Several Northern Virginia officials yesterday stood in the middle of the $55 million Four Mile Run Flood control project in Alexandria and said that, for the first time, all but the most devastating flood waters can be handled by a new 200-foot-wide channel.

"Many of you have waited a lifetime for this," said Dayton Cook, head of Alexandria's Transportation and Environmental Services department who acted as master of ceremonies. "This is a great day."

The project, started six years ago, is 90 percent complete, with remaining work to be finished in the fall, Cook said. He noted that the project as it stands is expected to handle any flooding during the summer and fall hurricane season.

For years the Arlandria section of the city, which adjoins Arlington, has been regularly inundated by flooding caused by heavy rains and the overflow from Four Mile Run, a creek that empties into the Potomac River near Washington National Airport.

The development of homes and offices near the creek prevents the waters from being absorbed into the ground, Cook said. Additionally, the only previous attempts at controlling floods channels built in the late 1800s that have proved inadequate, he said.

The results of flooding have been millions of dollars of damage to homes and businesses in the Arlandria area. Hurricane Agnes alone caused $14 million of damage during one furious week in 1972.

The floods themselves, or the fear of them, has stymied development of the area, which has lagged behind other sections of Alexandria and Arlington in new home and office construction, according to Alexandria City Manager Douglas Harman.

The heart of the project is the 200-foot-wide channel that runs from Mount Vernon Avenue on the west to the Potomac River on the east. The channel passes under seven railroad bridges, U.S. Rte. 1 and the Richmond, Fredericksburg and Potomac Railroad Yard until it enters the river near the airport.

Once the project is completed in the fall, boat docks will be built at Mount Vernon Avenue and Commonwealth Avenue, and a bicycle path will be opened, Cook said.

"This is an unprecedented moment," said former Rep. Joel T. Broyhill (R), who lobbied for federal funds for the project during his 22 years in Congress. "It is unprecedented for a member of Congress to work for a project while in office and then to see that it is completed when he leaves," he said.

Broyhill is chairman of the board of the Ohio Valley Construction Co., the prime contractor for the project. He joined the firm after leaving office in 1974.

Development of the project was held up for years by bickering between rival jurisdictions over who would pay for it, according to city officials. The Army Corps of Engineers completed a study of the area in 1966, and work was started in 1973.

Most of the $55 million cost of the project was borne by the federal government, with Alexandria contributing $6.5 million, Arlington about $2 million, and the RF&P Railroad about $500,000, according to officials. CAPTION: Map, New Four Mile Run channel runs from Mt. Vernon Ave. to Potomac River. By Richard Furno - The Washington Post