Northern Wisconsin State Fairgrounds Erosion Repair

Challenge

The Northern Wisconsin State Fairgrounds received a record-breaking amount of rainfall in a single 24-hour period – over seven inches. What resulted was two major washouts on a steep hillside at the perimeter of the fairgrounds. The eroded slope was approximately 40 ft. high and steep at a 1:5:1 slope. Approximately 2,000 cubic yards (CY) of earth were displaced during the washout; this is equivalent to approximately 150 dump truck loads of soil. The larger of the two washouts left a 10-15 ft. deep void in a camping area immediately above the hillside. The resulting void also exposed water, sewer and electrical lines servicing the area. Further adding complications, the impacted area is a decades-old dumping ground for old concrete block and other construction debris.

Another added challenge was that the rainfall and resulting damage occurred in the fall. As a result, Northern Wisconsin Fairgrounds, Inc. was in need of a project team that could create a design, attain regulatory approval and have the project under construction within 60 days. If the project was not underway during this time, the harsh Midwest winter would set in, freezing the soil and making the project more difficult.

Solution

Northern Wisconsin Fairgrounds, Inc. sought the expertise of SEH to begin an investigation of the washouts and the adjacent tributary drainage area to offer up solutions for repairing the site. After close examination of the area, the SEH project team concluded that a permanent stormwater management plan and immediate repair of the erosion areas would be necessary. In the initial phase of the project, and because of the complexity of the eroded landscape, three stormwater management options were considered for repairing and rebuilding the site. Two of the options included stormwater detention ponds. Because the fairgrounds use nearly all of their property area for events throughout the year, they opted against these options as they can take up a lot of much needed space.

The selected option included using large capacity stormwater collection piping along with an armored overflow spillway for significant rainfall events. This option avoided the loss of any fairground surface area.

Because of the steep nature of the hillside and its propensity for future washouts, the final solution was a two-stage system that would carry stormwater safely down the hillside using large capacity stormwater collection piping. The pipe system handled runoff from most rainfall events, while the armored overflow spillway would handle larger rainfall events and minimize the probability of future slope erosion.

Once the design concept was approved by the Fairgrounds and Wisconsin Department of Natural Resources, SEH developed plans and specifications within one week so the Fairgrounds could secure a contractor and begin work before the ground froze. It was also essential for the site to be repaired and ready for the spring snow melt and rain.

The stormwater sewer pipe system consists of 24-in. storm sewer pipe traveling down the steep slope. The project team also implemented two 20-25 ft. deep drop manholes extending vertically from the main storm sewer pipe to dissipate the water and help to slow the flow, minimizing erosion potential at the bottom outlet. The schematic below illustrates how the stormwater pipe system conveys water away from the area.

The void created by the erosion was filled in with compacted fill along with a layer of clay to help prevent the subsoil from becoming saturated during future rainfall events, a contributor to the slope erosion.

Because of the length and sharp angle of the slope, the team implemented a specialized long-reach backhoe to avoid further disrupting the area and performing the required restoration.

The project team placed an armored overflow spillway consisting of approximately 150 CY of large boulders and rock over the top of the storm sewer pipe system. The team also poured concrete over the rocks and boulders to further strengthen the slope against future erosion.