Effects Of Pool Drawdown And Wing Dams Pool 8 And Closure Dams Pool 13 On Navigation Channel Sedimentation Processes Upper Mississippi River

Construction of navigation locks and dams on the upper Mississippi River about 60 years ago submerged wing dam training structures, thereby reducing their effectiveness and increasing secondary channel and floodplain conveyance. The U.S. Army Engineer District, St. Paul, executed a drawdown of Pool 8 (upstream of Lock and Dam No. 8) near La Crosse, WI, during the summers of 2001 and 2002. Water levels were allowed to drop below normal minimum values to expose mud flats, promote seed germination, and benefit fish and wildlife. By lowering water levels during a drawdown, wing dam training structures submergence and floodplain conveyance will be decreased, and flow patterns around the training structures will be altered. This could result in sediment mobilization and scour in the navigation channel. During the spring of 2001, three closure dams were constructed in Pool 13 (upstream of Lock and Dam No. 13) by the U.S. Army Engineer District, Rock Island, near Savannah, IL. These closure dams are actually submerged weirs that should allow water to continue to flow into the backwater areas of the islands of Pool 13, but at reduced rates. At issue is whether the main channel might require reduced dredging in future years as a result of the construction of the closure dams and, also, whether the backwaters of the eastern side of the islands will fill with sediment.

The likelihood of 1 - or 2-foot drawdowns, and the area affected by such alternative drawdowns, was estimated for Pool 13 on the Upper Mississippi River. Minimum water surface (elevation) requirements were compared to computed water surface profiles to determine a critical low flow that would allow a navigation channel 400 feet wide and 10.5 feet deep. An upper limit on flow was established based on the flow at which open river conditions would exist for a given drawdown. The range in flows that would allow for a drawdown was used to estimate success rates using historical daily discharge data. Success rates were determined for a variety of drawdown durations between two time periods, May 1-August 15 and June 15-August 15. The greatest effect of the drawdown would be near Lock and Dam 13. Higher discharges, as well as distance from the dam, lessen the drawdown amount. Areas that would be affected by these two drawdown scenarios were predicted by overlaying maps of water surfaces with depths using a geographic information system. Although the drawdown effects on most physical and biotic components of Pool 13 are unknown, some general drawdown effects likely to occur because of the changes in water surface elevation are presented.