Cynthia N. Cudaback

Tidal asymmetry in an estuarine pycnocline: Depth and thickness

Tidal variations in estuarine stratification are revealed by the depth and thickness of the density interface. The depth of the interface may be predicted using an inviscid two-layer model that combines baroclinic estuarine circulation with barotropic tidal currents [Helfrich, 1995]. Here we present results from a two-layer model modified to include the effects of bottom friction and interfacial mixing. Modeled layer thickness and speed compare favorably with prior analytic studies [Farmer and Armi, 1986; Pratt, 1986]. We use a bulk Richardson number criterion to estimate the thickness of the pycnocline from two-layer model results; the predicted pycnocline depth and thickness compare remarkably well with observations. We also investigate the effects of changing bottom friction and barotropic currents on the pycnocline thickness.

Tidal asymmetry in an estuarine pycnocline: 2. Transport

Flood currents in shallow estuaries are driven by an along-channel barotropic and baroclinic pressure gradient that increases monotonically toward the bottom, while friction retards near-bottom currents. Therefore, in many estuaries there is a middepth maximum in flood currents. We explore this phenomenon 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 currents and simulates observed currents in the Columbia River entrance channel within about 10%. There is good qualitative agreement between model salinity 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 [Cudaback and Jay, 2000].

A RATE FOR THE SCAVENGING OF FINE PARTICLES BY MACROAGGREGATES IN A DEEP ESTUARY

234Th activity profiles in Puget Sound have been studied using a model that incorporates reversible exchanges between dissolved, fine particulate, and macroaggregate Th reservoirs. Macroaggregate settling is made responsible for the downward flux of Th and the vertical gradients of activity in measured profiles. Least squares fits of model to data yield rates/time scales for the exchange processes involved. Fine-particle scavenging by macroaggregates is found to occur with a time scale of 4–6 days over a large range of macroaggregate settling speeds, ws. Macroaggregate-disaggregation time scales are 1–4 days when ws is 100 m/d. Rates of sorption and remobilization characterizing the exchange between dissolved and fine-particulate forms of the isotope cannot be individually identified from these data, but acceptable model values include those measured in the laboratory. Rates of sorption that depend on particulate concentrations which increase to the seafloor result in profiles of dissolved Th having above-bottom maxima. Based on inferred exchange rates, the residence time for fine particles introduced at the surface of this deep (∼200 m) estuary is estimated to be 11–16 days when ws = 100 m/d.