In less than four years - on Jan. 30, 1982 we will celebrate the 100th anniversary of the birth of Franklin Delano Roosevelt. That would be a most appropriate time to dedicate a great memorial to the man who led the nation in two decisive "rendezvous with destiny" - the Great Depression and World War II.

There are two official proposals for F.D.R. memorials.

One would place it in Washington, between the Tidal Basin and the Potomac River, in the august company of the monuments to George Washington, Thomas Jefferson and Abraham Lincoln.

The other would be in New York City - on an island in the East River that used to be called Welfare Island and is now Roosevelt Island. It commands a magnificent view of the Manhattan skyline, and of the United Nations which F.D.R. helped create.

The Washington proposal is the culmination of an 18 year search by the F.D.R. Memorial Commission, which was established by Congress and is currently chaired by Engene J. Keogh, a former congressman from New York. By act of Congress, the commission must build the memorial on the site along the Tidal Basin, known as West Potomac Park. And that accounts for most of its troubles.

The site, to be sure, was selected by a committee of eminent urbanists - including Lewis Mumford - appointed by the Memorial Commission itself. The commission and the committee wanted the best without any notion just what that was, and the beauty of the site is beyond dispute.

But when a national competition yielded various designs for the West Potomac site, conscious and subconscious resistance developed.

Most of Washington's taste and opinion-makers were not ready emotionally to admit F.D.R. into the green shrine devoted to Washington, Jefferson and Lincoln. They were certainly not ready to allow a conspicuously modern sculpture - the brilliant Pederson and Tilney design that was dubbed "Instant Stonehedge" - into the almost sacred, lyric parkscape the Macmillian Commission created more than half a century ago.

A second design by Marcel Breuer, commissioned in 1966, when former senator Eugene McCarthy was the chairman of the Memorial Commission, did not appease the opposition. How could it? It was still an intrusion - esthetically and symbolically - and generated as much objection as the proposal to slash a six-lane freeway under the Lincoln Memorial and across the Tidal Basin.

The commission next decided to honor F.D.R. with a rose garden. But that seemed too slight. Roses bloom only part of the year.

So now we have arrived at a contradiction: a non-monumental monument, a memorial which is, in the words of its designer, painstakingly "unobstrusive or meant to be. The contradiction is, of course, that a memorial is supposed to obtrude itself into our memories.

The designer of this non-memorial is Lawrence Halprin, a noted landscape architect from San Francisco, who has created a formalized "happening," of the kind that was so new and exiting in '60s.

Halprin proposes a granite wall, about 14 feet high, that zogzags for some 1,000 feet down the landstrip between the river and the basin. Along the way are various stations, that, by means of sculpture, planting, gushing water, sound and light, will entertain and enlighten people.

Like so many post-modern designs that are intellectual rather than visual, the Halprin design relies on a plethora of rationalizations and verbi. It is therefore hard to tell from his age. It is therefore hard to tell from his drawings and words just what the effect of all this will be. You cannot anticipate the experience.

Emotional impact aside, what we have here is a sort of outdoor museum that will instruct us about Franklin Delano Roosevelt's life, partly with symbols (water tells us about his Navy experience), partly with inscriptions of things he said and partly with sculptures and bas-reliefs by Leonard Baskin, Neil Estern, Robert Graham and George Segal.

I find myself a little apprehensive about "a sculptural evocation of the Roosevelt administration's response to the Great Depression . . . a man and a woman in a room listening intently to a radio." A three-dimensional W.P.A. mural revival?

Equally hard to visualize is how the U.S. Congress, in these parsimonious days, will respond to this $46 million plus experience.

True, the Fine Arts Commission and the Planning Commission have given the idea a shrug-of-the-shoulder approval. But the hard questions - about the cost of operating and maintaining this museum, the sufficiency of its parking, the wisdom of drawing big crowds to such a small area, the airplane noise and the rerouting of the popular cherry blossom drive around the Tidal Basin - are likely to further stiffen resistance.

Again, I fear, the site is in the way of concept. If we really want an outdoor museum-memorial (there are some good ones in Israel), we don't really want it in West Potomac Park. And if we want more acitivity in that area, how about more tennis courts, walkways, benches and other amenities?

The proposal for Roosevelt Island in New York City is a monument plain and simple - and very beautiful. It was designed by Louis Kahn, who died in March 1974, and is considered one of America's greatest modern architects. The F.D.R. memorial was his last work.

The structure is essentially a rather small room, one the very tip of the narrow island, open to the sky and enclosed on three sides by thick granite walls. The open side frames a view of Manhattan with the river in the foreground and the open sea beyond.

The impact of this simple drama is heightened by a long processional approach, either on granite-paved walkways on either side of the promontory, or across a stretch of lawn in the center. Both walkways and lawn are shaded by linden trees.

Before you enter the "room" - a kind of sancturary with inscriptions of Roosevelt quotations on the Walls - you pass a small sculpture court containing an allegorical sculpture and a likeness of Roosevelt.

Four years ago, when this design was approved, the memorial was to cost $4 million. Even if that figure triples becasue of inflation, it will cost only about a quarter what Halprin's happening would cost, to say nothing of the maintenance and operation of his "interpretative center."

Louis Kahn's design need not be interpreted. The sky and the skyline will speak for it.

The New York F.D.R. memorial was conceived as a special attraction to the Roosevelt Island new town, which is now thriving. It was designed and is being built and managed by the New York State Urban Development Corp.

The memorial and its surrounding park are to be jointly funded by the City of New York, the state and private donations.The city's financial crisis forced the postponement of construction.

The Franklin D. Roosevelt Memorial Commission, I suggest, could easily bring its difficult mission to a glorious conclusion by abandoning West Potomac Park and shifting its efforts to New York's Roosevelt Island.

That may take some persuasion in a Congress which doesn't like to change its plans. But then, Keogh of New York is a most persuasive man.

And no one in Washington, I suspect, will put up much of fight to keep that granite wall.