Roanoke Times
Copyright (c) 1995, Landmark Communications, Inc.
DATE: SATURDAY, August 20, 1994 TAG: 9408220091
SECTION: VIRGINIA PAGE: C-1 EDITION: METRO
SOURCE: By CATHRYN McCUE STAFF WRITER
DATELINE: LENGTH: Medium
They may be celebrating the completion of the Roanoke River flood-control project. Maybe.
The project was envisioned 20 years ago. It's at least three years away from construction, and will take about three years to build.
By that time, it may not be as effective as it once would have been.
Since 1974, the estimated water level for a 100-year flood has risen in the Roanoke Valley by 2 1/2 feet. That means the flood plain has widened, taking in more businesses and homes and triggering building-code and insurance headaches.
The water probably will keep rising.
Every 10 years or so, the U.S. Army Corps of Engineers and the Federal Emergency Management Agency look at the valley's 100-year flood level, the general standard for establishing the flood plain, and recalculate it if need be.
Note to readers: You may want to use a paper and pen for the next few paragraphs.
From 1984 to last year, the level was 21 1/2 feet above the river bottom. In October 1993, it rose to 23 1/2 feet, said city engineer John Peters.
The flood-control project will widen and deepen the river channel to allow more water to move through. Designed in the mid-1980s, it lowered the 100-year flood level by 18 inches, down to 20 feet.
But now, with the higher estimate from the federal government, the project will reduce the level to 21 feet - almost back where we started. So ...
"If it doesn't go ahead and get built now, they may have to, in 10 years, maybe do some redesigning on it," said Ray Batchelor, in the Wilmington, N.C., district of the Corps of Engineers.
A number of things contribute to the rising 100-year flood level. The first thing engineers look at is development in the watershed.
"Every time you pave a parking lot, you add to the discharge into the stream," Batchelor said.
Development covers ground that would otherwise absorb much of the rain water during a storm, explained Marty Frengs, a FEMA engineer. In addition, homes, shopping malls and office complexes are designed so that water runs off the property into storm sewers or little creeks, which eventually dump into the river.
Then there's development along the river, which often requires filling in parts of the river, again reducing the area for water to go when it comes rushing down the channel.
Third, highway and railroad bridges catch debris and cause water to back up. They also require filling in areas sometimes.
"You can't blame the railroad, you can't blame Joe over there who built three houses, you can't blame the factory," Frengs said. "Each guy does his own little bit."
And each year's rainfall data are plugged into the engineers' computer models that determine the 100-year flood levels. The more information they have, the better the prediction.
The 1985 flood - of the 100-year variety - had a lot to do with last year's increase, Peters said. Several small tributaries, including Martins Creek, Deer Branch, Murdock Creek and Twelve O'Clock Branch, now have federally defined flood plains.
The valley's governments had no flood plain land-use management controls until 1978, Peters said. Since then, they require the bottom of structures built in the flood plain to be at least two feet above the flood level - a foot more than the federal government requires for its subsidized-insurance program.
In addition, Peters said, Roanoke and Roanoke County require developers who build near a waterway with 100 acres or more of contributing drainage to do a flood study, even if the development is not in the official flood plain.
Such measures should help cap the 100-year flood level, he said.
But there's no accounting for what nature holds in store.
by CNB