Beginning this weekend, four-foot high waves will start crashing on a concrete beach beside the Capital Beltway in Alexandria, opening the way for the city to become the surfing capital of Northern Virginia.

It's all thanks to a new $1.2 million wave pool at the new Cameron Run Regional Park, built by the Northern Virginia Regional Park Authority. Just last month the authority opened an $825,000 sports complex on the Occoquan River in Lorton and an $840,000 cluster of vacation chalets, with hot tubs, on the Potomac River in Loudoun County.

The eight-foot-deep wave-pool complex will include a small lake for canoeing and fishing -- it is part of the Cameron Run flood control project -- and an acre of sundecks where bathers can close their eyes, listen to the surf and get sunburns identical to those acquired at Rehoboth and Ocean City.

Alexandria Mayor Charles Beatley is scheduled to become the city's first surfer today when he arrives in his bathing suit to officially open the new pool. That is, providing the wave-making machine is working. Earlier in the week, it was malfunctioning, but park officials said they would have it working properly today.

The pool will not be open to the public until Saturday, however. The entrance fee will be $3.50.

Safety is a major factor in the design of the new Cameron Run wave pool, park authority executive director Darrel Winslow said. Not only will there be no undertow from the waves but also there will be no jellyfish or sharks.

By the same token, there can't be any sand castles for the children because there won't be any sand or sand dunes. But Cameron Run does have artifical grass running down almost to the water's edge, meaning that bathers will not have to wash off salt and sand -- or grass -- at the end of the day.

The wave pool did not arrive in Alexandria easily.

It is in the long, narrow Cameron Valley flood plain, between Duke Street and the Beltway, where some city officials have long dreamed of intensive high-rise development and environmentalists dreamed of a tree-lined stream valley.

In the end, the city won, and the man responsible was Dayton Cook, the longtime Alexandria director of public works.

Cook, for whom the little lake beside the wave pool is named, engineered the digging of the big flood control ditch for Cameron Run, the single most expensive public works project in the city's history. It has left hundreds of acres of largely private land high and dry for development and comes with a four-lane industrial road, Eisenhower Avenue, to serve them.

The 48-acre, city-owned park site, leased, to the park authority for 40 years, is on Eisenhower Avenue, off Telegraph Road, and is surrounded by vacant land for which developers already are bidding. The park authority opposed the four-lane industrial road as unsuitable beside a park but eventually agreed to develop an urban park when city officials made noises about quitting the 24-year-old regional park agency.

A swimming-pool complex was agreed upon, with ball fields nearby. Now the city is operating 17 acres of the site for playing fields. Originally, an indoor pool was proposed. That was eventually replaced by an outdoor pool that, in turn, was replaced by what park authority brochures say is "the newest piece de resistance of all recreational facilities throughout the country," a wave pool.

Opening coincidentally with Northern Virginia's wave pool this weekend is another one in the year-old Wild World theme park near the Capital Centre in Prince George's County. That one cost $2 million and is even larger than the Virginia one.

The million-gallon Wild World pool will have twice as much water as Cameron Run, and already its officials claim it is the world's largest wave pool. But admission to it will also cost more: $11.50, which includes access to the theme park's other mausements. CAPTION: Picture, $1.2 million pool at Cameron Run Regional Park