Sinkholes are rapidly appearing across Britain as downpours continue and geologists warn more are expected to open up in the coming months – even when the rains stop.
Dr Tony Cooper, of the British Geological Survey, said the number of sinkholes being reported has increased by almost five fold, from just a few a year since the storms began in December.
Sinkholes are closely associated with heavy rainfall, but Dr Cooper warns once the deluge stops and groundwater levels start to recede there is a risk of even more, as rocks lose support from the water they hold and heavier surfaces collapse into them.
As the deluge continued this weekend, a 20ft deep hole opened up in a garden in Hemel Hempstead on Saturday and almost 20 homes had to be evacuated. Last week part of the M2 had to be closed after a 15ft sinkhole opened up in the central reservation, and the month before a car was swallowed up when a sinkhole appeared in a driveway in High Wycombe as Britain was soaked in the wettest December and January in the south since records began.
“I expect if heavy rain continues to fall we will get more sinkholes,” said Dr Cooper.
“It is also quite likely that as things go back to normal we will get another spate of sinkholes.
“When you get heavy rainfall the ground fills up with water, and groundwater levels rise. The area above where the sinkhole is develops buoyancy and as the hole beneath the ground drains out the covering materials become heavier, and eventually the support from the ground beneath and the water in the rocks is gone, and the land collapses.”
The likelihood of sinkholes opening up depends on the underlying rock, making the South East, as well as parts of Yorkshire such as Ripon and Doncaster more susceptible.
In the South East there is a lot of chalk which dissolves easily making underground cavities bigger, while in Ripon, gypsum, which is very soluble, means the area is more at risk by being swallowed up by sinkholes.
He added: “The South East is fairly susceptible to sinkholes. From the south up to Norwich, that whole area is susceptible to both natural sinkholes and those caused by mining.”
Despite experiencing some of the worst of the flooding since before Christmas, the South West, which has less soluble underlying rock, is less at risk.
Dr Cooper said technically a sinkhole is naturally occurring, whereas when a hole develops above a manmade cavity where mining has taken place it is known as a dene or crown hole – but all types have been affected by the recent downpours. He said this is a key reason why sinkholes are opening up on areas of chalk, where the land is both soluble and likely to have been mined.
It is also not a coincidence that sinkholes in Britain are opening up next to significant developments such as houses and roads rather than beneath them, where water runs off onto the land and soaks through into the rocks below.
Dr Cooper added: “Drains coming off houses and down the sides mean when water leave the property it goes into the ground. You get cracked drains, or drains pulled apart around the perimeters of houses where the ground starts to sag.”