Two general contractors and the consultant employed by Metro to oversee subway construction were jointly blamed yesterday for the construction accident that flooded and closed part of the Blue Line in August 1977.

That assessment was contained in two reports the Metro staff submitted to the Metro board after months of investigation. The extra costs of construction because of the accident are expected to total more than $2 million, according to Metro sources.

The flood occurred when waters of the Washington Channel reached two construction dams and a supposedly watertight bulkhead, then moved through a completed but unused subway tunnel on the future Yellow Line to the L'Enfant Plaza station. There, the Yellow Line runs above the Blue Line.

Water rose to about 3 feet, clogging drains shared by both lines and backing up into the Blue Line, closing eight stations for four days.

The accident occurred on a $38.6 million project to complete the Yellow Line connection from the edge of the Southwest waterfront to the Pentagon. The plan was and is to sink tubes into the Washington Channel and connect them with concrete tunnels on each side of the channel.

The construction dams, called cofferdams, were supposed to hold back the waters of the channel while workmen excavated for the tubes.The sunken tube project is a $38.6 million joint venture of Perini Horn Morrison Knudsen. The Perini job abuts completed tunnel work running from L'Enfant Plaza to the waterfront. That contract was held by Traylor Brothers. The breach occurred between the Flagship and Hogate's restaurants.

Perini built the cofferdams; Traylor Brothers installed the bulkhead.

The entire Metro construction project is being supervised by Bechtel Corp., which received $19.5 million from Metro in 1978 for performing that work. The present Bechtel contract is being renegotiated from month to month.

Investigators found that the cofferdams had been flooded to a lesser degree on three previous occasions, yet the same construction techniques were continued. The bulkhead, they found, was inadequately fabricated. A drain pipe leading to the Metro tunnels from the channel was not capped as it was supposed to be.

Bechtel's supervisors were supposed to oversee the project. A consultant study commissioned by Metro from A.A. Mathews Inc., of Rockville, called the work of all three principals "less than satisfactory."

A report from John S. Egbert, Metro's assistant general manager for design and construction, said that "no single action was in itself the principal cause".

A spokesman for Perini declined to comment because "we have not read the report."

Thomas Traylor, president of Traylor Brothers, said "Our belief is that [the bulkhead] was damaged during construction subsequent to completion of our part of the construction."

Thomas G. Flynn, a San Francisco spokesman for Bechtel, said, "We take strong exception to the Mathews report. It is wrong, inaccurate and very incomplete.

"It is clear that [Metro] also does not agree totally with the report and has submitted its own. Beyond that, we believe [Metro] was completely correct in its position that further detailed discussion of the matter at this time is inappropriate."

Metro officials declined to comment beyond the reports.