Insurance and Groundwater Flooding

A number of clients have been in touch, via after buying a home, and then finding out that insurance terms are onerous because of perceived risk of groundwater flooding. Some insurers, as standard, exclude damage caused by ‘a rise in water level’. But shopping around, you can find insurers that don’t exclude this.

However, as with the companies providing homebuyers searches, some of these insurers aren’t applying groundwater susceptibility data appropriately. A recent client, despite having bought a home in an area of negligible risk of flooding from rivers or surface water, had an annual excess of £5000 for any damage caused by groundwater flooding. According to groundwater flooding susceptibility maps her property had ‘very high’ susceptibility. Fortunately I was able to provide a reality check on the map results that enabled my client to work with the insurers to get a sensible quote.

The property is in Walton-on-Thames, Surrey; and beneath Walton is a thick layer of gravel, and beneath the gravel is clay. In this geological environment high river levels may propagate through the aquifer to cause the water table to rise at a distance from the river, like in Oxford. Irrespective of the size of the river, or of the ground level relative to river flood height, on the groundwater flooding susceptibility map this geology will always be flagged as having ‘very high’ susceptibility.

By coincidence I did the risk assessment on 7 January 2014, and when checking on the Environment Agency website, the Thames at Walton was at its maximum historical level. My client’s garden wasn’t underwater, which was a good sign. After review of high resolution topography data, I found that her garden was actually 2.6 m above the maximum flood level of the Thames. Floods don’t tend to go uphill so I could say that the property was actually at very low risk of groundwater flooding. The insurers agreed and the annual excess was reduced from £5000 to the standard £250.

web lidar Elevation of part of Walton-on-Thames above the maximum flood level of the River Thames