We analyse a range of different datasets which are included as standard in our flood risk reports to ensure the potential purchaser has a comprehensive understanding of the property or land.

The main types of flood risk we investigate are:

River flooding

Also known as fluvial flooding, this occurs when a river or stream is unable to take on water draining in to it from surrounding land. The additional water causes the water to risk above its banks or retaining structures and subsequently flows onto the land.

Coastal flooding

This flooding is due to the accumulation of water along the coast caused by rising sea water above normal levels. Coastal flooding can result from a combination of high tides, stormy weather conditions and tidal surges in times of low atmospheric pressure.

Flood Zones

A flood zone is described as the area of land which will flood if there is river or coastal flooding. This data does not account for the presence of flood defences. This data displays the area of land that is likely to be flooded in such an event. The Flood Zones are in grouped into three categories:

  • Flood Zone 1 – Low probability less than 0.1% chance in any year (any area that is not considered at risk of flooding)
  • Flood Zone 2 (identified in green on the Groundsure flood maps) – Medium probability, greater than 0.1% but less than 1% from rivers and 0.5% from the sea.
  • Flood Zone 3 is split into 3a and 3b.
    • Zone 3a High risk (1% of greater from rivers and 0.5% or greater from the sea in any given year).
    • Zone 3b is the “functional floodplain” or used as a flood storage area – this is a very high flood risk area.

RoFRaS

Risk of Flooding from Rivers and the Sea data provides a broad brush assessment of the likelihood of flooding. It calculates the likelihood of flooding to areas of land within the flood plain of an extreme event (taking into account flood defences). This data is divided into 50m squares (impact cells) which report on the highest possible risk within that 50m square. This shows the area of land which is likely to flood if defences should fail.

The data is divided into 4 probability bands:

  • Less than 1 in 1000 (0.1%) chance in any given year Very Low
  • Less than 1 in 100 (1%) but greater than or equal to 1 in 1000 (0.1%) chance in any given year Low
  • Less than 1 in 30 (3.3%) but greater than or equal to 1 in 100 (1%) chance in any given year Medium
  • Greater than or equal to 1 in 30 (3.3%) chance in any given year High

RoFRaS takes into account the location, type and condition of local flood defences, and shows the probability that land, not individual properties will flood.

Surface water flooding

Also known as pluvial flooding results from overland flow before the runoff enters a watercourse or sewer. It is usually the result of high intensity rainfall, but can occur with lower intensity rainfall when the land has a low permeability and/or is already saturated, frozen or developed. Surface water flooding is becoming a regular issue due to the high rate of developments creating large impermeable surfaces. This data is based on a topographic computational model which is designed to address the question “where would water naturally collect under given rainfall/flood scenarios?”

Flood depths

The data within the Groundsure reports indicates the depth of which an area may expect to be flooded in a variety of rainfall events (1 in 200, 1in 1000 etc). The data is divided into 5m square cells and the model provides the maximum depth of flooding in each 5m cell. i.e. the 5m cell could be a combination of low and moderate flood depths. However, the highest possibility is reported upon so that 5m cell would be at a moderate flood risk.

Surface water flooding is divided into 8 probability bands:

  • Less than 0.1m in a 1 in 1000 year rainfall event – Negligible
  • Greater than 0.1m in a 1 in 1000 year rainfall event – Low
  • Between 0.1m and 0.3m in a 1 in 200 year rainfall event – Low to Moderate
  • Between 0.3m and 1m in a 1 in 200 year rainfall event – Moderate
  • Greater than 1m in a 1 in 200 year rainfall event – Moderate to High
  • Between 0.1m and 0.3m in a 1 in 75 year rainfall event – High
  • Between 0.3m to 1m in a 1 in 75 year rainfall event – Significant
  • Greater than 1m in a 1 in 75 year rainfall event – Highly Significant

Groundwater flooding

Occurs when the water table rises above normally expected and anticipated levels and intersects with the surface, this is usually after long periods of sustained rainfall. This dataset is a susceptibility indication which identified areas where geological conditions may result in flooding and where groundwater may come close to the surface. It is not an indication of flood risk, i.e. an indication of likelihood of such flooding occurring (it doesn’t provide information on the depth of the flood or the likelihood of occurrence of an event of a particular magnitude). It does show if the area is susceptible to groundwater flooding occurring based on the geological conditions. Properties with basements are more likely to be affected by groundwater flooding.

Groundwater Susceptibility is divided into 4 categories:

  • The area is not considered to be prone to groundwater flooding based on rock type.
  • There is limited potential for groundwater flooding to occur
  • There is potential for groundwater flooding of property situated below the surface
  • There is potential for groundwater flooding to occur at the surface

Additional, yet less common types of flooding include Reservoir failure and sewer flooding.

Reservoir Failure

Identifies areas that are most likely to flood following the sudden catastrophic failure of a reservoir. This data is not mapped for security reasons.

Sewerage Flooding

Extreme rainfall events may overwhelm sewer systems and cause local flooding. This is not something that can be predicted/modelled. Groundsure do not provide this information, but historical sewer flood events can be found on the DG5 ‘At Risk Register’ which is compiled by water companies.

The Environment Agency and Natural Resources Wales Flood Map

Shows the natural floodplain for England and Wales. This shows the same information as the EA Flood Zones within the Groundsure reports.  It doesn’t take into account surface water and for that reason shouldn’t be relied up to give an indication of the flood risk of an area.

We’ve also produced a handy flood guide which contains a lot of useful information.

 


Understanding flood risk

Different types of flood risk

Probability of flooding

What’s included in our reports?