Tidal asymmetry in an estuarine pycnocline 2. Transport

Flood currents in shallow estuaries are driven by an along-channel barotrop ic and baroclinic pressure gradient that increases monotonically toward the bottom, while friction retards near-bottom currents. Therefore, in many es tuaries there is a middepth maximum in flood currents. We explore this phen omenon using a simple three-layer model in which each layer has vertically uniform currents and constant density. In this model the middle layer is of intermediate density and grows by shear-induced entrainment from the other two layers. This very simple model produces a middepth maximum in flood cu rrents and simulates observed currents in the Columbia River entrance chann el within about 10%. There is good qualitative agreement between model sali nity transport and observed transport. The model pycnocline rises and falls tidally, in phase with the observed pycnocline, although pycnocline depth and thickness are better simulated using results from a two-layer model [Cu daback and Jay, 2000].