Coon Creek Watershed
The storm event that led to the dam failures and this PLAN-EIS process for both watersheds is summarized on the Project History page.
Coon Valley is located in the Driftless Area (unglaciated area) of Wisconsin and characterized by steep, high bluffs and numerous coulee reentrants. Coon Creek Watershed has an area of 90,601 acres (141.6 square miles) to the confluence with the Mississippi River. The focused planning area for the PLAN-EIS is 68,762 acres (107.4 square miles) and includes the village of Chaseburg. Approximately three-fourths of the watershed area is in Vernon County, the remaining area is equally divided between La Crosse and Monroe Counties.
The Coon Creek Watershed Work Plan was the first of its kind to be developed by the NRCS (formerly SCS) for the Coon Creek Flood Control Project in 1958 (supplemented in 1960 with revised supporting tables). The Project Agreement was signed by the NRCS and the La Crosse, Monroe, and Vernon County Conservation Districts. Conservation Districts are no longer organized as a subdivision of state government in Wisconsin, but their primary functions were deferred to land conservation committees under their respective County Boards.
The major problems in the watershed in 1958 were floodwater damages to crops and pasture, fences, farmsteads, machinery, buildings, livestock, county and township roads and bridges, and urban areas of Coon Valley and Chaseburg. Project measures implemented under the Watershed Work Plan included a multitude of land treatment practices to reduce erosion and improve the hydrologic condition of watershed; and 14 flood control dams with a total capacity of 1,160 acre-feet to regulate flood flows from 20.75 square miles, or 27 percent of the watershed above the village of Coon Valley. The benefit-cost ratio for the project was about 1.2:1.
On the night of August 27, 2018, seven watershed dams over-topped and three dams failed; Luckasson Dam (CC 21), Blihovde Dam (CC23), and Korn Dam (CC 29). Rainfall amounts up to 11 inches were reported on the night of August 27 and early morning of August 28. Additional rainfall amounts up to 7 inches were reported in the afternoon of August 28, after the dam failures.
The dams failed (breached) along the interface between the earth-fill and highly jointed Jordan sandstone bedrock. Each breach extended full depth to the valley floor. No one was injured or killed. Large debris fields were observed downstream of the dams for about ¼ mile. An unoccupied house was moved off its foundation. Agricultural lands and road crossings were damaged. Nearly identical dam failures were observed after the same storm event in the adjacent West Fork Kickapoo Watershed at Jersey Valley Dam (WFK1) and Mlsna Dam (WFK Pilot).
Historic failures of similar nature are also mentioned in SCS investigation reports at Dahlen Dam (CC41) after a 5 to 6-inch rainfall on July 1, 1978; Clockmaker Dam (WFK16); and Bad Axe Watershed Sites 12 and 33.
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Combined Investigation Report for Coon Creek and West Fork Kickapoo Watershed Structures. NRCS, 2019.