Author(s): M. Arthur Simanjuntak; Jörg Imberger; Keisuke Nakayama; Tadaharu Ishikawa
Keywords: Down-gradient flux; estuary; Richardson number; shear; turbulent closure
Tone River is a shallow, tidal estuary with a controllable upstream freshwater discharge via a barrage. The mixing characteristics in the benthic and interfacial regions of the Tone River estuary were investigated during a time window of maximum freshwater discharge and high shear, using temperature and velocity microstructure measurements. Although the production of turbulent kinetic energy from mean shear was high throughout the water column, the intermittency of both up- and down-gradient buoyancy fluxes resulted in negligible net down-gradient mixing. These opposing fluxes were incorporated into a turbulent closure scheme by using the concept of percentage down-gradient flux. A comparison of vertical diffusivity from two alternative closures suggests that the stratification imposed a length scale limitation on the vertical turbulent excursions that proved crucial for the transition region between the benthic and interfacial regions; it is argued that a Richardson-number based closure is both conceptually and practically adequate to represent the effect of length-scale limitation and percentage down-gradient flux.