Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

A new Mississippi River Delta is building in Atchafalaya Bay of southcentral Louisiana as a result of a upstream natural diversion in river course. The present study focuses on several distinct but complementary aspects of the physical processes of the modern Atchafalaya River Delta. Coherence analysis reveals that the subtidal fluctuations of water stage and current velocity in the estuarine area are closely related to atmospheric forcing at different time scales. The water stage fluctuations at Morgan City had significant variance at time scale 2.5-10 days which was driven by local longitudinal (north-south) winds. There were also fluctuations at periods of 3 days which were the result of up-bay propagation of sea level fluctuations generated by longshore (west-east) winds. In winter local longitudinal winds were dominant, and in summer coastal Ekman flux driven by longshore winds became more pronounced. It is possible to estimate by a linear model the subtidal current driven by winds if enough long-term records are available. Several characteristics (e.g. annual sediment flux, seasonal variation in suspended sediment, grain size variation, abnormally large floods and sediment transport, delta growth rate, and channel geometry etc.) of sediment input from the Atchafalaya River, importance for understanding deltaic depositional processes are examined. Cluster analysis, based on grain size distribution, is applied to three sets of sediment samples totaling 563 to determine the natural grouping of the sediments in Atchafalaya Bay. Clusters produced proved to be sedimentologically meaningful and environmentally significant. Each cluster is related to certain environment(s) of deposition identified by previous studies. Based on the classification established, sediment samples from unknown environments in the study area can be objectively identified based on their grain-size distribution. Based on nonparametric statistical tests, seasonal variation in sediment grain size differs among delta lobes. This can be explained by seasonal variations of atmospheric forcings and the difference in elevation and location of delta lobes.

Pages

207

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