Date of Award

1990

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Geography and Anthropology

First Advisor

Richard H. Kesel

Abstract

Although suspension is the dominant mode of material transport in rivers world-wide, concepts of suspended sediment transport have come mostly from studies of small rivers. This study describes relationships between discharge and suspended sediment in a large river system, the Mississippi-Atchafalaya in south Louisiana. The primary objectives are: (1) to determine whether the suspended silt-clay and sand follow different relationships with discharge because the silt-clay is supplied largely from the basin whereas the sand is supplied largely from the channel perimeter; and (2) to determine whether hysteresis effects differ in high as compared to low discharge years because of conditions including supply limitations and bankfull flows. Empirical relationships of discharge with suspended sediment, silt-clay, and, to a lesser extent, other variables in the Mississippi, Atchafalaya, and Old rivers show quadratic log-log relationships, while the Red River shows linear log-log relationships. The nonlinearity is strongest in the fine sediments and reflects the decreased availability of fine material at high discharges possibly related to the basin's climate, dams, and revetments, and removal of sediments by flushing. Further downstream in the Mississippi, velocities decrease at low discharges, fine material becomes increasingly abundant, and settling and resuspension of fines become more important processes. The Mississippi, Atchafalaya, and Old rivers show pronounced hysteresis effects with a lead of sediment peaks before discharge crests of as much as two months during high discharge years. The lead decreases with decreasing discharge maxima so that in low discharge years sediment peaks and discharge crests coincide. Even though in all years the Red's discharge is less than that contributed through Old River, time series show that the Atchafalaya's sediment signature, which shows numerous sediment peaks during falling and stationary discharges, is strongly influenced by the sediment-laden Red River in most years. Results show the need for further study of large rivers and that existing concepts regarding relationships of basin size and hysteresis effects, as well as the relative downstream movement of sediment waves and floodwaves should be reassessed.

Pages

195

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