The Yellow Line construction accident and flooding that crippled the Metro subway last weekend occurred on a route that may never go anywhere and may not be needed, but will cost at least $79 million.

The flooding, which curtailed operations on the existing subway lines for four days, also has raised serious questions about the adequacy of Metro's contract supervision. For such a flood to happen, three supposedly watertight barriers would have to fail. All three did.

The Yellow Line wasn't supposed to be of questionable usefulness: it just seems to be working out that way. The line was to run from the Spring-field-Franconia section of Fairfax County through Alexandria and Arlington, across the Potomac River on its own bridge, under the Washington Channel in its own tubes, through the heart of Washington and out to Greenbelt in Prince George's County.

But inflation, strikes, environmental concerns and decreasing expectations for Metro have forced a restudy of where the subway should go and how much should be completed. Both ends of the Yellow Line - from Alexandria to Springfield and from downtown Washington to Greenbelt - are candidates for extinction in that study.

As a result, there is the possibility that the Yellow Line's completed Potomac River bridge and Washington Channel tubes under construction will be nothing more than part of a glorified connection track between existing operating stations on the Blue Line: the Pentagon in Virginia and L'Entant Plaza in Southwest Washington.

The flood brought all that - plus Metro's construction difficulties - sharply into focus.

Metro board member Clcatus Barnett put the big question direftly to Roy T. Dodge, assistant general manager for construction and design, at Thursday's board meeting. "Was the Bechtel Corporation's supervision satisfactory?" Barnett asked. "If not, what action should be taken?"

The Bechtel Corp., a private contractor to Metro, is the supervisor for all subway construction here. It is supposed to oversee each piece of work and make sure it is done right. Bechtel is receiving about $23 million from Metro this year for that work, Dodge said. Bechtel reports to Dodge.

Dodge told Barnett and the board members that the question would be answered as part of a complete investigation. A. A. Mathews Inc., a Rockville construction firm that is not involved in the Yellow Line contracts, will assist Dodge in the investigation. Mathews is doing other construction work for Metro.

A chronology of the flood has been put together in interview with Dodge and engineers and contractors directly involved in the Yellow Line construction.

It was at 6 p.m. on Thursday, Aug. 25 when a cofferdam, or barrier, on the Yellow Line construction site in the Washington Channel sprang a leak and started the chain of events that partially closed the subway.

That construction project is the only uncompleted one in the link between the Pentagon and L'Enfant Plaza. It is a $38.6 million contract held by a joint venture of Perini, Horn, Morrison and Knudson to connect a tunnel at the northern edge of the Washington Channel between Hogate's and the Flagship restaurants with the new bridge that crosses the Potomac in the area of the 14th Street highway bridge.

Much of the contract involves preparing the bottom of the Washington Channel for the reception of giant steel tubes. When the preparation is done the tubes will be sunk into place. Eventually trains will run in them.

The rest of the contract is a more routine cut-and-cover operation to build a tunnel across. East Potomac Park linking the sunken tubes with the new bridge.

The cofferdam had been a problem from the day it was first driven.Material underlying the channel was very hard, and there was some splitting between the interlocking steel piles as they were hammered into the earth. Leaks occured several times and were plugged. More than $300,000 worth of grouting had been poured to seal splits, or leaks, at one time or another. Still, the dam did not hold.

About 7 p.m. an inner wall of the dam - a second line of defense - also failed. Water began to pour from the first compartment into the second and to lap against, then submerge two timber and steel bulkheads covering the entrances to the completed but unused Yellow Line tunnels.

Between 7:20 and 7:30, the eastern-most of those two bulkheads developed a large leak and water began to roll through both interconnected Yellow Line tunnels to the L'Enfant Plaza Station, a major stop on the Blue Line that started operation July 1.

Metro stopped running trains. At the construction site, Perini personnel put out a cry for help in other contractors in the area. Within 30 minutes, four 10-ton dump trucks from other Metro projects had appeared and unloaded rock and dirt into the cofferdam excavation around the inner barrier. By 10 p.m., 60 truckloads had been dumped and fabrication was beginning on a new bulkhead to seal the leaky tunnel entrance.

But the water continued to rolls. It reached l'Enfant Plaza, then plummeted through the drainage system there to the Blue Line tracks below. From there it rolled to the low point in that part of the system - the Federal Center SW Metro station. A small lake about four deep formed.

Sometime Friday, Aug. 26, the situation was stabilized. Water was still coming in, but emergency pumps were keeping it contained behing a sandbag barrier that had been thrown up in the LEfant Plaza station.

From that point forward, it was a question of cleaning up and getting a good seal on the tunnel bulkhead. Metro maintenance crews replaced silt-laden equipment in the tunnels and cleaned the tracks. They were ready to run trains Monday morning, but had to wait until Wednesday afternoon until the tunnels were declared safe to reopen seven of the Blue Line's 18 stations.

Lost subway revenue - the fares for something in excess of 40,000 passengers for four days - and the cost of substitute bus service will be only part of the total bill. Dodge said he expected Metro's insurance to pick up the tab. There is certain to be a lengthly wrangle involving contractors, insurance agents and lawyers before the bills are settled.

The Potomac River and Washington Channel crossing were added to Metro's planned subway in March, 1968, after a regional system containing only one river crossing was proposed at public hearings.

That one crossing - now part of the operating Blue Line - was to be between the Foggy Bottom station in Washington and the Rosslyn station in Arlington.

In the 1968 plan, the line went from Rosslyn directly to the Pentagon, then south through Alexandria and went to Springfield. Another line branched west from Rosslyn generally along the Interstate 66 corridor to the Beltway. That plan is similar to what Metro intends today. The Springfield line is open as far south as National Airport and the westward extension from Rosslyn is under construction to Glebe Road.

Newspaper files and interviews with politicians and planners who went through hearings on the 1968 plan show that there were two basic problems with the system as far as Northern Virginians were concerned.

There was no line on Columbia Pike, probably the most congested corridor at that time in Northern Virginia.

There was no river crossing near the 14th Street Bridge. Therefore, residents from Alexandria, Arlington and Southern Fairfax County would have to go out of their way - to Rosslyn and Foggy Bottom - to get to work by subway in the Southwest or on Capitol Hill, D.C. residents who worked in the Pentagon also protested about having to go the long way around to get there.

(Prosposals for a single crossing in the Memorial Bridge area and a Y-type junction to Rosslyn and the Pentagon were rejected in the mid 1960s for both engineering and operational reasons.)

A Columbia Pike line was deleted from the final system because of its high costs. It would have had to be tunnel construction through uncertain rock and the other two lines could be done for about the same money.However, the existing Pentagon station has been built so that a Columbia Pike line can be easily connected sometime in the future.

A Potomac crossing was added to give Alexandria and Fairfax residents a direct shot to the Southwest and, not incidentally, to assist Metro in winning approval of a bond issue from Fairfax County voters. The bonds passed overwhelmingly. That was 1968.

Since then, one of the most successful public transportation experiments in the U.S. in terms of moving people has occured on the 14th Street Bridge: the Shirley Highway express buses.

They have been popular from the beginning because they are fast, travel on and exclusive bridge of their own and do not encounter traffic problems until they are actually in Washington. Riders going to the Southwest and Capitol - those to be served by the added subway crossing - have a minimum traffic hassle.

Those going uptown, the Farragut Square area, will find that a transfer to the subway through Rosslyn is faster about 10 minutes than a rush-hour bus.

The obvious question that is now being asked was whether the 14th Street Metro crossing was needed, given the success of the buses.

Jackson Graham, former general manager of Metro and a man who ran it with an iron hand until he resigned in 1976, is given credit by all who observed him as being tremendously successful at getting things into the ground and never looking back. There is no evidence, in interviews with many long-time Metro planners, that anyone in the agency ever considered not building the extra river crossing just because the Shirley buses work.

The last contract for the crossing was awarded in April, 1976, three months after Graham left. The crossing is a fait accompli.

The fate of the ends of the line will be decided in the next few months along withe several other segments. The Yellow Line is vital to the future operation of a planned but unfunded line from Anacostia to Greenbelt, because the Yellow Line contains the only track connections needed to run it. But that line, too, is under study.

Metro planners still feel that the Yellow Line will be useful and will attract many passengers from the Alexandria corridor who wouldn't ride a bus anywhere and who wouldn't take the train the long way around.

"Yes its justified," said Mathew Platt, a top Metro planner. "From a service point of view, it's justified."

The crossing will cost $79,765,546. It's average cost will be $7,222 a foot.

The Rossyln crossing, contracted for before the Arab oil crisis, cost $6,247 a foot, even though it is in deep rock - supposedly the most expensive construction.