NEAR-BED TURBULENT KINETIC ENERGY AND DISSIPATION IN SMALL TIDAL CHANNELS

A 4-day field experiment was carried out in April 2011 in a salt marsh in the St Jones River watershed, part of the Delaware Estuary in Kent County, Delaware. The study area was located at the confluence of first- and second-order tidal channels that intersect the salt marsh. The goal of the field study was to investigate spatio-temporal variability in the hydrodynamics of small tidal channels. Three stations with in-situ sensors that measured flow velocity, sediment concentration and water depth were established in two channels. Results of the experiment show that the peak velocities occur near high and low tide, at the highest and lowest water levels. The ebb flow velocities in the secondary channel were found to be up to 0.5 m/s and significantly larger than the flood flow velocities, which were typically below 0.3 m/s. Near-bed velocity profiles of all three velocity components (u, v and w), were collected at 60 Hz using a Nortek Vectrino II – profiling velocimeter. Near the bed in the tertiary channel, peak ebb and flood velocities were similar in magnitude, with the flood velocities slightly larger. From these velocity profiles the turbulent kinetic energy and stress profiles were determined in the lower 30 mm of the water column, at 1 mm increments. The turbulence dissipation profile was computed from the velocity profile data using the frequency spectrum and the structure function method. Although the velocities were higher during flood, the turbulent kinetic energy profile shows larger values during ebb, at low water levels, than during flood. Additionally, the Reynolds stress and turbulence dissipation profiles show a peak during the same stage of the ebbing tide as the turbulent kinetic energy peak.