Why the risk of flood increases after drought
Preparedness for extreme weather has increasingly become a priority: as the UK recorded its hottest June on record, according to the Met Office, coupled with a decline in reservoir and groundwater levels and river flows in some areas, drought is an ever-growing concern.
While a bout of sudden and heavy rainfall may be welcome after a prolonged dry and/or hot spell, it has inherent risks, namely, flash flooding from surface water.
So, why does this happen?
When the ground becomes very dry, it becomes compacted. This means it is difficult for the rain to soak in because it has to “force” the air out of the soil. The result is surface runoff because the arid land quickly becomes overwhelmed by the large volume of rain and cannot disperse or absorb it fast enough.
Dr Rob Thompson, a hydrologist at the University of Reading, demonstrated perfectly in this video how a sudden deluge affects very dry conditions and can overwhelm the ground.
Drains are also unable to cope with the volume of water, and they can flood, too.
Properties at risk
According to the National Infrastructure Commission, about 325,000 properties in England alone are at the highest risk from surface water flooding, which means there is at least a 60% chance they will flood in the next 30 years. The Commission also warns that up to 295,000 more properties could be put at risk without action.
In its report, Reducing the Risk of Surface Water Flooding, published in 2022, it wrote:
“The impact of a sudden flooding incident on health, livelihoods and wellbeing for affected residents and businesses can be profound. And if you’re in that situation, you don’t really care where the water has come from – you just want it to stop.
“Surface water flooding is the flood risk we know the least about. It is highly localised and hard to predict. A highly local problem needs local solutions.”
Why trees can help
Because all types of plants and trees help regulate water absorption and retention, if the vegetation becomes stressed or dies off during a drought, fewer plants can capture and slow down heavy rainfall.
Planting more trees can help mitigate the risk of surface water flooding. According to the Woodland Trust, trees reduce surface runoff by 80% compared to asphalt – as trees mature and the canopy grows, rain is prevented from hitting the ground at high speeds. This so-called interception spreads the effect of rain. Some studies have suggested it also enables up to 30% of the water to evaporate directly from the canopy before it even touches the ground.
Tree root systems also enable rainwater to penetrate more deeply into the soil faster–resulting in less potential for surface runoff.
What we can do in our properties
The increase in impermeable surfaces – such as roads and entirely paved driveways – results in harder surfaces that cannot absorb rainwater so quickly.
After widespread flooding in England in 2007, Sir Michael Pitt carried out a review for the government to see what lessons could be learned. Although there were myriad factors, he identified the paving over of front and rear gardens as a major issue when it came to surface water flooding in urban areas.
According to the report, about two-thirds of the flooding in 2007 was because of surface water runoff.
As a quarter of all front gardens in the country are completely paved, according to the Royal Horticultural Society, removing even some of the paved or sealed surface is a good idea and adding some planting. Every small plant or shrub helps to mitigate against surface water flooding.
How does flash flooding relate to climate change?
It is largely accepted that extreme weather is becoming more frequent and more intense because of human-induced climate change.
The State of the UK Climate 2021 report by the Met confirmed that climate change is not just a problem for the future and that it is “already influencing the conditions we experience on the earth’s surface”.
The UK’s climate is changing, with recent decades being warmer, wetter and sunnier than the 20th century. All of the UK’s top 10 warmest years since 1884 have occurred in the 21st century.
While global warming can lead to increased intense rainfall, it can also contribute to prolonged and/or severe droughts, which causes the risks around surface flooding when there is a heavy downpour.
There are myriad reasons behind it: unsustainable development and the degradation of soils result in the natural environment being able to absorb vast volumes of water from heavy rainfall.
There are also concerns about rising sea levels, caused in part by the rise in global temperatures leading to the melting of glaciers and ice caps. Coupled with increased numbers of storms, it means there is a greater chance of coastal flooding when there are extreme weather events.
GeoSmart Information produces flood risk assessment reports to support planning applications, property transactions and insurance for architects, planning consultants and developers, and the legal market. We also produce unique, market-leading data for property professionals to understand groundwater flood risk.
Contact one of our specialist team members today to learn more about the flood risk services we offer.