WASHINGTON—Filling a large number of bags with sand and then placing them side by side next to a body of water remains the nation's most sophisticated method for flood prevention, a two-month FEMA study concluded Tuesday. According to FEMA's findings, floods—natural disasters that have occurred since the beginning of time yet still destroy hundreds of American homes each year—are most often combated by scooping clumps of sand into burlap sacks and binding them with pieces of twine. "Perhaps bigger bags are the answer," FEMA head R. David Paulison said. The second-most effective U.S. anti-flood technology remains getting in the car and driving to an area that is not currently being flooded.