When it rains, even solid mountainsides can pour

When heavy rain is in the forecast and flash flood watches and warnings are issued, those of us who live above flood plains can be guilty of being a little smug.

Here's something that should deflate that smugness a bit: landslides.

According to the U.S. Geological Survey, the Appalachian region is the nation's largest with a high susceptibility for landslides, even though pricey homes sliding toward the Pacific in California downpours are often the image that comes to mind when "landslide" is mentioned.

The National Weather Service in Blacksburg has been reacquainting both the public and its own meteorologists with this sometimes forgotten danger.

Steve Keighton, science operations officer at the weather service's Blacksburg office, calls it "our region's EF-5 tornado - our big disaster that could happen, but fortunately is going to be rare."

"Our overall awareness of the landslide threat has increased over the last few years," Keighton said. "It started in 2004 with Frances, Ivan and Jeanne, and continued in 2009, with the 40th anniversary of Camille, when we did some research and presentations on that."

As Keighton's mention of a couple of past events underscores, rare does not mean unprecedented.

In 1969, the remnants of Hurricane Camille turned entire solid mountainsides into muddy goo in Nelson County and vicinity as some locations received more than 2 feet of rain in a few hours. More than 150 people died in subsequent flooding and mudslides.

In 1940, Watauga County in rugged northwestern North Carolina was hammered with multiple landslides, dislodging many houses and killing several people.

Especially rainy periods, like September 2004 when the region was affected by the remnants of hurricanes Frances, Ivan and Jeanne, and even this summer, have produced smaller but troublesome landslides in the Appalachians.

A localized cloudburst on July 3 created multiple mudslides that closed U.S. 221 southwest of Roanoke for a short time.

For the first time, the National Weather Service in Blacksburg issued a seasonal statement in July reminding residents of the threat of landslides. The office has also developed a public information statement it can couple with some flash flood watches when the threat of landslides is determined to be especially high.

That determination is the difficult part.

"The tricky thing is being able to predict what scale and size these can happen," Keighton said.

Generally, landslides do not occur without an enormous amount of rain in a relatively short period of time. An inch or two in an hour may cause creeks to overflow and streets to flood, but rain more on the order of 3 inches in an hour or 5 or more inches over several hours is typically needed to trigger landslides.

Remnants of tropical systems are typically what dumps these kinds of rain amounts on our region, but Keighton said there is also landslide potential from especially torrential rain in localized storms and even from heavy rain falling on top of winter snowpack.

Virginia has not yet taken on the in-depth mapping of landslide danger that North Carolina has, Keighton said, but the worst zones for potential landslides include those where previous landslides have occurred, those disturbed by construction, and stream beds through steep slopes.

If you live on or underneath a steep slope (as I do) with heavy rain in the forecast, keep an eye on the conditions around you. Are trees starting to slide or tilt on the hillside? Are there loud popping noises as rocks and boulders move? Is mud starting to flow down your hillside?

If there is any doubt and rain is continuing, moving to another location may be in order until the threat has passed. If you know in advance your hillside has been affected by landslides in the past or is susceptible to them because of erosion or human-caused disturbance above you, you might want to consider going somewhere else if extreme rains, as with a tropical system, are expected.

To read more on landslide danger, visit this U.S. Geological Survey website: http://landslides.usgs.gov/

Taking a break

I am taking a break from the weather column through Sept. 25. Lynchburg weather blogger Keith Huffman is covering the Weather Journal blog for me through Sept. 18.

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