Abstract

Flood and ebb tidal cycles in the Altamaha Estuary produce differing bedforms, sedimentary structures, thicknesses of sedimentary units, and grain size distributions. Differences are the result of changes in bed shear, flow regime, and mechanisms of sediment transport. The salt wedge developed on flood cycles produces a stratified estuary with highest flow velocity below the highest rate of salinity change. This relation results in upper flow regime as predicted by the densimetric Froude relation; trochoidal in phase waves to 2 m in height are formed. Surface waves and internal waves are seen in the salinity stratification. Ebb flow modifies the sand. wave surface, and sediment transport is by ripples and dunes in the lower flow regime. Large-scale planar cross-bedding is produced by flood flow, small-scale ripple and dune structures are developed by ebb flow. The estuary is an effective mechanism for size segregation. Suspension populations are removed by both flood and ebb flow. There is a net inland transport of suspended sediment with deposition on tidal flats and marshes. A single log-normal source population is fractionated into several differing populations by bedload transport, suspension and recycling during successive tidal cycles. Characteristic log-probability size distributions are developed in different environments.

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