Searches User Guide: Flooding

What is flooding?

Flood definition: An overflow of water onto normally dry land. 

Flooding can occur anywhere, not just at the bottom of a hill or on a floodplain. Extreme flooding can cause property damage, contamination, potential decrease in value and insurability issues at the property. 

Risk of Flooding from Rivers and the Sea

This dataset is known as RoFRaS in England and FRAW in Wales: 

RoFRaS: Risk of Flooding from Rivers and Seas

FRAW: Flood Risk Assessment Wales

Rivers: A river floods when the water normally flowing in the channel overflows its banks and spreads out onto the surrounding land.

Seas: When the coast is flooded by the sea. A flood starts when waves move inland on an undefended coast, goes overtop or breaches the coastal defence works like dunes and dikes.

This dataset broadly assesses the likelihood of flooding from both river and tidal sources, and calculates the likelihood of flooding to areas of land within the floodplain of an extreme flood event. This dataset takes into account flood defences and considers the likelihood that these defences will over top or breach.

Risk rating: Very low, Low, Medium, High:

Historical Flood Events

This dataset shows records of historic flooding from rivers, the sea, groundwater and surface water. Records began in 1946 when predecessor bodies started collecting detailed information about flooding incidents, although limited details may be included on flooding incidents prior to this date. Takes into account the presence of defences, structures, and other infrastructure where they existed at the time of flooding, and includes flood extents that may have been affected by overtopping, breaches or blockages.

Flood Defences

This dataset shows records of flood defences owned, managed or inspected by the Environment Agency and Natural Resources Wales. Flood defences can be structures, buildings or parts of buildings. Typically these are earth banks, stone and concrete walls, or sheet-piling that is used to prevent or control the extent of flooding.

Areas Benefiting from Flood Defences

Areas that would benefit from the presence of flood defences in a 1 in 100 (1%) chance of flooding each year from rivers or 1 in 200 (0.5%) chance of flooding each year from the sea.

Reduction in Risk of Flooding from Rivers and Sea due to Defences

This shows the area where there is a reduction in risk of flooding from rivers and the sea due to flood defences taking account of the condition they are in. The data is based on modelled river and/or sea water levels alongside information for about 175,000 flood defence assets, such as crest level, defence type and condition. The model calculates how much water would overtop or breach each defence, considering the defence type, height and condition. 

This dataset came into use by Groundsure on 28th March 2024. 

Flood Storage Areas: part of floodplain

Flood Storage Areas may also act as flood defences. A flood storage area may also be referred to as a balancing reservoir, storage basin or balancing pond. Its purpose is to attenuate an incoming flood peak to a flow level that can be accepted by the downstream channel or to delay the timing of a flood peak so that its volume is discharged over a longer period.

Proposed flood defences

The data includes all Environment Agency/Natural Resources Wales projects over £100,000 that will change or sustain the standards of a flood defence in England and Wales over the next 5 years. It also includes the equivalent schemes for all Local Authority and Internal Drainage Boards. Please note schemes identified in this dataset are not taken into consideration in the Risk of Flooding from Rivers and Sea (RoFRaS) section until the proposed schemes have been constructed. This dataset includes information on schemes between 2011 and 2016 only. 

Flood Zone 2

The Flood Zones (2 & 3) dataset is designed to be used for development and planning purposes and is known as The Flood Map for Planning. It provides an assessment on the likelihood of flooding from rivers and the sea if no flood defences were present. 

Areas of land at risk of flooding, when the presence of flood defences are ignored. Covering land between Flood Zone 3 and the extent of the flooding from rivers or the sea with a 1 in 1,000 (0.1%) chance of flooding each year.

Flood Zone 3

Areas of land at risk of flooding, when the presence of flood defences are ignored. Covering land with a 1 in 100 (1%) or greater chance of flooding each year from rivers or a 1 in 200 (0.5%) or greater chance of flooding each year from the sea.

River flooding (Scotland only)

This is an assessment of flood risk for rivers in Scotland produced using modelled data, provided by Ambiental Risk Analytics. It also takes account of flood defence information provided by the Scottish Environment Protection Agency (SEPA). It shows the chance of flooding from rivers presented in the following categories: 1 in 30 year (3.33%), 1 in 100 year (1%), 1 in 250 year (0.4%) and 1 in 1,000 year (0.1%). The return period at any given location shows the highest return period for that location e.g. if a site is

considered susceptible to flooding in both a 1 in 100 year event and a 1 in 30 year event, the 1 in 30 year return period will be shown. The data table given in the report also shows the worst case flood depth for each given return period.

Coastal flooding (Scotland only)

This is an assessment of coastal flood risk in Scotland produced using modelled data, provided by Ambiental Risk Analytics. It also takes account of flood defence information provided by the Scottish Environment Protection Agency (SEPA). It shows the chance of coastal flooding presented in the following categories: 1 in 30 year (3.33%), 1 in 100 year (1%), 1 in 250 year (0.4%) and 1 in 1,000 year (0.1%). The return period at any given location shows the highest return period for that location e.g. if a site is considered susceptible to flooding in both a 1 in 100 year event and a 1 in 30 year event, the 1 in 30 year return period will be shown. The data table given in the report also shows the worst case flood depth for each given return period.

Surface water flooding

Surface water flooding is caused when the volume of rainwater falling does not drain away through the existing drainage systems or soak into the ground, but lies on or flows over the ground instead. This type of flooding is usually short lived and associated with heavy downpours of rain, thunder storms etc.

Ambiental Risk Analytics surface water (pluvial) data identifies areas likely to flood as a result of extreme rainfall events, typically this is topographical low spots which are naturally vulnerable to surface water ponding or flooding. This data set was produced by simulating 1 in 30 year, 1 in 100 year, 1 in 250 year and 1 in 1,000 year rainfall events. Modern urban drainage systems are typically built to cope with rainfall events between 1 in 20 and 1 in 30 years, though some older ones may flood in a 1 in 5 year rainfall event.

The return period at any given location shows the highest return period for that location e.g. if a site is considered susceptible to flooding in both a 1 in 100 year event and a 1 in 30 year event, the 1 in 30 year return period will be shown.

The data table given in the report also shows the worst case flood depth for each given return period. The depths are banded into the following generalised depths:

• Between 0.1m and 0.3m

• Between 0.3m and 1.0m

• Greater than 1.0m

Groundwater flooding

Groundwater flooding is caused by unusually high groundwater levels. When the natural underground drainage system cannot drain rainfall away quick enough, causing the water table to rise above the ground surface. It occurs when the water table rises above the ground surface or within underground structures such as basements or cellars. Groundwater flooding tends to exhibit a longer duration than surface water flooding, possibly lasting for weeks or months, and as a result it can cause significant damage to property.

This risk assessment is based on a 1 in 100 year return period and a 5m Digital Terrain Model (DTM). It shows the risk of groundwater flooding in the following categories: Negligible, Low, Moderate, Moderate- High, High: