CHANGES IN DUNE GEOMETRY AND BED ELEVATION IN THE LOWER MISSISSIPPI RIVER BETWEEN SUCCESSIVE ANNUAL PEAK FLOWS

The dynamics of dunes in large rivers is still not fully understood. In particular, our knowledge of changes in dune geometry with increasing/decreasing flow depth and velocity, and fluctuation in the overall bed level over which these dunes migrate, require more investigation. This is because assessing sediment storage and transport within river systems, as well as sediment delivery to coastal zones, is of utmost importance. Recent studies on dune morphodynamics have looked at the probability distribution of bed elevations over a train of dunes to estimate both the characteristics of dunes (e.g., mean height) and sediment transport conditions (e.g., high versus low stage). This study continues building on these findings and present results on temporal variation in dune geometry and bed elevation in a large river, at various time scales over a 14-month period.

Seven surveys were conducted where bed-elevation profiles were recorded along a 10-km stretch of the Mississippi River located at about 370 km from the river mouth. River stage ranged from 3 m to 10.5 m (ref. to NGVD), and we used an ADCP to record flow velocity profiles along the survey line (only preliminary results about this will be presented here). Sediment in the channel is composed of very well sorted fine sand.

Results show that the riverbed is reworked and dunes adjust their geometry to different flow conditions within days or weeks, and that there is nothing like dunes superimposed on remnant larger dunes. The dunes from the highest peak flow were about 4m high, and brought the largest bed-level variations (and hence remobilization of deeply stored sediment from the previous peak). Subsequent dunes, and low-variability bed levels, re-transported sediment previously deposited at higher bed level (crests of big dunes). These results indicate substantial bedload sediment transport in the Mississippi River.