A set of daily streamflow-routing models are used to simulate streamflow at 10 sites along the Wisconsin River for water years 1915-76, to determine the effects the reservoir system has on flood discharges. Streamflow is simulated under the following two conditions: (1) No reservoirs are in the system and (2) all of the present reservoirs are in place and operated with current rules.
At Wisconsin Dells, 20 miles upstream from Portage, daily streamflow hydrographs are estimated for the 10-, 50-, 100-, and 500-year floods. These were determined from statistical analysis of the simulated daily streamflows for the condition of all reservoirs in place.
The reservoirs have a significant impact on floods. The mean annual flood peak at Wisconsin Dells is lowered about 20% from 43,000 cubic feet per second for the simulated, unregulated condition to 34,000 cubic feet per second for the simulated, regulated condition. The 100-year flood peak at Wisconsin Dells is reduced about 10% (92,000 to 82,000 cubic feet per second) between the simulated, unregulated and simulated, regulated conditions.
The 100-year flood peak at Wisconsin Dells, computed from the simulated, regulated streamflow data for the period 1915-76, is 82,000 cubic feet per second, including the effects of all the reservoirs in the river system, as they are currently operated. It also includes the effects of Lakes Du Bay, Petenwell, and Castle Rock which are significant for spring floods but are insignificant for summer or fall floods because they are normally maintained nearly full in the summer and fall and have very little storage for floodwaters. (USGS)
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