Why do we get flash floods in Wirral when it rains heavily?

Yesterday, Wirral witnessed some of the most memorable flash floods in living history. Trains came to a standstill, the M53 was reduced to a one-lane crawl in parts, and many roads were submerged and impassable to both vehicles and people.

Photographs and video of most areas in Wirral, including Heswall, Bebington, Prenton and notably Borough Road in Birkenhead, which coincidentally is very low lying and follows the course of a river that used to be there, were included in our report from yesterday.

But, just why do we get floods in Wirral when it rains more than usual?

Contrary to much social media speculation, it is fully understood that flash flooding due to heavy rain is not normally caused by blocked drains. Most flood waters will drain away through the roadway drains given time and the flooding was simply due to there being so much rainfall that the drains became overwhelmed.

It will be evident if a drain is blocked or not if the water fails to drain away after a few hours, or in low-lying areas within a day. If the drains were blocked, the water would never drain away. Blocked drains would not drain water away even with light rainfall and would back up and overflow quite quickly.

Basically, flash flooding happens when rain falls so fast that the underlying ground cannot cope, or drain it away fast enough. So, if the rain is falling too heavily for the ground or drains to cope, there is a risk of flash flooding.

Urban areas experience “surface water” flooding because these areas have a lot of hard surfaces, such as paved front gardens, roads, and car parks. When rain falls on them it can’t soak away into the ground as it would do in the countryside. This leads to surface overflow and can often overwhelm local drainage systems.”

Climate change makes extreme rainfall more likely – a warmer atmosphere can hold more moisture and so these storms become more intense and lead to flash flooding as witnessed in Wirral yesterday.

Dr Liz Stephens, hydrologist and associate professor of climate resilience, University of Reading, said following recent flooding around the country, “A landmark study in 2014 found that the intensity of summer rainfall events in the UK was expected to increase as a result of climate change, but the surface water flood hazard maps for the UK have not been improved since 2013.  These urgently need updating. 

“The current accuracy of surface water flood maps reflects an investment choice and not what is possible with the state-of-the-art science. I hope that this is the wake-up call that the government needs to take this risk of climate change seriously.”

Flood risk consultants, UNDA said that people paving over their front gardens to create off-road storage space for their cars came with an environmental cost. “Paving increases the risk of flash flooding – instead of grass and soil soaking up the rain, it runs straight off paving and overwhelms drainage systems and floods roads.

“Reversing the trend that has seen 4.5 million front gardens totally paved over is “vital for the nation’s health”, the RHS said, “for wildlife, to mitigate against pollution and heatwaves and to protect the UK’s homes from flooding”.

Alistair Griffiths, director of science at the RHS, said the charity “had an idea of the increase in paving front gardens, but not as much as this. We need to tackle this issue and while plants won’t solve the whole thing, they are certainly part of the answer.”

Main image: Kings Lane Bebington – Image credit: Victoria Demirer

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