MULTI-SCALE DUNES OF THE MISSISSIPPI RIVER: THE ANALYSIS OF TIME VARIATION OF PROBABILITY DISTRIBUTIONS OF BED ELEVATION AS A STEP TOWARD THE UNDERSTANDING OF BEDLOAD SEDIMENT TRANSPORT

Recent developments of sediment continuity models have demonstrated the relationships between dune morphology, especially trough-scour depth, and the composition and rate of sediment transport. The probability distribution of bed elevation over a train of dunes is a key element of these new depth-continuous models, i.e. without distinct bed layers. These models are yet essentially theoretical and there exists only limited experimental data to implement them. This paper presents much needed results from the field.

Data from multibeam surveys (USACE) during the annual highest discharge of the Mississippi River was analyzed. The study site, located at the Red Eye Crossing, covers a stretch 5700-m long and 75-m wide corresponding to the thalweg and an area of about 350 m x 50 m over a point bar. Sediment in the channel is composed of very well sorted, fine sand.

The bathymetry shows multi-scale dunes that are not always superimposed on each other but adjacent, although all migrate on macro sandwaves scaling with channel width (1 km) and planform. Mean flow depth was 27 m and velocity was about 1.8 m/s. Preliminary results give length of 120 m and height of 3 m for symmetric dunes with superimposed asymmetric dunes about 11-m long and 1-m high. Similar ‘small’ dunes migrate on the point bar but others in the channel are twice as long for similar height. Dunes near the river bend average 69-m long and 2.6-m high, with celerity up to 2 m/h. The analysis of the probability distributions of bed elevation allows encompassing these multi-scale dunes and/or selecting specific river sections. The probability-distribution curves vary little for a 24-hour interval, hence suggesting steady sediment supply and transport rate. The dimensionless curves for the thalweg data compare well to experimental results at high sediment transport stage. Depth-continuous models for sediment continuity are promising for scale-integrated studies of sediment transport. Future research includes relating probability distributions of bed elevation to estimates of sediment transport from acoustic measurements and to the texture and structure of preserved stratigraphy.