Heatwaves and drought – why they are setting us up for severe flash flooding incidents?

Europe has been experiencing an intense heatwave with temperatures rising to 35°C in some parts of the UK in the last few weeks . This follows hard on the heels of the July heatwave, and many parts of Europe reaching record-breaking temperatures. Whereas this would once have been good news for those planning to holiday at home, we are now experiencing drought and heat more frequently and on a more intense scale than was previously the norm for this time of year. The ground is very dry, and reservoirs are running empty, with many hosepipe bans in place. The very short recent rain and thunderstorms may have provided a small amount of much needed water, but we will need a lot more rain to recharge water stocks and our desperation for rain can come at a cost.

Summer heatwaves and flash flooding

The summer is no stranger to extreme weather events, and during the past week or so parts of the UK have been struck by thunderstorms and flash floods. These flooding events can be isolated and only affect certain locations within an area that falls within one of the Met Office’s warning areas, but can also be very costly. This is likely to be the case if further thunderstorms and intense rainfall are witnessed over the coming days or weeks (and possibly months).

The recent high temperatures and lack of precipitation have hardened the ground and subsequently reduced the infiltration capacity of soils. This poses a huge threat when there is heavy rainfall, as previously permeable surfaces are likely to act as impermeable surfaces, leading to increased surface water runoff and flash flooding events. Add in the ever-increasing footprint of impermeable manmade surfaces and the flash flooding risks will be hugely exacerbated.

Can flood models identify the pluvial risk?

There are several things to consider here. National scale flood hazard models will provide an indication of the risk of surface water flooding under different flood frequencies, however these models are likely to underestimate the risks under our current circumstances. Within flood models, different ground surfaces will be parameterised on their infiltration capacity (for example fields can absorb more water than concrete), which under normal conditions are fairly reflective of water absorption rates. However, given our present ground conditions, we are likely to see most surfaces acting as ‘impermeable’ surfaces. This may lead to pluvial (surface water) flooding occurring in places not modelled/ perceived to be at risk and/or lead to an increase in the magnitude of the risk.

This is where having an accurate view of the potential flood risk is crucial or perhaps even multiple views of the potential risk. By utilising high-resolution LiDAR topographic data, ground surface features can be better identified, and this allows for more defined flow paths and an increased accuracy in the representation of the potential pluvial flood risk.

The way that the risk of flooding to property or infrastructure is assessed is also relevant here. Different approaches to correlating flood hazard to building exposure all have their strengths and weaknesses – particularly when it comes to understanding pluvial flood risk. Ambiental/ RoyalHaskoningDHV have been doing a lot of research into this, so please contact us if you would like a further discussion on this topic.

It is also worth noting that the Environment Agency (EA) have a national pluvial hazard map as well , which by working with lead local flood authorities (LLFAs) also provides a reasonably good view of the risk. This data from the EA coupled with another modelled view, could help to provide a more accurate picture of the risk.

What can be done?

Ensuring your flood model(s) are fit for purpose will provide some reassurances here by helping to exclude the majority of risks in the first instance. By utilising the most accurate input datasets into a flood model (such as LiDAR data), coupled with constant model validation and refinement, will provide increased confidence in risk selection prior to any flooding events. That is not to say that the current antecedent conditions could still lead to unexpected losses from flash flooding.

Over the last year, Ambiental has increasingly been looking into claims and model validation, along with how past experiences can be used to help refine underwriting strategies and guidelines. If you therefore have any concerns over your existing flood model or would like a more in-depth discussion on model parametrisation, then please do feel free to contact us.

The Ambiental/ Royal HaskoningDHV team are on hand to assist with any concerns you may have prior to these forecast storm events or post storms, should you experience any ‘unexplained’ flooding claims. As part of this, Ambiental is willing to conduct an independent assessment on your existing view of risk or we can also conduct a claims evaluation post any event. Please email us on [email protected] or call +44 (0)1273 006 966 for more details.