Salem, SD - Like several Midwestern states, in 1993 South Dakota experienced flooding along its major rivers. In Salem, the McCook County seat and home to about 1,325 people, 68 homes were damaged, 22 businesses were affected, utilities were lost, streets were closed, and residents evacuated to shelters.
It was not the first damaging flood in the agrarian center, which is located on the prairie about 35 miles west of Sioux Falls, the state’s largest city. Significant floods also had occurred in 1969, 1984, 1985, and 1986. To significantly reduce the flood risk in certain areas, in 1995 the city built a diversion channel along Snake Creek that accommodates overflow during heavy rains and flooding.
“The flood protection is much better because the diversion freed up a lot of our drop inlets, which take in water here in town,” said Ken Scott, who was public works director when the project was installed and mayor in 2010. “Before, there was just too much water coming into the stream.”
The effectiveness of the diversion channel was clearly demonstrated in 2008 when 5.75 inches of rain fell over a short period of time. This exceeded the event that had caused the 1993 flood, but Salem reported only nuisance flooding. Similarly, in 1995, 2009 and 2010, the area drained by the diversion channel avoided flooding despite heavy rains.
“The ditch helped tremendously,” Scott said.
Going approximately 1½ miles of the creek’s length, the diversion comprises about 1,900 feet of pipe and 4,600 feet of open ditch. While the original channel flows through the city diagonally from the northeast to the southwest, the diversion channel follows a slightly different path. It first heads due west to the edge of the developed area and then it flows south and west to rejoin Snake Creek near the southwest corner of the city.
The project also included the reshaping of certain portions of Snake Creek to create more efficient flows. Reshaping involved straightening some sections that were each less than 50 feet long and removing obstacles that restricted flow. The diversion approximately doubled the capacity of the original channel and each can carry about 200 cubic feet per second. Together, the original channel and the diversion provide protection from a flood that has a 2-percent chance of occurring in any year, which is also known as a 50-year flood.
Approximately 72 percent of the $902,612 cost of the diversion was provided by the Hazard Mitigation Grant Program (HMGP) of the Federal Emergency Management Agency (FEMA). FEMA paid $654,739 and the remaining $247,873 came from local sources.
The Snake Creek diversion is part of a planned comprehensive flood control system that is a work in progress. The city constructed another element, consisting of drainage improvements on the south end of Salem, around 2005 and provided additional improvements there in 2010.