Insight User Guide: Flooding

Risk of Flooding from Rivers and the Sea (RoFRaS)

This dataset shows the chance of flooding from rivers and/or the sea, based on cells of 50m. Each cell is allocated one of four flood risk categories, taking into account flood defences and their condition; Very Low (less than 1 in 1,000 chance in any given year), Low (less than 1 in 100 but greater than or equal to 1 in 1,000 chance), Medium (less than 1 in 30 but greater than or equal to 1 in 100 chance) or High (greater than or equal to 1 in 30 chance).

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.

Flood Storage Areas

Areas that act as a balancing reservoir, storage basin or balancing pond 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.

Flood Zone 2

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

Ambiental Risk Analytics surface water (pluvial) FloodMap 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

Sample data table for river, coastal and surface water:

Return period	Worst case depth
1 in 30 year	0.1m-0.3m
1 in 100 year	0.3m-1.0m
1 in 250 year	0.3m-1.0m
1 in 1,000 year	Greater than 1.0m

Groundwater flooding

Periods of abnormally high rainfall can result in groundwater flooding of basements and the emergence of groundwater at the ground surface, causing damage to property and infrastructure. 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: High, Moderate- High, Moderate, Low and Negligible:

Category	Description
High	Groundwater levels have the potential to emerge above ground level in a 1 in 100 year event to a depth of greater than 0.25m. Within this zone there is a risk of groundwater flooding to both surface and subsurface assets. Groundwater may emerge at significant rates and has the capacity to flow overland and/or pond within any topographic low spot.
Moderate-High	Groundwater levels are between 0.01m and 0.25m in a 1 in 100 year event. Within this zone there is a risk of groundwater flooding to surface and subsurface assets. There is the possibility of groundwater emerging at the surface locally
Moderate	Groundwater levels have the potential to impact basement structures in a 1 in 100 year event.
Low	Groundwater levels have the potential to impact basement structures in <1 in 100 year event.
Negligible	Groundwater is not considered a risk at the site.