Reynolds stress and TKE production in an estuary with a tidal bore

Abstract

We report new measurements of the turbulent properties of the flow in a tidally energetic estuarine channel of almost uniform cross-section. A high-frequency (1.2 MHz), bottom-mounted Acoustic Doppler Current Profiler (ADCP) has been used to observe the velocity field at a sampling rate of 10 Hz in parallel with measurements of the surface elevation by tide gauges. Our data have been analysed using the Variance Method to determine turbulent kinetic energy (TKE), shear stress and TKE production over the tidal cycle with a time resolution of 60 s. During the highly energetic but brief flood period, when the surface axial velocity reaches ∼2 m s −1, we observed large values of stress (>2 Pa) and shear production (∼5 W m −3). TKE is also input through the release of energy in the bore itself which results in a brief but intense injection of energy at the bore front with large transient TKE levels (∼100 J m −3). Subsequent input by shear production maintains TKE levels which are generally lower (∼20 J m −3) than the strong peak associated with the bore for the rest of the flood. On the ebb, the flow is relatively tranquil with maximum speeds ∼0.5 m s −1 and peak TKE production rates of ∼0.1 W m −3. The flow and elevation data have also been used to estimate the energy fluxes into and out of the estuary. Short (∼1 h), intense energy inputs (∼8 MW at springs) on the flood flow are largely balanced by longer, less intense seaward energy flow on the ebb. The net energy input is found to be ∼0.1 MW at springs which is consistent with estimates of upstream dissipation. Peak dissipation in the bore itself may exceed the mean energy input but it is active only for a small fraction of the tidal cycle and its average contribution does not exceed ∼12% of total dissipation.