By Steve Vogel
Washington Post Staff Writer
Wednesday, June 28, 2006
The rainfall drenching the Washington region has been a once-in-200-years event, and numerous and destructive flash floods have raced down area creeks and streams. But flooding on the Potomac River is not expected to even approach that of major deluges of the past, forecasters said yesterday.
During flooding caused by Hurricane Fran in September 1996, the Potomac River crested in Georgetown at 13.75 feet, and it reached 17.72 feet during a record flood in October 1942.
By contrast, the crest in Georgetown for this week's event was likely to be between seven and nine feet during high tide late tomorrow morning and again tomorrow evening, a prediction which, if it holds, would not be expected to cause major damage, forecasters said. At the Wisconsin Avenue gauge in Georgetown, seven feet is considered a minor flood and eight feet a moderate flood. Major damage could begin at 10 feet.
Still, the record rainfall, combining with a storm surge, was likely to cause minor to moderate flooding along the tidal Potomac, including in Old Town Alexandria, according to forecasters and hydrologists. Yesterday evening, water was over the edge at Washington Harbour in Georgetown, where the floodwall had been raised to protect shops and restaurants.
Rich Hitchens, a hydrologist with the National Weather Service, cautioned that the projections might change. "The rain's not done yet," Hitchens said, as tropical rain approached Washington.
A major reason for the relatively low level of Potomac flooding expected is the near-drought conditions that preceded the rain. Last week, the Potomac River at Little Falls was running at less than a quarter of its normal June flow.
"We started the week in a very, very dry condition," said Jim Kolva, a hydrologist with the U.S. Geological Survey. "We were at a rainfall deficit, the ground was dry. Quite simply, a lot of the rainfall soaked right into the ground."
The flow at Little Falls yesterday was about 30,000 cubic feet per second, more than three times normal. Hitchens projected that Little Falls might reach its flood stage of 10 feet tomorrow, but it is not expected to reach anywhere close to the 19.29 feet reached during the January 1996 flood, caused by melting snow pack, which severely damaged the C&O Canal.
Had this week's rain come at a different time of the year -- in the winter instead of a dry summer -- the story might be different, Kolva said. "The trees and vegetation were stressed," he said. "With them in full leaf, a lot of the water was soaked up."
From 7 a.m. Friday through 8 a.m. yesterday, 12.11 inches of rain had been recorded at Reagan National Airport. "This amount of rain in four days should occur once every 200 years, and we just lived through it," said Jim Lee, meteorologist in charge at the National Weather Service in Sterling.
Another major factor for the lack of expected major flooding is that the high rainfall totals in the Washington region were not spread over the vast Potomac River watershed, which drains 14,670 square miles, including portions of Maryland, Virginia, West Virginia, Pennsylvania and the District.
"Those kind of rainfall totals, first of all, didn't happen over the entire basin," Kolva said. "Some areas got one or two inches. The system is able to absorb that."
Although streams and creeks can flood quickly, a major river such as the Potomac "integrates the whole watershed," Kolva said.
By contrast, Hurricane Agnes in June 1972 dropped six inches or more over much of the watershed, triggering a major Potomac flood that registered 15.45 feet at the Wisconsin Avenue gauge and 22.03 feet at Little Falls.
Although much of this week's rain came Sunday and Monday, the Potomac is not expected to crest until early tomorrow because it takes time for rain across the watershed to reach the Potomac and flow downriver. "Small streams flood much more quickly because the distance the water has to travel is much less," Kolva said.
In addition to rainwater flowing downriver, officials were keeping a nervous eye on a storm surge that could cause coastal flooding on the tidal Potomac and western shore of the Chesapeake Bay. The tropical disturbance that blew into Washington last night could drive water ashore. "You have an upriver wind," Hitchens said. "That's what makes it extra challenging to predict."
The last storm surge during Hurricane Isabel in 2003 caused more than $10 million in damage in Alexandria and Fairfax County after a nine-foot wall of water coursed through Old Town and the Belleview and New Alexandria neighborhoods in Fairfax County, damaging 2,200 homes and more than 60 businesses.
Alexandria officials were monitoring levels of the Potomac yesterday from an emergency command center that has been open since Sunday evening and will stay open until the crisis has passed, said Brian Hannigan, a spokesman for the city.
City officials sent leaflets to businesses in Old Town yesterday, warning that the river could rise one to two feet above normal at peak high tide during the next few days.
Bob Lorenson, owner of the Virginia Shop on South Union Street in Old Town, was carting stock from the first floor of his business to the second floor yesterday. Even though the end of his block often floods during high tide, he hadn't taken the extra precaution of moving all his wine bottles and gift items upstairs since Hurricane Isabel, he said.
"I'm not that concerned," Lorenson said. "I think the floor might get wet."
Staff writer Annie Gowen contributed to this report.