Measuring Suspended-Sediment Concentrations, Grain Sizes, and Bedload using Multiple Single-Frequency Acoustic Doppler Profilers and Echologgers in the Lower Chippewa River, Wisconsin.

Science Center Objects

The Upper Mississippi River (UMR) provides critical habitat for hundreds of aquatic species and provides Minnesota with a transportation link to the rest of the world. Reliable measurements of sediment are important for making decisions as part of maintaining the channel. In 2014, sediment deposition in the navigation channel caused channel closures of the UMR delaying commercial navigation for 3 weeks. The U.S. Geological Survey (USGS), in cooperation with the U.S. Army Corps of Engineers, proposes to begin real-time monitoring of suspended-sediment concentrations, loads, and bedload on the lower Chippewa River, a major source of sand to the UMR.  New techniques, recently developed by the USGS uses multi-frequency arrays of single-frequency acoustic Doppler profilers and echosounders to measure suspended-sediment concentrations. Bedload will be quantified by measuring dune movement using echosounders mounted on bridge piers and multibeam sonar surveys. This study will provide current data from which to accurately quantify total sediment loads for the lower Chippewa River and will serve as an early warning to mitigate future closures of this important navigation channel.

Background & Importance

Sand dunes on the bed of a Chippewa River sidechannel

Ripples of sediment on the bed of a Chippewa River sidechannel. (Credit: David Dean, USGS. Public domain.)

The Upper Mississippi River (UMR) provides critical habitat for hundreds of aquatic species and also serves as an important economic resource by providing a transportation link for Minnesota to the rest of the world. The St. Paul District of the U.S. Army Corps of Engineers (USACE) is responsible for lock and dam operation and for channel dredging on the UMR between Minneapolis, Minnesota and Guttenburg, Iowa. The USACE developed a district-wide bed material sediment budget (U.S. Army Corps of Engineers, 2003) using available information on sediment transport at U.S. Geological Survey (USGS) gaging stations, long-term channel dredging data, studies of sediment deposition, and hydraulic data. While useful for describing average annual sediment loads, the sediment budget cannot provide information on sediment loads at smaller time scales such as seasonal flood events. Sand-sized material transported on the bed and in suspension is of particular interest due to the costs associated with navigation channel dredging and because of sediment deposition in backwater areas.

The USACE is interested in obtaining measurements of suspended-sediment and bedload sediment on the lower Chippewa River (lower image) to improve their ability to quantify sediment loads and to adjust navigation channel programmatic decisions to avoid future closures of the channel due to high sedimentation rates. A study by Rose (1992) identified the Chippewa River as a major contributor of sand-sized sediment to the UMR below Lake Pepin. Data collected from 1976 to 1983 indicated that the average annual total-sediment load and the percentage transported as bedload was 1,073,000 tons and 61 percent at the Chippewa River at Durand and 940,000 tons and 44 percent at the Chippewa River near Pepin (near Chippewa River and Mississippi River confluence).

General Methods

The lower Chippewa River at Highway 35, WI, looking downstream

The lower Chippewa River at Highway 35, WI, looking downstream, with trees at the edge of the river (Credit: David Dean, USGS. Public domain.)

Using new techniques developed by the USGS, suspended-sediment concentrations, loads, and grain sizes will be measured using sidelooking acoustic Doppler profilers permanently mounted within the river. Acoustic signals will be calibrated using physical depth-integrated sediment samples. Suspended-transport data will available in 15-min increments.

Bedload will be quantified using single frequency echosounders to track dune movement, and these at-a-point measurements will be calibrated using episodic repeat multibeam sonar surveys which provide the ability to track dune movement across the entire river width.

References Cited

Rose, W. J., 1992, Sediment transport, particle sizes, and loads in lower reaches of the Chippewa, Black and Wisconsin Rivers in Western Wisconsin, US Geological Survey, v. 4124.

U.S. Army Corps of Engineers, 2003, Bed Material Budget for the St. Paul District Reach of the Upper Mississippi River, Anoka, Minnesota to Guttenburg, Iowa. U.S. Army Corps of Engineers, St. Paul District, 190 East Fifth Street, St. Paul, MN 55101-1638.