The heaviest flow in the upper Potomac River since 1954 has burst an earthern dike and a cofferdam, delaying work on a $153 million dam designed to protect the Washington area from future droughts.

Army Corps of Engineers officials said yesterday it is too early to estimate the cost and time it will take to repair the damage Monday night to the dam site near Bloomington, W.Va.

Dean Miller, a corps engineer, said the dam contractor took a "calculated risk" in building the cofferdam that rainfall would not be excessive while the temporary dam was being built. If the rains had come two weeks later the reservoir formed behind the cofferdam would have been large enough to handle the storm's waters, he said.

Miller, who is supervising construction work by the Perini Corp. of Massachusetts, said the heavy rains were unusual for July.

Miller said a 12-foot-high by 600-foot-long dike had been closed across the Potomac on May 22, sending the river flowing through a 16-foot diameter tunnel cut through a mountain to bypass the dam site.

With the dike in place, a cofferdam was begun to protect the construction of a permanent 300-foot-high dam. The cofferdam, which will be 50 feet high on completion, had been built to about 30 feet. Miller said the river began to top the dike at about 9 a.m. Monday, flowing in a foot-deep sheet across the top of the dike.

Floor waters quickly filled the area between the dike and the cofferdam and began to overflow the cofferdam.