Shih-Nan Chen

Axial Wind Effects on Stratification and Longitudinal Salt Transport in an Idealized, Partially Mixed Estuary*

Abstract A 3D hydrodynamic model [Regional Ocean Model System (ROMS)] is used to investigate how axial wind influences stratification and to explore the associated longitudinal salt transport in partially mixed estuaries. The model is configured to represent a straight estuarine channel connecting to a shelf sea. The results confirm that wind straining of the along-channel salinity gradient exerts an important control on stratification. Two governing parameters are identified: the Wedderburn number (W) defined as the ratio of wind stress to axial baroclinic pressure gradient force, and the ratio of an entrainment depth to water depth (hs/H). Here W controls the effectiveness of wind straining, which promotes increases (decreases) in stratification during down-estuary (up-estuary) wind. The ratio hs/H determines the portion of the water column affected by direct wind mixing. While stratification is always reduced by up-estuary wind, stratification shows an increase-then-decrease transition when down-estuar...

A Nearshore Model to Investigate the Effects of Seagrass Bed Geometry on Wave Attenuation and Suspended Sediment Transport

The effects of seagrass bed geometry on wave attenuation and suspended sediment transport were investigated using a modified Nearshore Community Model (NearCoM). The model was enhanced to account for cohesive sediment erosion and deposition, sediment transport, combined wave and current shear stresses, and seagrass effects on drag. Expressions for seagrass drag as a function of seagrass shoot density and canopy height were derived from published flume studies of model vegetation. The predicted reduction of volume flux for steady flow through a bed agreed reasonably well with a separate flume study. Predicted wave attenuation qualitatively captured seasonal patterns observed in the field: wave attenuation peaked during the flowering season and decreased as shoot density and canopy height decreased. Model scenarios with idealized bathymetries demonstrated that, when wave orbital velocities and the seagrass canopy interact, increasing seagrass bed width in the direction of wave propagation results in higher wave attenuation, and increasing incoming wave height results in higher relative wave attenuation. The model also predicted lower skin friction, reduced erosion rates, and higher bottom sediment accumulation within and behind the bed. Reduced erosion rates within seagrass beds have been reported, but reductions in stress behind the bed require further studies for verification. Model results suggest that the mechanism of sediment trapping by seagrass beds is more complex than reduced erosion rates alone; it also requires suspended sediment sources outside of the bed and horizontal transport into the bed.

Cyclone-driven deep sea injection of freshwater and heat by hyperpycnal flow in the subtropics

China (973 Program) [2009CB421200]; Program of Introducing Talents of Discipline to Universities [B07034]; Academia Sinica Thematic Program AFOBi, Taiwan [NSC 98-2116-M-001-005]

Estuarine Exchange Flow Quantified with Isohaline Coordinates: Contrasting Long and Short Estuaries

AbstractIsohaline coordinate analysis is used to compare the exchange flow in two contrasting estuaries, the long (with respect to tidal excursion) Hudson River and the short Merrimack River, using validated numerical models. The isohaline analysis averages fluxes in salinity space rather than in physical space, yielding the isohaline exchange flow that incorporates both subtidal and tidal fluxes and precisely satisfies the Knudsen relation. The isohaline analysis can be consistently applied to both subtidally and tidally dominated estuaries. In the Hudson, the isohaline exchange flow is similar to results from the Eulerian analysis, and the conventional estuarine theory can be used to quantify the salt transport based on scaling with the baroclinic pressure gradient. In the Merrimack, the isohaline exchange flow is much larger than the Eulerian quantity, indicating the dominance of tidal salt flux. The exchange flow does not scale with the baroclinic pressure gradient but rather with tidal volume flux. T...

Asymmetric Estuarine Responses to Changes in River Forcing: A Consequence of Nonlinear Salt Flux

AbstractA linear theory for estuarine adjustment to river forcing as put forth by MacCready is extended to allow for quantification of nonlinear salt flux induced by gravitational exchange flow. It has been shown that, under a steplike change of river forcing, the estuarine responses are asymmetric, with the salinity field adjusting faster during the rising discharge. The asymmetry arises because the up-estuary salt flux due to exchange flow is a nonlinear function of estuarine length ∝ L−3. During the rising discharge, the estuary is longer, and the salt flux is comparatively less sensitive to the length variations. As a result, the up-estuary salt transport cannot keep pace with the rate of discharge changes, leading to a larger net salt flux and thus a shorter response time. A simple theory accounting for the nonlinear effect is then applied to Hudson-like systems and shown to capture the asymmetric response. The asymmetry is generalizable to other estuarine regimes where up-estuary salt fluxes are exp...

Generation of Upwelling Circulation under Downwelling-Favorable Wind within Bottom-Attached, Buoyant Coastal Currents

AbstractA two-dimensional modeling study by Moffat and Lentz recently reported that downwelling-favorable wind can induce cross-shore upwelling circulation within a bottom-attached, buoyant coastal current. Here, we extend the problem to three dimensions. The driving mechanism and the sensitivity for the upwelling circulation are studied, using a primitive equation ocean model and an analytical model. After the initial downwelling adjustment that steepens the isopycnals and compresses the coastal current, the cross-shore flow can switch to steady upwelling circulation. This reverse circulation coincides with a vertically well-mixed water column and persists until interrupted by the arrival of river plume bulge from upstream. During the upwelling phase, the ageostrophic cross-shore flow follows the Ekman balance. The sense of cross-shore circulation is governed by a dimensionless parameter, the shear ratio, which measures the relative size of geostrophic shear and velocity shear supported by the wind in th...

Bathymetric Influences on the Estuarine Equilibrium Length and Adjustment Time

AbstractLinear theories are extended to enable investigations of how exponentially convergent width and sloping bottom affect the sensitivity of estuarine equilibrium length and adjustment time. This study focuses on the response to river forcing and considers a regime dominated by gravitational circulation, but the results are generalizable. For a range of forcing and bathymetric profiles, the predicted equilibrium length and adjustment time compare favorably with numerical solutions from a width-averaged model. The main findings are that 1) convergent width and sloping bottom reduce the sensitivity of equilibrium length to river forcing. The sensitivity is governed by a dimensionless parameter that measures the degree of width and depth changes sampled by the intrusion length. Hence, the sensitivity is not a constant in a system but varies with forcing: when discharge increases, a shortened estuary experiences less bathymetric changes over its intrusion. The sensitivity therefore increases progressively...