11/26/2014

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Sinkhole science: How prone is the Northwest?

Sinkhole science: How prone is the Northwest?

PORTLAND, Ore. - A sinkhole swallowed a Florida man right from his bedroom on Friday and you have to wonder whether something like that could happen here in the Northwest.

We do get the occasional sinkhole on the West Coast but they are much smaller than what you see in other places around the country.

Why is that?

The big difference is the water table. In Florida, it's very high and close to the surface - sometimes just 10 feet down. In the Northwest it's much lower - it can be hundreds of feet beneath the surface.

But the water table is just part of the equation. It also depends on what kind of soil and bedrock is underground. Other parts of the country are sitting on limestone, salt or gypsum - rock that dissolves much easier in water.

And in Florida, the ground used to be filled with water - enough to hold up the land on top of it. But over the years, swamps have been drained and that's left a big problem.

"They've lowered the ground water table to make some of the land more productive," said Scott Burns, a geology professor at Portland State University. "You build something on top of it - the weight is there and it just falls through."

Burns said here in the Northwest, the problem almost always stems from a broken water pipe. But in the West Hills, which is the most likely place for a sinkhole in the Portland area, it's something different.

More than 100 years ago, city engineers built wells in the West Hills and drained the water into reservoirs. To keep the land stable, they put in wooden supports and now some of those supports are wearing out.

"A lot of that wood is rotting, so periodically those caverns under the ground collapse and we have a sinkhole," said Burns.

There are parts of the Northwest that are on limestone - mainly in the Wallowa and Kalama mountains where there is not much of a population base. Which means there may be limestone cave sinkholes in Oregon that nobody has seen.


Map of States that are Prone to Sinkholes

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