Date of Award

1986

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Sedimentological data, and high-resolution current velocity time-series were acquired nearshore along the muddy coast of Louisiana's chenier plain. Results suggest that the 2 to 10 cm thick, laminated and graded silt/shell + mud beds which characterize shoreface mudflat sedimentation form through intermittent dumping of fluid mud suspensions. Cohesive and non-cohesive sediments are first separated and concentrated by shear within a wave boundary layer (WBL). Rapid sedimentation, or "freezing", of the mud fraction occurs when the yield strength of the fluid mud suspension equals that of the WBL shear stress. Incident waves were well described by shallow water linear wave theory, but exhibited the progressive nearshore attenuation and absence of breaking characteristic of muddy shorelines. An onshore decrease in wave shear stress results, which is the reverse of that on surf-dominated coasts. Fluid mud layers, characterized by significant yield strength, are proposed to form when high concentrations of suspended sediments are introduced nearshore, but are prevented from depositing by WBL shear stresses close to the bed. In cross-section, WBL sorting causes a near-bottom discontinuity in sediment concentration, and the associated vertical yield strength gradient. Expanded spatially, this point of weakness forms a plane along which the fluid mud layer can move in response to shear exerted at its upper margin by an accelerating flow. Adjustment of the steady alongshore current velocity profile to the fluid mud layer restricts coastwise sediment transport. Significant crossshore transport of fluid mud may occur, however, under the influence of unsteady flows. Such low-frequency flows were found to be associated with shore-amplified water level oscillations which have characteristics of a standing wave with an antinode at the shoreline. The onshore decreasing wave shear stress gradient leads to preferential deposition at the shoreface. Widespread renewal of chenier plain shoreline advance through mudflat accretion is predicted when the Atchafalaya delta, now forming 100 km to the east in Atchafalaya Bay, emerges on the inner shelf, and the nearshore supply of fine-grained sediment is dramatically increased.

Pages

163

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