SOUTH CHESAPEAKE BEACH -- In the spring of 1987, the year after Nanette and Jonathan Sheldon moved into their new home on a cliff overlooking the Chesapeake Bay, a neighbor's house fell into the water.
Six months ago, a piece of concrete from another neighboring house's foundation dropped into the bay. And now the foundation of the Sheldons' home is cracked, and their front deck is halfway over the cliff.
When their neighbor, Mary Bethune, first settled in the small South Chesapeake Beach community of Randle Cliff, in 1955, her front yard, supported by the cliff, extended out over the bay by more than 30 feet, almost as far as a nearby pier.
Now, the cliff's edge is only three feet from her living room. Within about three years, she expects the cliff to be under the house. Mrs. Bethune says she will have to demolish the living room to buy more time for the stability of the rest of the house.
Beach and cliff erosion along the nation's backwater bays and ocean shores drives many waterfront homeowners to buy erosion insurance policies, which are underwritten by the National Flood Insurance Program.
The Federal Emergency Management Agency (FEMA), which administers the insurance program, has issued some 2.5 million policies amounting to $220 billion of coverage.
Jim Taylor, assistant administrator of the Federal Insurance Administration, an arm of FEMA, said that in fiscal 1990, FEMA paid out more than half a billion dollars in claims -- about $100 million more than average -- mostly because of Hurricane Hugo.
To reduce these staggering claims, the federal agency contracted with scientists at the University of Maryland at College Park and in four other locations to design technology that will, for the first time, accurately forecast the annual rate of erosion.
This technology, known as metric mapping, is planned to be a "cornerstone" in future erosion management programs, said A. Todd Davison, senior program specialist at FEMA.
Last May, the House of Representatives passed the National Flood Insurance, Mitigation and Erosion Management Act of 1991, legislation intended to encourage wider participation in the flood insurance program and to establish new coastal erosion management programs.
The Senate Banking Committee is to hold a hearing on a companion bill Sept. 17.
If the measure becomes law, FEMA will, for the first time, designate erosion zones along U.S. coastal areas and also provide local communities with forecasts of annual erosion rates for their area.
"The philosophy behind the bill is to reduce future losses of the government in erosion-prone areas by encouraging those most at-risk to relocate, and to discourage future development in those areas," Mr. Taylor said.
FEMA is trying to enlist communities in erosion management programs, including restrictions called "setbacks" that require new housing to be built a certain number of feet from the shoreline, depending on the area's erosion rate.
Although participation in erosion management will be voluntary, the federal agency will use sanctions on local governments to try to force compliance.
Residents of those erosion zones where local government has not begun erosion management practices will eventually lose their insurance coverage and other benefits, such as relocation of their homes at government expense.
But FEMA and homeowners need to know where the risks are greatest.
"The foundation of erosion management has to be accurate identification of the hazard," Mr. Davison said.
That's where College Park's coastal geographers come in.
"We have developed state-of-the-art computer erosion mapping software," said Stephen P. Leatherman, a geography professor who is director of UM's Laboratory for Coastal Research.
With a FEMA grant of $267,000, Dr. Leatherman and his colleagues are examining erosion rates along Calvert County's Chesapeake Beach shoreline.
As part of the mapping process, the lab uses aerial photographs of shorelines from as early as the late 1930s and compares them to more recent photographs. But aerial photos are only good as reference points because they contain errors of spatial relationship.
"The airplane is never horizontal, and scale variation occurs across the photograph," said Gregory T. French, a coastal geographer in the lab who works with the computer system.
The lab developed mathematically based computer programs that check the spatial relationship between designated geographical markers -- street intersections, canals, and other points -- that can be seen in the photograph and can also be located on a map.
"Then we tell the computer, 'Match them up,' " Mr. French said.
The critical points, representing the shoreline markers, are moved to their correct location and all other points in between shift during the move.
In the process, the shoreline gets realigned to its correct position.
The geographers also use historic maps from the 1840s made by the old Coast and Geodetic Survey, which is now known as the National Ocean Survey.