USE OF AN INSTRUMENTED TRIPOD SYSTEM TO EXAMINE SEDIMENT DYNAMICS ANDFINE-SCALE STRATA FORMATION IN MUDDY SURFZONE AND NEARSHORE ENVIRONMENTS

A prototype instrumented tripod system designed for quantifying physic al processes in muddy surfzone and nearshore regions was tested on tid al flats north of the Amazon River mouth. Physical data (i.e., wave ch aracteristics, current velocities, suspended-sediment concentrations a nd seabed characteristics) recorded during a 15 hr deployment at 1-3.5 m water depth on this energetic, non-barred, fine-grained deposit dem onstrate the value of this system to make field observations of sedime nt dynamics over underconsolidated cohesive boundary. Significant sedi ment transport is occurring via resuspension of fine grained particles and aggregates by wave-induced velocities up to 100 cm sec-1. Tidal c urrents (0-35 cm sec-1) are shorenormal with a low-frequency (residual ) component advecting material alongshore. Physical data are supported by simultaneous coring which allows documentation of reworking for th e uppermost seabed over a semi-diurnal tidal cycle. A fluid-mud layer (< 10 cm) thick covers the ''seabed'' and changes in density and thick ness with varying benthic shear stress. Muddy deposits accumulating in this setting are composed of physically stratified laminations of sil t/clay (mu- to cm-scale).