Summer-flash-flooding-tree

Is the current dry weather setting us up for severe flash flooding incidents?

In Newsby ambrisk

Is the current dry weather setting us
up for severe flash flooding incidents?

The UK has been experiencing one of the longest heatwaves in fifty years, with a number of places across England not seeing significant levels of rainfall for 56 days. Temperatures in London today reached 33°C, with most of the South East of England breaking the 30°C mark. This is obviously good news for those who plan to holiday at home this year, but the ground is very dry, and reservoirs are running empty. Rain and thunderstorms are forecast and seem desperately needed now to top up reservoirs and give gardens a much-needed watering. However, the want for rain and these storms could come at a cost.

Summer heatwaves and flash flooding

The summer is no stranger to extreme weather events, and last summer Ambiental reported on the flash floods that swept through the village of Coverack in Cornwall on the 18th of July. The village experienced the equivalent of two weeks rainfall in one hour, causing damage to 50 properties and incurring a cost of more than £1m.

As seen in Coverack, these flooding events can be very isolated, but also very costly. This is likely to be the case if thunderstorms and intense rainfall are witnessed over the coming days or later in the summer.

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 should we see 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 has 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 launched their pluvial hazard map in 2013, which by working with lead local flood authorities (LLFAs) 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 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)203 857 8543 for more details.

About the Author

Mark has a solid understanding of Ambiental’s products and the flood modelling processes behind them. He has a strong technical understanding and base knowledge of flood mapping and GIS processes, which is frequently put to good use to help assess clients’ particular needs and respond to any technical questions.

Mark Nunns