FLOW AND SEDIMENT TRANSPORT IN A SHALLOW BAR-BUILT ESTUARY, NORTHERN GULF-OF-MEXICO

A field experiment was undertaken in Lake Barre, a shallow estuary wit hin the Louisiana coastal zone, in November 1991 with the primary goal of acquiring a better understanding of time-mean flow and sediment tr ansport in a shallow Louisiana bay on the periphery of a deteriorating marsh. A network of four monitoring stations recorded current velocit y and water turbidity at various levels within the water column while water level, wave, salinity, and wind speed and direction data were ac quired at a single station. Results show a response of the water colum n and the seabed to winds, waves, and tides during the period of study . Variations in velocity profiles, roughness lengths, and to a lesser extent, friction velocities suggest the existence of at least two dist inct flow regimes during the periods of study that encompassed four co ld fronts. One, characterized by small roughness and weak vertical vel ocity shear, occurred near the time of cold front passages when winds were northerly. The other, identified by large and frequently variable roughness and vertical velocity shear distributed uniformly through t he water column, was predominately associated with tidal flows. The fo rmer was attributed to current ripples while the latter appeared to be related to the activity of benthic organisms. A large positive vertic al sediment gradient that appeared during the first cold front was att ributed to advection of sediment from an area of rapid degradation ups tream. Sediment transport calculations reveal fluxes of sediment from the marsh into the study area during cold front passages, the largest magnitudes associated with the earliest cold fronts. Flux magnitude ge nerally decreased with each succeeding cold front. Quantities of fine- grained sediment moving landward during strong prefrontal events are a vailable for nourishing the marsh and contributes material for upward building of a bayou mouth bar. However, long-term subsidence rates are sufficient to mask the effects of deposition. Because much of the sed imentary material within Lake Barre is of sandsize, the bar is armored against the effects of subsequent storms from either direction.