Podcast: Heavy rain, flooding and sinkholes

Sinkhole

A sinkhole that left a huge crater in the middle of a residential area in Schmalkalden, Germany. The hole was about 20m deep.

4 March 2014 by Sue Nelson

This week in the Planet Earth podcast, Tony Cooper and Vanessa Banks of NERC's British Geological Survey (BGS) give us a unique insight into how sinkholes form and explain why the worst of them may be still to come.

To assist those who find text-based content more accessible than audio, a transcript of this recording is available below.

Sue Nelson: This time in the Planet Earth podcast, that sinking feeling, the sudden appearance of huge holes in the ground across the UK. I'm Sue Nelson and I've come to the British Geological Survey on the outskirts of Nottingham in Keyworth to find out about the recent increase in sinkholes.

In February alone there were at least three times more sinkholes than is normal for a whole year. These included a four to five meter hole on the M2 in Kent and another similarly sized one in a driveway in High Wycombe, a massive seven and a half metre sinkhole which caused three homes to be evacuated in Yorkshire and 20 homes were evacuated after a giant hole appeared in someone's back garden in Hemel Hempstead.

Well with me to discuss why these sinkholes are occurring our principle geologist, Tony Cooper, and engineering geologist, Dr Vanessa Banks. Tony, if I could start with you first. I know sinkholes are a hole in the ground but is there a technical definition of them?

Tony Cooper: Well where the ground has collapsed and strictly speaking a sinkhole is where soluble rock has dissolved underground and it is a natural phenomenon and has just collapsed, but we also have a lot of mining collapses and ancient workings, deneholes in the south of England that have also collapsed and the manifestation of both these types is that you end up with a hole in the ground with the ground collapsing. In places it is very difficult to tell the difference between a manmade sinkhole and a natural sinkhole.

Sue Nelson: And a denehole is one of these manmade holes?

Tony Cooper: A denehole is a hole where a shaft has been dug to extract chalk, usually, and they dug it below the covering materials down to the good rock, the good rock was less weathered, it had more minerals in it and they would use it to fertilise the ground in medieval times. So they would dig down and then enlarge the area underneath and put other tunnels running out from it and when they had finished they would just choke the top up and cover it in really, so there's no expression at the surface of them.

Sue Nelson: Now, I gave a few examples of where these sinkholes have been happening over the UK. They've ranged from Kent to Buckinghamshire, Yorkshire, Hertfordshire, there seems as if there is no apparent connection, but is there something that connects all these places?

Tony Cooper: There is a connection in that they are all soluble rocks, but we've basically got four types of soluble rocks in the UK that give us sinkholes. Starting with the least soluble rocks we've got the chalk and then the limestone and most people are aware of limestone caves and have been into show caves and so on. These take a long time to dissolve to make those big caves that we go in. Then we've got gypsum which is the raw material for plaster. That rock actually dissolves very quickly and that also makes caves. It dissolves so quickly that if you took a block about three metres cubed, the size of a large van, put it in a river it would dissolve in about 18 months. Then we've got salt and most people are aware of how quickly salt dissolves - rock salt, table salt, they all dissolve at a similar sort of rate and there are parts of Cheshire where we've got salt deposits that also dissolve and now and again cause sinkholes.

Sue Nelson: Now, one of these special features, I think, is the best way to put it of the BGS in Keyworth here is this avenue, this lovely pavement area here with rocks on either side. Vanessa, you are standing beside one of them alongside Tony and myself - is this one of the offending rocks?

Vanessa Banks: This is indeed. This is an example of the chalk and you will see that it is a white, fine grain material which is made up of micro organisms that have broken down and inter bedded with the chalk you actually see these layers of flint. Very, very early workings were indeed after the flint as a material to construct arrowheads and other cutting devices.

Sue Nelson: So what sort of research do you do then here that helps you able to study or know which areas are more likely to have sinkholes or not?

Vanessa Banks: Well our interests lie very much in understanding the way in which the water moves into these soluble rocks because it tends to be focused and concentrated on areas where dissolution can take place and how that water moves through the ground and how it emanates from the ground as springs, and how it changes in chemistry as it moves through the ground. We put a lot of effort into trying to understand were those flow paths go, so one of our specialists undertakes things which we call 'dye tracing tests', so we have an idea of the way in which this water moves through the ground.

Sue Nelson: So you effectively put some dye in at the top end and then see where it comes out and then you know perhaps which route it has taken?

Vanessa Banks: Absolutely so. There may be multiple pathways, so we're monitoring lots of springs and it may not just simply be a dye it may be an optical brightener or it may be something else that we pass through the system in an inert way as possible so we don't impact on water quality.

Sue Nelson: So, basically, the fact that we've had the wettest January in centuries and a pretty soggy February, it sounds like the ingredients for a perfect storm really for sinkholes.

Vanessa Banks: Well that is indeed the case. There is this strong association between the December/January rainfall figures for the south and south-east of England and it has been associated with a number of triggers of collapsed features, possibly not related to the increased rate of dissolution but more to the disturbance of the surface materials that are covering these voids.

Sue Nelson: Tony, how do you go about reassuring people who perhaps live on land that is made up of chalk or gypsum as another rock that is particularly soluble that a sinkhole isn't going to appear in their back garden anytime soon?

Tony Cooper: Well, if you actually look at the country and look at the amount of damage from the storms and the amount of people that have been flooded, the number of people affected by sinkholes actually is very small. So even in the worst parts of the country where we've got lots of sinkholes you are more likely to suffer damage from other things, so that puts it in perspective. It's just more of an unusual sort of thing to happen and it is very drastic if it does happen under your house but there aren't that many houses that actually get hit by them.

Sue Nelson: When they do, Vanessa, do you effectively act as a hit squad, do you go out to study them as and where they occur?

Vanessa Banks: We don't actually have a response team to all of these events-

Sue Nelson: A sinkhole response unit! What a great thought.

Vanessa Banks: But we do where we can go and visit for scientific purposes and collect information and indeed the Rainham Mark Grammar School occurrence in Gillingham, our photographer went out there, liaised with the people on site and came back with images that we can use then to share with the public to inform them in that case about what deneholes are chalk wells may look like.

Sue Nelson: Tony, I suspect the insurance industry would be very interested in the research that you do?

Tony Cooper: We've actually looked at the whole country to see the distribution of the soluble rocks and the places where they are more prone to dissolution and we've produced a product called 'Geosure' for the insurers industry. It is based on our scientific research, it is backed up by our databases and it doesn't really affect people's insurance premiums very much but it gives the insurance business a handle on where their liabilities lie.

Sue Nelson: What about the future then? With climate change and the possibility of wetter winters and wetter weather ahead for the UK, does this mean that sinkholes are likely to be a more frequent occurrence? Vanessa.

Vanessa Banks: I think they will be focused in specific areas in response to these types of events that we've seen. They have occurred in the past in response to prolonged rainfall so during the 1980s, for instance, there were a significant number of occurrences in Norwich. It would seem that some of the storm tracks have changed, so last year the storm tracks were slightly further north and that was associated with this increased propensity for landslides and the sinkholes weren't picked up then, so it would very much depend on where these increased storm events occur.

Sue Nelson: And in terms of you've got all this research here and you've got all this knowledge about the UK, would it affect building, for example, because if somebody is applying for permission to build a whole range of houses without any knowledge perhaps of what's beneath them, it would seem again that this is where this research is extremely useful.

Tony Cooper: We do feed our research into the building industry. They have to take account in certain places of the sinkhole situation, so in the city of Ripon, for example, there are special planning routines which involve detailed investigation, they have to put forward mitigation measures and they have to sign the sites off as a competent person to ensure that what they are designing fits the bill. Now the problem comes in actually interpreting the geology and whether or not they put in robust enough mitigation measures so there is some discussion going on about what are the suitable mitigation measures for some of these sites and whether they should be built on at all, and there are parts of certain towns that are so bad that they shouldn't be built on, not very many towns, but there are small parts of them that are very bad.

Sue Nelson: Dr Tony Cooper and Vanessa Banks from the British Geological Survey in Keyworth near Nottingham, thank you very much for joining me and that's the Planet Earth podcast from the Natural Environment Research Council. You can follow us on Facebook and Twitter where we will post a few pictures of our recording today next to the avenue of rocks at the site. I'm Sue Nelson, thanks for listening.