Partnership helps prevent flooding along Minnehaha Creek

Gray's Bay Dam connects Lake Minnetonka to its only outlet, Minnehaha Creek. Courtesy Minnehaha Creek Watershed District

A partnership between the Minnehaha Creek Watershed District and the National Weather Service is helping to prevent flooding along the 22-mile creek, which flows from Lake Minnetonka through Southwest Minneapolis and into the Mississippi River.

The watershed district receives detailed precipitation forecasts and predictions of inflow and water levels on Lake Minnetonka from the weather service forecast office in Chanhassen. It uses the information to determine how much water to discharge out of Gray’s Bay Dam, which connects the lake to the creek. That in turn helps prevent flooding along the creek as it meanders from Minnetonka through Hopkins, St. Louis Park, Edina and Minneapolis.

The partnership began after the flooding of 2014, which caused over $1 million in damage along the six main streams in the watershed. Gray’s Bay Dam was submerged underwater, and water flowed into Minnehaha Creek at record levels.

“We weren’t able to control anything,” said Tiffany Schaufler, project and land manager for the watershed district.

Schaufler said the interagency partnership may or may not have made a difference when it came to the 2014 flooding. But she said the district wanted to take some of the lessons it learned and refine its dam-management process.

Four pieces of information go into dam management, according to Schaufler, including lake levels, the month of the year, the creek capacity and rain forecasts. The first three variables are known, she said, but the moving variable is always the forecast.

“What the weather service has allowed us to do is really refine a very specific forecast for Lake Minnetonka,” she said.

The weather service provides the watershed district with seven-day forecasts broken down into six-hour increments. The district releases more water from lake a few days before large forecasted rain events and slows the discharge a couple days before the rain. That allows the additional water to pass through the creek, ensuring that the creek doesn’t flood.

“Any change takes about two days for it to make its way through 22 miles down to Minnehaha Falls,” Schaufler said.

Schaufler said the partnership was helpful during the snowstorm this past April, when about 16 inches of snow fell during a four-day span. The district knew the lake levels were on the higher end of what they should be in the spring, she said, and knew of the incoming precipitation, because of the weather service information. It applied for a Minnesota Department of Natural Resources permit to aerate, or pass air through, the dam, thereby clearing ice and allowing water to pass through before the spring snow melt and rainfall.

Schaufler noted another piece in the partnership, which comes in the form of additional data from Hennepin County Emergency Management. The department has developed what it calls the Hennepin West Mesonet, which is a series of 30-foot towers across the county that provide real-time weather data. Schaufler said there are two towers on watershed district property and that the system helps the district better understand inflow into Lake Minnetonka.

NWS Service Hydrologist Craig Schmidt said the watershed district partnership also fulfills a major goal of his agency, which is to help people make better decisions when it comes to infrastructure, public safety and emergency management. He said the weather service also does a lot of work to help the U.S. Army Corps of Engineers with its navigation systems and helps smaller cities and counties with weather information for their outdoor events.

Event managers already have a lot to handle when executing an event, Schmidt noted, from managing traffic to the actual event itself. He said his agency’s work gives the managers peace of mind and allows them focus on their events.

“They know that someone is watching this for them and paying attention to this for them,” he said.

Like Schaufler, Schmidt also touted the success of the interagency partnership, noting particularly wet years in 2016 and 2017. Both years saw record-setting precipitation events, and 2016 was the wettest year on record in the watershed, according to the watershed district. Still, the agencies eliminated almost all flooding downstream, Schmidt said.