
                         Roanoke Times
                 Copyright (c) 1995, Landmark Communications, Inc.

DATE: SATURDAY, June 24, 1995                   TAG: 9506260061
SECTION: VIRGINIA                    PAGE: A-6   EDITION: METRO 
SOURCE: CATHRYN McCUE STAFF WRITER
DATELINE:                                 LENGTH: Medium

WATCH OUT FOR SIGNS OF EROSION

The typical scenario for an earthen dam failure goes like this: "They go very slowly at first. It would not look dangerous for an hour, then two hours later the dam would be gone, or something like that."

Michael Duncan is a civil engineering professor at Virginia Tech who has studied dams for years. He didn't know details of Thursday night's dam break in Campbell County, but he was able to guess pretty much what happened.

``[For] 30-year-old dams and older, overtopping is the major cause of failure," he said.

Old, small dams often don't have large enough spillways to drain rushing water during a heavy downpour. Water starts coursing over the top, washing the soil away. Once erosion starts, it just gets worse, weakening the structure until huge chunks give way, Duncan said.

Concrete dams don't have that problem. Most hydroelectric dams are made of concrete, including Claytor Lake and Smith Mountain Lake dams, said Appalachian Power Co. reservoir supervisor Mike Thacker.

They are regulated and inspected by the federal government and must be built to handle the "probable maximum flood," Thacker said.

And how much is that? "Like something you've never heard of before," he said - an inundation of almost biblical proportions, a 1,000-year or even 2,000-year flood.

Since 1982, all earthen dams in Virginia have had to meet certain design standards set by the Division of Dam Safety, part of the Department of Conservation and Recreation.

The state requires permits and yearly inspections for earthen dams more than 25 feet that hold at least 50 acre-feet of water. In all, there are 460 such dams in the state, said agency spokesman Gary Waugh.

These are mostly built for flood control, water supply, irrigation or recreational use, such as the Timber Lake dam.

Roanoke owns two earthen dams, on Falling Creek and Beaver Creek in Bedford County, said city engineer Bane Coburn. They were built around the turn of the century. Although both dams were topped in 1978 and 1985, they remain stable, he said.

It's hard to predict how much it has to rain before a dam fails, Waugh said. "It's like asking how fast does a car have to be going before you get hurt in an accident," he said. It all depends on the topography, what's upstream, rain conditions, dam construction and other variables.

Dam owners should make sure they have an adequate spillway and an emergency spillway. They should also check for any signs of erosion, Waugh said. State engineers will come out to check dams on request. Call the department at (804) 786-2065.

by CNB