Located on the Potomac River about a mile upstream from Washington, DC, the Little Falls Dam has been the site of 22 drownings since 1975. To unknown boaters, it seemed harmless. Floating downstream toward the dam, all they could see was a small drop of about 2 feet and then a few mild rapids. The killer was hidden under the surface. The dam was designed as an Ogee spillway, with a dam end sill at the downstream base of the dam. This 2-foot-high toe section and the sloping downstream face of the dam were designed to dissipate hydraulic energy so the riverbed below the dam would not erode. However, this created a violent vertical roller effect in the deep water below the dam. Floating debris--or people--could get trapped in this vertical roller for several minutes.
After performing model tests that showed the procedure would eliminate the undertow turbulence, the U.S. Army Corps of Engineers decided to place grout-filled bags along the downstream face of the dam to create a spillway with a more gradual slope. The polyester bags were stacked 5 bags deep in a stair-step fashion, with the longest bags placed first and the shortest bags last. Using a metal frame that held the bags in proper place while they were being filled with grout, divers placed three bags in a row, all at the same time. Then, to help tie these bags to the bags already in place underneath them, the divers forced 2 1/2-foot-long epoxy-coated reinforcing bars down through the bags at about 6-foot spacings.