Construction Controls Minnesota Floodwater

By Ivy Chang | May 9, 2011

Wet weather, accompanied by spring floods, has once again saturated soils in the Upper Midwest to create overflowing rivers and lakes that spread to roads, homes and entire towns. Minnesota state agencies, such as Department of Transportation, Department of Natural Resources, and Pollution Control Agency, have stepped in to provide repairs that prevent further damage in areas of severe flooding.

In the northwestern part of Minnesota, some lakes are overflowing and hindering traffic because roads are flooded. “There are quite a few lakes in the area that are land locked,” said Seth Yliniemi, project engineer for Mn/DOT. “Sand Lake is one lake that has been on a steady rise for many years. Last fall, the lake threatened to overflow Highway 59. We added four inches of asphalt on the highway and that got us through the winter. Now, it continues to rise and is going over the highway.”

The two-lane highway has 12-foot driving lanes and eight-foot shoulders. The nearby lake rose faster than predicted and keeps rising from the wet conditions.

“We’ve been in wet season for quite a long time,” said Yliniemi. “The water can’t go anywhere, and Mn/DOT selected the preferred alternative based on a consultant’s recommendation, which is to build a force main and lift station to pump the water to the Pelican River which connects other lakes and to other river systems.

“We have the plan together and we’re under emergency order because water is over topping the road. Pelican River is about one and a half miles from Sand Lake. Without a pump system, we couldn’t get the water out.”

Lake management

Hough Inc., Detroit Lakes, Minn., is the general contractor that installed pipes, the force main, and the lift station.

“The intent of this project is to manage Sand Lake down to its ordinary high water mark,” said Mike Hough, owner. “We notified our suppliers and subcontractors because this project is under emergency declaration.”

On April 11, Hough began installing the outlet and gravity pipes. “The outlet is at the Pelican River where the water is flowing and will come out at that point. The pipes are reinforced concrete with a hole and screen over the hole to keep debris out and keep animals from going in,” explained Hough.

“The water can’t flow directly into the river because there could be problems with turbulence and scouring in the river bottom. It has to come into some kind of a structure that will force the water to go up in a less erosive way.”

Hough said the project proceeds backward to the beginning with about 10 workers preparing the site to install the lift station. Workers changed the 24-inch pipe to an 18-inch dual wall polyethylene pipe at a control structure and installed the pipes going uphill about 3,000 feet in the ditch adjacent and parallel to Highway 59, and then to the high point.

“We excavated along the ditch by the highway to install trench boxes first, then installed the pipes. The soil was questionable in spots varying from sand to clay to organic materials. In some areas, we sub cut
the pipe and take out additional soil
underneath to put in select granular materials. With any pipe, we have to make sure there’s good foundation underneath it,” said Hough.

Hough installed the 14-inch PVC force main that carries water from the lift station to the project’s high point. “We have to force the water to the project’s high point with a pump. We will install a vertical turbine type of pump at the lift
station. The water will be pumped
into a concrete structure where it connects to gravity and flows out of the gravity pipe, about 3,000 feet long, that connects the high point to the Pelican River,” said Hough.

Dewatering wetlands

The pipes are completely installed along Mn/DOT’s right of way to the project’s high point. But the wetlands near the lake, about 1,000 feet long, had to be dewatered before the project continued. Hough hired Blake Drilling in Blaine, Minn., which installed dewatering equipment to draw down the water to an elevation that’s suitable for installing pipes on a stable foundation and offer frost protection.

Hough passed over the 1,000 feet of wetlands that must be dewatered to install more than 6,000 feet of force main. “Most of the water from the dewatering project will go through the force main and into the Pelican River through screened wellpoints to extract sediments.”

By the end of April, Hough installed
the lift station and prepared the lift station site, which included a driveway, a fenced area, and electrical power supply that enters a control panel to provide power for the pump. Before the pump will be allowed to operate, the Department of Natural Resources has to inspect the area and give permission to Mn/DOT to begin pumping water out of the lake.

The pump will take out 1,800 to 2,200 gallons per minute to bring down the lake to the normal high-water mark. Highway 59, covered with floodwater, may close and traffic will be redirected until pumping has a positive impact to open the road to traffic.

End of year completion

“It takes time to draw down an estimated 676 million gallons of water,” Hough said. “We figure the time required is about nine months working 24/7 to lower the lake by four to five feet. Our job now is to make sure the pump and lift station are working, ensure the outlet is clear, and maintain all the structures. If a problem arises, we have a 30-hour response time.”

By the end of this year, Hough will have restored the site, installed erosion blankets and seeded the area.

Yliniemi said during the floods, some farmers are pumping floodwaters to irrigate their fields. “Sand Lake used to have an area to land boats about 80 to 90 feet from the highway,” Hough said. In the last few years, there’s only floodwater at the lake and many water projects to deal with high water.