March 2013
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Sinkholes in karst landscapes
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How do sinkholes form?
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Case study: Impacts of strawberry farming in Hillborough County, Florida
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Managing sinkholes
In the news: Golfer swallowed by sinkhole
A 43-year-old American golfer fell 18 feet underground into a sinkhole on a golf course in Illinois, USA. "I was standing in the middle of the fairway," he reportedly said. "Then, all of a sudden, before I knew it, I was underground."
The golfer fractured his shoulder and needed a ladder to get out of the hole. The surface opening to the underground sinkhole was newly formed and only around one meter in diameter. It was expected that the hole would have fully collapsed the next time it rained, due to the additional weight of waterlogged soil. Source: ABC News (12/03/13)
Sinkholes occur in Karst landscapes, where acidic water dissolves one or more layers of the bedrock. Areas with limestone geology are particularly vulnerable to chemical erosion and therefore form Karst landscapes, which are characterised by cracks, fissures, underground drainage systems and sinkholes.
In a Karst landscape, the bedrock becomes chemical weathered by weak acids. This weakens the bedrock, creating cavities and widening already existing fissures. Unable to support the weight of soil and surface objects (such as buildings, cars, roads etc.) above, the bedrock collapses to form a sinkhole.
The geophysical processes that lead up to the formation of a sink hole can take thousands of years. However, the physical collapse of the bedrock can take place within a matter of minutes or hours. Sinkholes can be a deep as 662 metres (Xiaozhai Tienkeng, China) and as wide as 75 miles across (The Qattara Depression, Egypt). They can also be considerably smaller, at just a few metres in diameter.
The USA is the world’s largest producer of strawberries and Florida is the nation’s leading winter strawberry producer due to a mild winter climate. The main area for farming strawberries in Florida is in Hillborough County, which covers around 5800 acres.
To protect plants from overnight frost-freeze events, sprinkler irrigation is commonly used. This sprays the plants with water, which forms a layer of ice on the crops to protect them from cold air temperatures. However, this requires large quantities of water to be intensively extracted from underground aquifers through pumping.
The rapid removal of water from the limestone bedrock encourages sinkhole formation. Since 1950, over 3,000 sinkholes have been reported to the Florida Geological Survey. The majority of these occurred between 1980 and 2010.
In fact, between 1980 and 2010, the local population more than doubled, and the acreage of strawberry fields increased almost fourfold – thus increasing the risk of sinkholes as a hazard to people. To further highlight the impact of water extraction, research has found that the majority of sinkholes reported to the Florida Geological Survey were found within ¼ mile of strawberry fields. Source: Plos One (2013)
Sinkholes are not easy to predict. They take thousands of years to form, yet can collapse suddenly with no warning. Extreme rainfall events and poor drainage are common causes for collapse. This can cause soil to become waterlogged and heavy, which increases the weight needing to be supported by the bedrock.
The lack of evidence of sinkholes on the earth’s surface makes future sinkholes difficult to pin-point. However, planners and property developers can use maps of geology to steer clear of Karst landscapes. As a result, the majority of sinkholes are in rural areas.
In urban areas, property owners may be able to detect the presence of a large underground cavity (and therefore an impending sinkhole) if nearby trees and gates are sloping or if doors and windows no longer shut properly. Ultimately, people need to look around for signs that the land is beginning to sink.
In the USA, companies exist to ‘fill in’ cracks and caves in the land - just like a dentist might fill in a tooth cavity. By injecting concrete into cracks in the bedrock, people can ensure that the ground is sturdy enough to build on. However, some commentators warn that this may not solve the root cause of sinkhole formation.