Potomac '100-year flood' hits twice in eight months Increased stream flow damages towns, washes more sediment into bay

September 11, 1996|By Frank D. Roylance | Frank D. Roylance,SUN STAFF

Take three months' worth of normal rainfall, drop it all onto a mountainous watershed in just one weekend, and you get the sort of flooding residents along the Potomac River have suffered from the remnants of Hurricane Fran.

The mud-brown, roiling torrents have damaged riverside communities and carried billions of pounds of sediment and nutrients toward the Chesapeake Bay.

"The stream flows we measured this past weekend are among the highest we've ever seen in the Maryland area," said Robert W. James Jr. of the U.S. Geological Survey (USGS) regional stream gauging program.

Parts of the upper Potomac basin recorded what hydrologists call a "100-year-flood." That's a volume of water with a 1 percent chance of occurring in any given year. On average, it's expected only once a century, but like hitting the lottery, it can happen anytime.

It was the second 100-year flood the Potomac watershed has seen in eight months.

The first was recorded on Wills Creek near Cumberland after the January blizzard, when record snows began to melt and engorged the Potomac and its tributaries. The second came with Fran, on the South Branch of the Potomac, near Springfield, W.Va.

"This is the first time it's ever happened -- that we've had two floods of this magnitude in one year," said Martha A. Hayes, a USGS hydrologist in the Towson office, which covers Maryland, Delaware and Washington.

"Ever," in this case, means since 1931, when the USGS began keeping regional data.

The center of Hurricane Fran made landfall near Cape Fear in North Carolina, then moved inland to the north-northwest across Virginia and West Virginia.

Rainfall amounts totaled 8 to 12 inches in the lower elevations of those states. But where the tropical air was lifted higher and cooled by the Appalachians, rainfall increased to 12 to 16 inches.

That water landed in a region that was already well-watered. Water tables in Garrett, Washington and Montgomery counties were at above-average to record levels last month.

Flows into the Chesapeake Bay from all tributaries since January have been nearly double those of last year, according to USGS data.

During August alone, the flow of fresh water from the Potomac to the bay averaged 6.7 times normal -- the most in USGS records.

So Fran's added torrents could do little but run off. Most-affected were the South Branch of the Potomac, which begins in West Virginia, and joins the North Branch below Cumberland; and the Shenandoah River in Virginia, which enters the Potomac near Harper's Ferry.

Near Washington, Fran's runoff ranked as a 20-year event -- one with a 5 percent chance of occurring in any one year. It was slightly smaller than last January's 25-year flood. The Potomac's peak flow near the capital was 202 billion gallons per day. That compares with 224 billion in January. The record -- 312 billion gallons per day -- was set in 1936, during an event triggered by snowmelt and rain.

USGS officials were gathering samples yesterday to help them measure the amount of sediment, fertilizer and other nutrients flowing into the bay.

In January, the raging Potomac swept an estimated 2.5 billion pounds of sediment to the Chesapeake, including 20 million pounds of nitrogen and 2 million pounds of phosphorus.

Fran's erosion could be worse, since the ground this time is not frozen.

But the damage to bay life may be limited. Although increased sediment can block sunlight and smother bay grass and other water plants, USGS hydrologist Scott Phillips said those species are dying off at this time of year anyway as part of their natural life cycle.

Pub Date: 9/11/96

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