Axial Wind Effects on Stratification and Longitudinal Sediment Transport in a Convergent Estuary During Wet Season (original) (raw)
Related papers
2005
Detailed field observations from the York River estuary, Virginia are used to examine the processes governing vertical density stratification and to evaluate the importance of spatial and temporal variations in turbulent mixing on estuarine dynamics and sediment transport. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away stratification, this study demonstrate that the wind can play a more important role in “straining” the along-channel estuarine density gradient. As a result, down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase stratification. Conversely, up-estuary winds tend to reduce, or even reverse the vertical shear, reducing stratification. While wind straining can play a dominant role in governing the overall degree of turbulent mixing at sub-tidal time scales, tidal straining of the along-channel density gradient can result in asymmetries...
Spatial differences in wind-driven sediment resuspension in a shallow, coastal estuary
Estuarine Coastal and Shelf Science, 2018
Two locations approximately 11 km apart along the axis of the New River Estuary near Jacksonville, NC USA were continuously monitored for eight years. Included in the observations are vertical profiles of turbidity, temperature, salinity, chl-a, dissolved oxygen, pH and water velocity as well as local wind velocity. Differences between the two sites result from a number of factors, including bathymetry, wind strength, direction and fetch, estuarine morphology, tidal currents and sediment properties. The site near the head of the estuary, Morgan Bay, is deeper, experiences generally weaker winds and has less fetch in most directions. Stones Bay, the down-estuary site, is shallower, experiences stronger winds and has longer fetch, particularly in the prevailing wind directions. Current speeds also differ along the estuary with the down-estuary Stones Bay site being more tidal. The observations were used together with a simple wave model to analyze the estuarine turbidity response to different forcing mechanisms. Results suggest that sediments are resuspended primarily by wind-wave generated bottom stress at both locations. While turbidity is generally higher in Stones Bay than in Morgan Bay, turbidity as a function of the local wave-induced bottom stress (including forcing from all directions) is similar at both locations at low stress but diverges at higher stresses. At higher bottom stresses, turbidity in Stones Bay responds primarily to winds from the NE, S and NW while turbidity in Morgan Bay responds primarily to winds from the NW and S. Accounting for sediment resuspension within an approximate spatial advection scale around each of the observation sites, yields a similar turbidity vs bottom stress response curve for the three primary directions in Stones Bay and the S direction in Morgan Bay but a greater turbidity response for winds from the NW in Morgan Bay. In the latter case, waves are crossing the section of the New River Estuary just downstream of the confluence with the New River and are presumably encountering sediments that are more easily resuspended. Average sediment export is down-river with more sediment leaving Stones Bay than Morgan Bay.
Near-bed sediment transport in a heavily modified coastal plain estuary
International Journal of Sediment Research, 2014
Numerous estuaries of the world have been strongly modified by human activities. These interferences can make great adjustments of not only sediment transport processes, but also the collective behavior of the estuary. This paper provides a typical case of a heavily modified coastal plain estuary of Sheyang on the China coast, where a sluice barrage was built in 1956 to stop the intrusions of storm surges and saline water. Four sets of instrumented tripods were simultaneously deployed along a cross-shore transect to continuously observe near-bed flow currents and sediment transport. The in-situ surveys lasted over a spring and neap tide cycle when a strong wind event occurred in the neap tide. Comparisons of flows and sediment transport between tide-dominated and wind-dominated conditions demonstrated the important role of episodic wind events in flows and sediment transport. The wind-induced currents, bottom stresses, and sediment transport rates were significantly greater when wind was present than corresponding quantities induced by the tides. The long-shore sediment transport induced by winds exceeds the cross-shore component, especially near the river mouth bar. These results indicate the noticeable importance of wave-dominated coastal processes in shaping topographic features. A regime shift of estuarine evolution under highly intense human forcing occurs from fluvial to marine processes. This finding suggests that the management strategy of the estuarine system should focus on the restoration of estuarine processes, rather than the present focus on inhibition of marine dynamics.
Modelling hydrodynamics and sediment flux within a macrotidal estuary: problems and solutions
The Science of The Total Environment, 2003
A model of estuarine circulation and sediment transport is described. The model uses a 2-D longitudinal and vertical grid to predict the distribution of tidal elevation, current velocity, density and sediment concentration. It has been developed for relatively narrow estuaries that have a large tidal range and potentially high river flow. The advantage over simple 1-D and depth-averaging 2-D models is that it can give reasonable representation of both gravitational circulation and the vertical distribution of suspended sediment concentrations, thereby providing better estimates of the bottom stress and sediment flux, particularly in the deep channel. The model has been developed as a management tool to provide some of the advantages of a full 3-D model, but with smaller development and running costs. It can be used to predict concentrations of contaminants in estuaries that have a significant load of suspended cohesive sediment, and can differentiate between dissolved contaminants and those that are attached to suspended sediments. It is a potentially useful tool for predicting the distribution of relatively short-lived contaminants that are affected by sediment concentration, such as bacteria and some radionuclides. ᮊ
Three-Dimensional Modeling of Fine Sediment Transport by Waves and Currents in a Shallow Estuary
Journal of Geophysical Research: Oceans
A suspended sediment transport model is implemented in the unstructured-grid SUNTANS model and applied to study fine-grained sediment transport in South San Francisco Bay. The model enables calculation of suspension of bottom sediment based on combined forcing of tidal currents and wind waves. We show that accurate results can be obtained by employing two-size classes which are representative of microflocs and macroflocs in the Bay. A key finding of the paper is that the critical calibration parameter is the ratio of the erosion of the microflocs to macroflocs from the bed. Different values of this erosion ratio are needed on the shallow shoals and deeper channels because of the different nature of the sediment dynamics in these regions. Application of a spatially variable erosion ratio and critical shear stress for erosion is shown to accurately reproduce observed suspended sediment concentration at four-field sites located along a cross-channel transect. The results reveal a stark contrast between the behavior of the suspended sediment concentration on the shoals and in the deep channel. Waves are shown to resuspend sediments on the shoals, although tidal and wind-generated currents are needed to mix the thin wavedriven suspensions into the water column. The contribution to the suspended sediment concentration in the channel by transport from the shoals is similar in magnitude to that due to local resuspension. However, the local contribution is in phase with strong bottom currents which resuspend the sediments, while the contribution from the shoals peaks during low-water slack tide.
Journal of Geophysical Research: Oceans, 2019
A numerical model with a vortex force formalism is used to study the role of wind waves in the momentum budget and subtidal exchange of a shallow coastal plain estuary, Delaware Bay. Wave height and age in the bay have a spatial distribution that is controlled by bathymetry and fetch, with implications for the surface drag coefficient in young, underdeveloped seas. Inclusion of waves in the model leads to increases in the surface drag coefficient by up to 30% with respect to parameterizations in which surface drag is only a function of wind speed, in agreement with recent observations of air-sea fluxes in estuaries. The model was modified to prevent whitecapping wave dissipation from generating breaking forces since that contribution is integrally equivalent to the wind stress. The proposed adjustment is consistent with previous studies of wave-induced nearshore currents and with additional parameterizations for breaking forces in the model. The mean momentum balance during a simulated wind event was mainly between the pressure gradient force and surface stress, with negligible contributions by vortex, wave breaking (i.e., depth-induced), and Stokes-Coriolis forces. Modeled scenarios with realistic Delaware bathymetry suggest that the subtidal bay-ocean exchange at storm time scales is sensitive to wave-induced surface drag coefficient, wind direction, and mass transport due to the Stokes drift. Results herein are applicable to shallow coastal systems where the typical wave field is young (i.e., wind seas) and modulated by bathymetry. Plain Language Summary Water circulation and pollutant flushing in estuaries, bays, and similar coastal environments depend mainly on tidal currents, winds, and density differences between fresh and saltwater. However, winds normally coexist with waves, and the impact of waves on circulation patterns is often overlooked in most numerical models. In this study we used a numerical model to explore how Delaware Bay responds to a typical storm event. The model was configured to consider or ignore the effects of waves on circulation, and we used both options to contrast and compare the simulated results. The model that ignored waves could not reproduce the mechanisms that have been shown to control the energy transfer from the atmosphere to the water column. In agreement with recent observations in estuaries, the model indicates that wind waves in shallow water have an important regulating effect on the roughness of the sea surface, which is key for surface drag and circulation. We also report that even when the wind speed is the same above the water surface, the spatial distribution of waves determines the effective amount of energy that gets transferred from the air to the water column.
Effects of wind on a stratified estuary
Marine Chemistry, 1991
Before the onset of wind in the Krka Estuary, a sharp halocline extends from 2.5 to 4 m below the surface, separating the upper brackish water layer from the lower marine layer. Strong wind (20 m s-l) induces a tilt of the halocline surface so that downwind (5-km wind fetch) the halocline is pushed to 4.6 m below the surface. Wind-driven surface currents induce a vertical gyre within the upper layer and a gyre in the opposite direction in the lower marine layer. As a consequence, the halocline close to the coast on the downwind side is not only deeper but is also steepened. The entrainment of marine water into the upper brackish water layer is intensified on the windward side. The possible role of wind in oxygenating the bottom layer is discussed.
The effect of sediment stratification on tidal dynamics and sediment transport patterns
Journal of Geophysical Research, 2005
1] The western tip of southwest Korea is characterized by a tidally dominated, turbid, coastal environment. There are well-developed tidal flats along the coast and around islands, together with offshore sand ridges several tens of kilometers long in the west. The effects of bottom boundary layer (BBL) sediment stratification on sediment-transport and tidal dynamics in this environment were examined using a sediment transport model coupled with a three-dimensional tidal hydrodynamic model. Model experiments using two scenarios, with and without the effect of sediment-induced stratification, showed that BBL sediment-stratification influenced the spatial distribution and reduced the magnitude of net sedimentation. The presence of a sediment-stratified BBL also led to a reduction in suspended sediment fluxes and an increase in the vertical gradient of sediment concentrations in the water column. These variations occurred because sediment-induced BBL stratification leads to not only a reduction in bottom shear stress but also a decrease in buoyancy production of turbulent kinetic energy and an associated dampening of turbulence. The significant reductions in turbulence and bottom shear stress result in changes to the vertical-current structure of the M 2 tide, including alteration of the tidal ellipse configuration and an increase in vertical shears of the tidal current amplitude and phase. These reductions also lead to a slight increase in tidal amplitudes due to the decreased tidal-energy dissipation. The model results indicate that feedback between the sediment-transport dynamics and hydrodynamics is an important factor in modeling sediment transport dynamics in tidally dominated, turbid, coastal environments. Such environments include the west coast of Korea and the Yellow Sea. A suitable approach to simulating the effects of this feedback may be the use of a modified bottom-drag coefficient as a stability function, together with the inclusion of the effect of sediment stratification on the hydrodynamics. Citation: Byun, D.-S., and X. H. Wang (2005), The effect of sediment stratification on tidal dynamics and sediment transport patterns,
Interaction of lateral baroclinic forcing and turbulence in an estuary
Journal of Geophysical Research, 2003
Observations of density and velocity in a channel in northern San Francisco Bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross-channel baroclinic pressure gradient and vertical mixing due to turbulence. Profiles of velocity, salinity, temperature, and suspended sediment concentration were measured in transects across Suisun Cutoff, in northern San Francisco Bay, on two days over the 12.5-hour tidal cycle. During flood tides an axial density front developed between fresher water flowing from the shallows of Grizzly Bay into the northern side of Suisun Cutoff and saltier water flowing up the channel. North of the front, transverse currents were driven by the lateral salinity gradient, with a top-to-bottom velocity difference greater than 30 cm/s. South of the front, the secondary circulation was weak, and along-channel velocities were greater than to the north. The gradient Richardson number shows that stratification was stable north of the front, while the water column was turbulently mixed south of the front. Time-series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle. In estuaries, longitudinal dynamics predict less stratification during flood than ebb tides. These data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry.
Estuaries, 2005
Observations from the York River Estuary, Virginia, demonstrate that the along-channel wind plays a dominant role in governing the estuarine exchange flow and the corresponding increase or decrease in vertical density stratification. Contrary to previous findings that suggest wind stress acts predominantly as a source of energy to mix away estuarine stratification, our results demonstrate that the wind can play a more important role in straining the alongchannel estuarine density gradient. Down-estuary winds enhance the tidally-averaged vertical shear, which interacts with the along-channel density gradient to increase vertical stratification. Up-estuary winds tend to reduce, or even reverse the vertical shear, reducing vertical stratification. In two experiments each lasting approximately a month, the estuarine exchange flow was highly correlated with the along-channel component of the wind. The changes in stratification caused by the exchange flow appear to control the amount of vertical mixing as parameterized by the vertical eddy viscosity. The degree of stratification induced by wind straining also appears to play an important role in controlling the effectiveness of wind and tidal mixing.