Factsheet: Surface water flooding

What is 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.

The data is modelled and supplied by Ambiental Risk Analytics to produce a 5m Digital Terrain Model (DTM) using LiDAR where available to map the topography.

LiDAR (Light Detection and Ranging) is a remote sensing technology that uses laser pulses to measure distances and create 3D models.

What is the data based on?

This data set was produced by simulating 1 in 30 year, 1 in 100 year, 1 in 250 year and 1 in 1000 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 older ones may even flood in a 1 in 5 year rainstorm event.

The model does not include all flood protective measures (i.e. additional drainage that may have been installed at this location) including those perhaps undertaken by the developer/builder during planning/construction or those perhaps incorporated into the property by previous/present owners.

Additionally, Groundsure has made no visit to the site with the report representing a desktop study. This report is an entry level desktop study designed to satisfy Law Society Practice Notes on Flood Risk and utilises the best commercially available flood models from a variety of external sources.

The data contained within the report is not a predictive model defining that flooding will definitely occur. Rather it is an indicative model, which calculates the potential for flooding if certain modelled criteria were to occur.

What should a purchaser do next (residential)?

The recommended next steps are based on the property age, and the presence of a basement. These will be detailed within the report but may include:

Ensure buildings and contents insurance covering flood risk is available and affordable;
Make enquiries of the seller (e.g. in the TA6) and other nearby residents on any flooding that may have occurred;
Sign up to the Environment Agency's Flood Warnings and Alerts https://www.gov.uk/sign-up-for-flood-warnings;
Investigate the various forms of flood resistance and resilience measures that will help protect your property in the event of a flood;
Create a flood plan https://check-for-flooding.service.gov.uk/plan-ahead-for-flooding;
The assessment is based on the highest flood risk found within the site boundary. Whilst the maps within the report provide a visual representation, we are able to provide a manual flood assessment breaking down the risk to the main dwelling/building and the outside areas. Please contact us at info@groundsure.com

If the property was built before 2009 it may be eligible for the Flood Re scheme. Whilst the property may qualify the decision to insure through Flood Re rests with your insurance provider. You may wish to enquire with them further on this. Properties built after 2009 are not eligible as the property should have been designed in a way to protect against flooding.

If the property has recently been constructed, the risk assessment within this report will not take into account measures put in place by the developer. This should be factored in when making any purchase decisions.

What should a purchaser do next (commercial)?

The recommended next steps are based on the property age, and the presence of a basement. These will be detailed within the report but may include:

Ensure buildings and contents insurance covering flood risk and business interruption is available and affordable;
Make enquiries of the seller and other nearby businesses on any flooding that may have occurred;
Sign up to the Environment Agency's Flood Warnings and Alerts https://www.gov.uk/sign-up-for-flood-warnings;
Investigate the various forms of flood resistance and resilience measures that will help protect your property in the event of a flood;
Create a flood plan, including evacuation and business continuity https://check-for-flooding.service.gov.uk/plan-ahead-for-flooding. The flood maps within this report may be of assistance in identifying higher risk areas;

If the property has a basement this is an additional entry point for flood waters and could increase the severity of any damage caused. We recommend seeking specialist advice on ways to mitigate or reduce the ingress of water.