2 deposits are perceived to be susceptible to flooding in more extreme events such as very heavy and prolonged periods of rainfall and/or extreme high tides, with high wind speeds and storm surges. They are the areas where secondary flooding may occur. Older and higher river terraces have been excluded as they tend to lie outside the geologically defined floodplain and relate to older generations of floodplains extending back over hundreds of thousands of years in some cases.

Areas at risk from coastal inundation are similarly characterised by a range of estuarine or marine deposits that include, for example, tidal flats and low-lying raised beaches.

Flooding zones

Geological indicators of flooding map example

The two types of flooding, fluvial and coastal, are each separated into two zones as below:

 

Fluvial Zone 1 & Zone 2

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.

 

Coastal Zone 1 & Zone 2

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.

Rainfall and standing water

Both classifications are also subject to an element of pluvial flooding from land as a result of an episode of heavy intense rainfall. We do not have a specific category to identify this type of event.

BGS pluvial flooding research, with the Centre for Ecology and Hydrology (CEH), is ongoing.

GIF does not currently identify the presence of permanent standing bodies of 'water' such as lakes or canals.

Environment Agency flood risk maps

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.

Further information

Contact

For more information e-mail flooding@bgs.ac.uk.

Contact Digital Licensing or 0115 936 3012 to get a Geological indicators of flooding (GIF) data licence.

Additional information can be obtained through Enquiries@bgs.ac.uk

Hutton field: well correlation diagram.