IN DEPTH| 27 February 2015
Sinkholes: Can we forecast a catastrophic collapse?
By Daniel Cossins
It’s the stuff of nightmares. Around 11pm on 28 February 2013, in a suburb east of Tampa, Florida,
37-year-old Jeff Bush was asleep in bed when the ground opened up and swallowed him. Hearing an
almighty crash, Jeff’s younger brother Jeremy rushed into the bedroom. “Everything was gone,” he
told CNN. “My brother’s bed, my brother’s dresser, my brother’s TV. My brother was gone.” Jeremy
jumped into the hole in a desperate attempt to find his brother. But as the floor began to collapse
further, piling more rubble into the void, a sheriff’s deputy arrived to haul Jeremy to safety. Jeff’s body
was never recovered.
The opening that took Jeff Bush was a sinkhole. These sudden events occur across the world –
earlier this week, two people in South Korea survived being swallowed by one – but areas like Florida
are particularly susceptible because of the local geology. Hundreds appear in the Sunshine State
every year.
Should we see these huge holes as unpredictable acts of nature, or could there be a way to see them
coming? In future, others might be spared Bush’s fate: geologists are busy identifying areas that are
particularly prone to sinkholes, while Nasa scientists have test-driven a system that might even
identify when the next catastrophic sinkhole might appear. You might assume that such efforts would
have the universal support of local residents – but some claim to have good reason to oppose a
sinkhole forecasting service.
Clint Kromhout, a geologist at the Florida Geological Survey (FGS) in Tallahassee, likes to tell people
that there are certain things you can be certain of in Florida: sunshine, beaches, hurricanes – and
sinkholes. “Sinkholes are just a fact of life here,” he says.
They form where groundwater dissolves soluble bedrock over hundreds or thousands of years to form
underground cavities. Sometimes, when the ceiling of a cavity can no longer support the weight of the
overlying sediments, it slowly subsides – or suddenly collapses.
Florida is particularly prone to the events because it lies on a vast limestone slab though which an
aquifer percolates, gradually eating away at the bedrock.
Sinkholes are a natural phenomenon but human activity can exacerbate the problem. Excessive
pumping of groundwater, for example, can remove support from the walls of a cavity, leading to
collapse. Weather is also a factor. Droughts lower the water table while heavy downpours dump
tonnes of water, adding to pressure on the ceilings above cavities and often resulting in multiple
collapses.
That’s what happened in Florida in June 2012, when Tropical Storm Debby passed through after
months of drought, leaving hundreds of sinkholes in its path. The damage was severe enough for the
state division of emergency management to ask the FGS to help them prepare for similar scenarios in
future.
So in August 2013, Kromhout and his colleague Alan Baker received $1 million to develop a sinkhole
vulnerability map. It’s based on a statistical method called ‘weights of evidence’. Basically, that means
combining evidence for known sinkhole occurrences with geological data – depth of limestone, for
example, or make-up of overlying sediment – to calculate a prediction about the relative vulnerability
in a particular location. “We’re out in the field gathering data on a list of geological features,” says
Baker. “Then we feed it all into our model, which is validated by testing on existing sinkholes, and it
gives us a scientifically defensible outcome.”
When complete in autumn 2016, the map will show several categories of ‘sinkhole favourability’ in
different colours at a scale of roughly 1 square kilometre. That’s not high-resolution enough to tell you
there’s likely to be a hole below your house, but it will provide clues to be followed up with ground
monitoring.
“The main goal is to help the Department of Emergency Management prepare for the next tropical
storm that comes through and creates a whole bunch of sinkholes,” says Kromhout.
Inevitably, though, homeowners across the state will be tempted to use the map to see whether
there’s a chance they might be sitting on a sinkhole-waiting-to-happen. And that’s where it might get
tricky.
On one level it’s clearly in the homeowner’s interest to learn about a potential problem – particularly
given that there are companies that can help. For instance, GeoHazards Inc., in Gainesville, Florida,
can fill the void by injecting it with grout – “like a dentist filling a cavity in tooth”, saysAnthony
Randazzo, emeritus professor of geology at the University of Florida and co-principal at the firm.
But the work is not cheap, often running to tens of thousands of dollars. And even when it’s done, the
value of a property can be slashed. In October last year, for example, Ginny Stevens of New Port
Richey, Pasco County, told the Tampa Bay Times that despite making all the necessary repairs to her
sinkhole-damaged house, its value had plummeted from $350,000 to $125,000.
In Hernando, Pasco, and Hillsborough counties, often referred to as ‘sinkhole alley’, insurance
premiums have risen sharply over the past few years. Worse still, a new state law passed in 2011 to
crack down on spurious sinkhole claims means that insurance companies are no longer obliged to
perform extensive geological testing. Nor do they have to pay out until the damage includes “leaning
or buckling” of load-bearing walls and signs that the home is “likely to imminently collapse”. “That’s
usually too late,” says Randazzo, “because it gets to the point where the homeowner doesn’t care if
the insurance company pays or not; they just want to get out alive.”
Then there are the psychological costs of discovering you might have a sinkhole problem. “If you’re
living in one of these houses it’s stressful and anxiety ridden,” says Randazzo. “You hear cracks and
nail pops, moaning and creaking. You know the house is not well supported and you start to wonder if
it might fall in.”
Understandably, then, some Floridians have raised concerns about the sinkhole vulnerability map.
What’s more, as Randazzo points out, “if it’s not precise, you might be worrying people
unnecessarily.”
Eye in the sky
A better solution might be to spot sinkholes as they develop – and Nasa may have found a way to do
so from high above, with Interferometric Synthetic Aperture Radar (InSAR), which detects tiny
movements in the ground. For years researchers at Nasa’s Jet Propulsion Laboratory (JPL) in
Pasadena, California, have been using InSAR to monitor the coast of Louisiana, which is sinking into
the Gulf of Mexico.
When a large sinkhole formed unexpectedly at Bayou Corne in August 2012, swallowing land and
prompting hundreds of local residents to leave for good, they went back and analysed radar scans
taken in the area before the collapse. They noticed that the ground had moved significantly, shifting
horizontally by 10 inches (25cm) towards the centre of where the sinkhole opened, a month before it
gave way. Watching out for the same telltale changes in other sinkhole-prone areas could form part of
an early-warning system to help residents evacuate before a potentially dangerous collapse.
“Not all sinkholes have surface deformation prior to collapse, so this wouldn’t be a magic bullet,”
cautions Ron Blom, principal scientist at JPL and co-author the Bayou Corne study. “But it would
certainly be useful as one part of a monitoring system. You could have InSAR data for regions prone
to sinkholes and if you happen to see movement, you go to the site and see what’s going on.”
Neither Blom or his co-author Cathleen Jones, also at JPL, are aware of any plans to implement such
a system in the United States. But Jones gets the impression that authorities are beginning to take
the sinkhole problem more seriously. That’s not necessarily because sinkhole formation is becoming
more frequent, she says, but because urban sprawl means the holes that do appear are increasingly
likely to affect people and property.
And so while there are good reasons why people who already own property in sinkhole-prone areas
might be wary of new mapping and prediction plans that could threaten their investment, those
thinking of building in these areas will probably be more enthusiastic. “I wonder, for example, if the
National Corvette Museum in Kentucky [where a giant sinkhole swallowed eight rare Corvettes in
February 2014] would have been built there had they realised there was a large cavity underneath,”
says Blom. “That would have been useful information to have.”