Researchers combining high-tech laser mapping and economic analysis are part of a new effort to understand a very old problem in Maryland: the effects of the steady retreat of coastlines.

A study released in August by the Department of Natural Resources estimated the impact rising sea levels might have on three low-lying Maryland communities. Towson University researchers determined that the south Anne Arundel County community of Shady Side, hit hard by flooding during Hurricane Isabel last year, could face even greater risks if sea levels in the Chesapeake Bay continue to rise.

Their study found that a two- or three-foot rise over the next hundred years would not put Shady Side permanently underwater, a danger for some communities on the Eastern Shore. But the researchers determined that rising waters could make the neighborhood even more prone to flooding, potentially causing tens of millions of dollars in damage.

Counties, universities and state agencies also are creating detailed elevation maps of areas near water. They say these will be useful for predicting which areas will be slowly inundated and which are likely to flood in the next big storm.

"We're all trying to make good decisions, and we need good data," said Ken Miller, who oversees the mapping efforts for the Department of Natural Resources. "Without this kind of information, you're kind of shooting in the dark."

Scientists say the Chesapeake's waters have been rising here for thousands of years: In fact, they helped create the Chesapeake itself, by permanently flooding the Susquehanna River about 10,000 years ago.

"It's essentially a drowned river valley," said Kerry Kehoe, coastal program manager for the Department of Natural Resources.

In more recent centuries, the Chesapeake's rising waters have turned solid islands into marshes and made other islands disappear. The best-known example is Sharps Island, once a 600-acre landmass with farms, a school and a hotel. Its only remnant is a leaning and decayed lighthouse.

In Anne Arundel County, the Maryland Geological Survey has noted that in the mid-1800s a set of small islands called the Three Sisters lay off the Shady Side peninsula. But those also vanished, probably by the turn of the 20th century, said Jeffrey Halka, chief of coastal and estuarine geology.

The reasons for rising sea levels are complex, officials say. Even setting aside theories about human causes of global warming, scientists say that Earth is in a naturally warm period, in which glaciers are melting and sea levels are rising about 1.5 millimeters per year worldwide.

In this area, the Chesapeake rose about one foot in the past 100 years. The land is also sinking because of a natural process called subsidence, lowering about one millimeter per year.

"The combination makes the Chesapeake have a higher rate of sea level rise than many other places," Halka said.

The study released in August was an attempt to take preliminary snapshots of how selected Chesapeake communities might be affected if water levels continue to go up. The three chosen were Shady Side, a dense and eclectic neighborhood on a peninsula south of the West River; the watermen's villages on Upper and Middle Hooper Islands on the Eastern Shore; and the semi-suburban communities of Piney Point and St. George's Island in St. Mary's County.

Researchers from a Frederick County company, EarthData, flew over each site in a small plane, aiming a laser at the ground. The plane sent down 22,000 laser pulses per second, then recorded the height at which each bounced back.

The data were analyzed to eliminate all sites where the laser hit a house or other structure, and researchers were left with an accurate map of how high the ground stood.

The margin of error was about half a foot, officials said -- very accurate compared with the topographical maps familiar to hikers, which mark only every 20 feet, officials said.

The researchers then compared the elevation map to an overhead view of the neighborhood and noted the elevations of houses, businesses and other lots. Then began the difficult task of determining which areas would flood, when they might flood, and whether the water would recede or stay for good.

On the Eastern Shore, the study found that the Hooper Islands were in serious danger of permanent flooding, which scientists call "inundation." The study found that if waters were to rise three feet over the next century, 153 properties could be inundated there -- more than at either of the other two sites.

Regarding the St. Mary's County communities, the study found that some roads might be inundated, along with 63 properties, if the sea level rose by three feet.

In Shady Side, by contrast, very little of the neighborhood sits below three feet of elevation. Instead of inundation, officials said, the main danger would come from flooding -- which would essentially be given a head start by a permanent rise in the water level.

"If Isabel comes again in 30 years, it will be this much worse," said Jeffrey A. Michael, a Towson University economics professor who worked on the study.

How much worse? Michael's study estimated that Shady Side would incur about $44 million more in flood damage during storms over the next century if the sea level rose two feet, and up to $60 million more in damage if it rose three feet. He said the estimates were made using flood insurance rates set by the Federal Emergency Management Agency that are based on an area's estimated potential for flood damage.

Sections of Anne Arundel suffered serious damage during Isabel last year, when floodwaters as high as six feet swept over bayside communities on the county's northern and southern ends.

"We learned that there's a lot of properties in this county that are still below basic flood elevation," said Bill Bryant, the county's administrator of inspection services.

In Shady Side, some people are still living in trailers or other temporary housing because of flood damage.

Trying to prepare for another such storm, state officials say they are eager to have laser-assisted elevation maps made of all of the state's low-lying areas. Maps are being prepared for all of Howard, Anne Arundel, St. Mary's and Charles counties, and they have been completed for many counties on the lower Eastern Shore.

Authorities said the maps will be used to create emergency management plans so police and fire departments will know which areas to focus on during floods. Talbot County, on the Eastern Shore, has given maps to residents and firefighters that show how much flooding could be expected from hurricanes of various sizes.

In many places, including Anne Arundel County, regulations require houses to be built at least one foot above estimated flood levels. But Kehoe of the DNR said that "we can't necessarily just build our way out of the problem."

The reason: Even if houses stay dry, the neighborhoods can't survive if roads, lawns, wells and septic systems are underwater.

For areas such as Shady Side, the data are making state officials wonder how much more development should go in.

"Do you want to keep putting people in there," asked Miller of the DNR, "when we know it floods when it rains hard?

A study says Shady Side could incur an extra $44 million in flood damage in the next century if the sea rises two feet, or $60 million with a three-foot rise. The site of the destroyed Marshall home, on Drum Point Road in Deale.Marshall's home, at far left, was razed after flooding from Hurricane Isabel. His new home, at left, was built above estimated flood levels. In Shady Side, north of Deale, a study found that rising waters could make the area even more prone to flooding, potentially causing tens of millions of dollars in damage.Bill Bryant, the county's administrator of inspection services, shows an elevated first floor and heat pumps and an anchored oil tank at the new waterfront home of James Marshall in Deale in south Anne Arundel County.