Hydrology and Home Insurance: Widespread Flooding after Storm Callum

Storm Callum has recently caused rivers in Wales to burst from their banks and flood nearly 200 homes. As a result, Operations Manager Huwel Manley of Natural Resources Wales (NRW) said, “There will be a large review about the flooding issues and flood protection.” First Minister Carwyn Jones said NRW's review will decide whether "there needs to be a rejigging of priorities in terms of where the work is done in the future".

The clean-up and reinstatement process may take months or even years for some, however the review may provide some reassurance for people and businesses whose properties have suffered damage. Thankfully for property owners, the costs will be covered by insurance, for those that have it. Although not on the same scale as Storm Desmond in December 2015, the damage caused by Storm Callum is likely to cost the insurance industry millions of pounds. Unfortunately, the possibility of recovery isn’t always open to insurers if the event was a true ‘natural disaster.’ However, insurers and their representatives should bear in mind that there may be a possibility of third party liability for the damage if someone’s action--or inaction-- caused or contributed to the flooding.

“In widespread flooding events,” says hydrologist Richard Keightley of Hawkins’ London Office, “there can be a perception that flooding was a ‘natural disaster’, and that there is no prospect of recovery.” Richard has, however, investigated many flooding incidents which simply cannot be blamed on nature alone, including:

  • Accidental ‘filling in’ of a surface water drainage ditch;
  • Culverting a previously ‘open’ watercourse;
  • Poor design or failure of property flood defences (such as flood-proof doors);
  • A lack of maintenance of infrastructure;
  • Blockages;
  • Inadequate operation of, or failure of, infrastructure such as sluices and pumps;
  • Failure of flood defences;
  • Inappropriate development (such as development in the floodplain)

During investigations of flooding events, Richard has been able to show whether or not the rainfall was severe enough that flooding would have still occurred, irrespective of any alleged action or inaction. Hydrologists analyse rainfall and other data to work out the ‘Average Recurrence Interval’ (ARI), or return period, of an event.

The distribution of rainfall over time varies between individual events, and weather systems that cause persistent rainfall over a long duration rarely produce high intensity, short duration rainfall. Therefore a single storm will often have a different ARI depending on the duration that is considered. Small urban catchments are typically susceptible to flooding from short duration events, while large river catchments are typically susceptible to flooding from long duration events. A depth duration frequency model, such as that shown in the example below, is used to determine the ARI of rainfall over a range of durations.

This model is for Abergorlech, which is located in the catchment of the Afon Tywi, in Wales. Preliminary data suggests that during Storm Callum, approximately 140mm of rain fell in 48 hours at Abergorlech. The DDF model shows that this has an ARI of approximately 100 years.

The rainfall has often stopped, and the floodwater already drained, by the time a forensic hydrologist reaches the scene. News and social media reports provide a valuable resource for information about the flood event. Computer modelling can also be used to replicate a flood, using LiDAR (Light Detection and Ranging, a type of aerial laser scanning) to recognise the topography of an area. This allows us to understand how floodwater would have travelled across the ground surface.

It is also worth noting, that the financial Ombudsman Service receives around 350 complaints about building insurance every month. Nearly half of complaints are regarding damage caused by weather events, such as flooding - with a marked increase in complaints following severe storms. 

After a flood, it can take years before a property becomes habitable again. Hawkins understands that investigating the root cause of damage due to a significant flood is an important step toward helping the community after a storm.

Please get in touch if you would like to enquire further about how a hydrologist can help in a flooding claim.

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