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How dry conditions can lead to severe flooding

Dry Conditions

Following a period of hot and dry weather, why have we seen such dramatic flooding? After weeks of drought conditions during the height of summer, we have seen the weather change rapidly to long and heavy showers which can have significant impacts on people and property.

July was a particularly hot, dry month. Temperatures exceeded 40°C for the first time on record and several wildfires were recorded. It was the driest July across England since 1935 and the driest July on record for East Anglia, southeast and southern England. From January to July 2022, the UK had experienced just 69% of the average rainfall levels, which led to a drought status in 8 of the 14 areas in England1.

However, this period of dry weather shifted in mid-August. Low pressure systems began to dominate our weather, resulting in significant spells of rain and reports of thunderstorms which have been observed across the country over the past week. This sharp transition from excessively dry conditions to wet weather often leads to flash flooding and the pooling of water. But why is this the case, when the ground is at its driest and starved of water?

The drying of soils causes them to harden and become less receptive to the absorption of water, which leads to far higher levels of run off. This video features an experiment carried out by University of Reading meteorologist Dr Rob Thompson, which shows the effects a heatwave can have on soil absorption rates2. This process is exactly what we have witnessed over the past week, with examples of flooding across many regions of the country.

Worksop in Nottinghamshire recorded 93mm of rainfall during a three-hour period between 5pm and 8pm on Tuesday 16th August 2022, almost twice the average monthly rainfall of 54mm. This combination of unusually high levels of rainfall and extremely dry ground conditions caused flash floods which affected at least 30 homes and business premises in the region. In addition, an eight-foot-wide sinkhole was reported in a Matalan car park3.

Flash flooding was also observed in the south of the country, resulting in train station closures and flooded roads in London4. Similar conditions were also noted in Kent and Sussex. In addition, heavy rain overwhelmed sewage systems, leading to pollution warnings at more than 40 beaches and swimming spots in England and Wales 3.

Landmark Information Group use modelled data from leading flood risk data providers, including JBA Consulting, the Environment Agency and Geosmart to identify areas potentially at risk of flooding from several sources (including river, coastal, surface water, and groundwater).

Potential depths and extents of flooding are used to determine risk ratings for a given property/site. This data is checked and corroborated by a qualified consultant at Argyll Environmental to inform you of the potential risk from flooding. We will also recommend potential next steps in terms of what you can do to best protect yourself and your home or property from flooding.

Written by Dillon Fiolet, Assistant Consultant &  George Hearn, Assistant Consultant at Landmark Information


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