ERDC/CHL CHETN-VII-5
December 2003
Effects of Drawdown and Structures
on Bed-Load Transport in Pool 8
Navigation Channel
by David Abraham and Jon Hendrickson
progress made in using multibeam bathymetric data to determine the effects of a pool drawdown and
structures on bed-load transport in the Pool 8 navigation channel. Work was conducted as part of the
Monitoring of Completed Navigation Projects (MCNP) program.
BACKGROUND: The channel training structures (wing dams and closing dams) that are currently
in place on the Upper Mississippi River were constructed more than 100 years ago to increase flow
in the navigation channel and cause scour to occur resulting in a deeper channel. Initially, these
structures accomplished this goal as evidenced by the islands and sandbars which formed around
them. Construction of the locks and dams, 60 years ago, submerged the training structures, reducing
their effectiveness and increasing secondary channel and floodplain conveyance. Both training
structure submergence and floodplain conveyance are a function of longitudinal position within the
pool, generally increasing from the upstream to the downstream end of the pool.
In cooperation with the U.S. Fish and Wildlife Service (USFWS) and the Departments of Natural
Resources from Minnesota and Wisconsin, the U.S. Army Engineer District, St. Paul, executed a
drawdown of Pool 8 on the Upper Mississippi River near LaCrosse, WI, during the summers of 2001
and 2002. Water levels were allowed to drop below normal minimum values at lock and dam
(L&D) 8 to expose mud flats, promote seed germination, and benefit fish and wildlife. The pool is
normally drawn down to an elevation (el) of 630 ft1 at L&D 8. In 2001, the drawdown was to
el 628.5, and thus was 1.5 ft lower than normal. In Pool 8, most of the dredging is done in the middle
reach of the pool between river mile (RM) 691 and 688. Figure 1 shows a satellite view of the study
area from RM 688 (bottom of figure) to RM 690 (top of figure). This is a reach where the combi-
nation of training structure submergence, high floodplain conveyance, and coarse sediment avail-
ability results in sediment deposition. By lowering water levels during a drawdown, training
structure submergence and floodplain conveyance will be decreased which could result in sediment
mobilization and scour in the navigation channel.
However, it was unknown how the flow and sediment movement in the vicinity of these structures
might change during such a drawdown. In order to quantify the effects of this water level manage-
ment on hydrodynamic and sediment transport processes, a monitoring plan was developed. Infor-
mation derived from a well thought out monitoring plan would allow navigation channel managers
to better assess potential costs and/or potential benefits of a water level drawdown. By measuring
hydraulic and sediment parameters before, during, and after the drawdown, comparisons can be
1
All elevations (el) cited herein are in feet referenced to the National Geodetic Vertical Datum (NGVD) (to convert feet
to meters, multiply number of feet by 0.3048).
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