CHANNEL-BED CHARACTERISTICS IN THE MISSISSIPPI RIVER AT THE RED EYE CROSSING

Recent developments in 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. In the Mississippi, trains of multi-scale dunes are observed superimposed on, or adjacent to, each other as they migrate over channel-scale bedwaves. The analysis of the probability distribution of bed elevation of selected stretches of the river is a powerful tool for estimating (quantitatively) the height of different groups of dune and assessing (qualitatively - -for now) the relative sediment-transport conditions. We surveyed a major sailing line of the Mississippi River several times, recording bed elevations with a 1MHz transducer and high-precision echo sounder, and flow-velocity fields with an ADCP. Results show that the ratio of dune height/flow depth is low (about 0.1) for the largest dunes in the thalweg and even lower (<0.06) for the small superimposed dunes. This may be due to the complexity of the flow field and/or grain size (fine sand). Future research will include relating the probability distribution to vertical sorting, velocity profiles and estimates of sediment transport from acoustic measurements, as well as associated stratigraphy.