Chen Shih-Nan

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  • Article
    Lateral circulation and sediment transport driven by axial winds in an idealized, partially mixed estuary
    (American Geophysical Union, 2009-12-03) Chen, Shih-Nan ; Sanford, Lawrence P. ; Ralston, David K.
    A 3D hydrodynamic model (ROMS) is used to investigate lateral circulation in a partially mixed estuary driven by axial wind events and to explore the associated transport of sediments. The channel is straight with a triangular cross section. The model results suggest that driving mechanisms for lateral circulation during axial wind events are different between stratified and unstratified conditions. When the water column is largely unstratified, rotational effects do not drive significant lateral circulation. Instead, differential advection of the axial salinity gradient by wind-driven axial flow is responsible for regulating the lateral salinity gradients that in turn drive bottom-divergent/convergent lateral circulation during down/up-estuary winds. From the subtidal lateral salt balance, it is found that the development of lateral salinity gradient by wind-induced differential advection is largely counterbalanced by the advection of salt by lateral circulation itself. When the water column is stratified, the lateral flow and salinity structures below the halocline closely resemble those driven by boundary mixing, and rotational effects are important. Lateral sediment flux and the event-integrated sediment transport are from channel to shoals during down-estuary winds but reversed for up-estuary winds. Potential impacts of wind-generated waves on lateral sediment transport are evaluated with two cases representing event conditions typical of upper Chesapeake Bay. Accounting for wind wave effects results in an order of magnitude increase in lateral sediment fluxes because of greater bottom stresses and sediment resuspension.
  • Article
    Tidal and groundwater fluxes to a shallow, microtidal estuary : constraining inputs through field observations and hydrodynamic modeling
    (Springer, 2012-05-30) Ganju, Neil K. ; Hayn, Melanie ; Chen, Shih-Nan ; Howarth, Robert W. ; Dickhudt, Patrick J. ; Aretxabaleta, Alfredo L. ; Marino, Roxanne
    Increased nutrient loading to estuaries has led to eutrophication, degraded water quality, and ecological transformations. Quantifying nutrient loads in systems with significant groundwater input can be difficult due to the challenge of measuring groundwater fluxes. We quantified tidal and freshwater fluxes over an 8-week period at the entrance of West Falmouth Harbor, Massachusetts, a eutrophic, groundwater-fed estuary. Fluxes were estimated from velocity and salinity measurements and a total exchange flow (TEF) methodology. Intermittent cross-sectional measurements of velocity and salinity were used to convert point measurements to cross-sectionally averaged values over the entire deployment (index relationships). The estimated mean freshwater flux (0.19 m3/s) for the 8-week period was mainly due to groundwater input (0.21 m3/s) with contributions from precipitation to the estuary surface (0.026 m3/s) and removal by evaporation (0.048 m3/s). Spring–neap variations in freshwater export that appeared in shorter-term averages were mostly artifacts of the index relationships. Hydrodynamic modeling with steady groundwater input demonstrated that while the TEF methodology resolves the freshwater flux signal, calibration of the index– salinity relationships during spring tide conditions only was responsible for most of the spring–neap signal. The mean freshwater flux over the entire period estimated from the combination of the index-velocity, index–salinity, and TEF calculations were consistent with the model, suggesting that this methodology is a reliable way of estimating freshwater fluxes in the estuary over timescales greater than the spring– neap cycle. Combining this type of field campaign with hydrodynamic modeling provides guidance for estimating both magnitude of groundwater input and estuarine storage of freshwater and sets the stage for robust estimation of the nutrient load in groundwater.
  • Article
    Sediment transport and deposition on a river-dominated tidal flat : an idealized model study
    (American Geophysical Union, 2010-10-16) Chen, Shih-Nan ; Geyer, W. Rockwell ; Sherwood, Christopher R. ; Ralston, David K.
    A 3-D hydrodynamic model is used to investigate how different size classes of river-derived sediment are transported, exported and trapped on an idealized, river-dominated tidal flat. The model is composed of a river channel flanked by sloping tidal flats, a configuration motivated by the intertidal region of the Skagit River mouth in Washington State, United States. It is forced by mixed tides and a pulse of freshwater and sediment with various settling velocities. In this system, the river not only influences stratification but also contributes a significant cross-shore transport. As a result, the bottom stress is strongly ebb-dominated in the channel because of the seaward advance of strong river flow as the tidal flats drain during ebbs. Sediment deposition patterns and mass budgets are sensitive to settling velocity. The lateral sediment spreading scales with an advective distance (settling time multiplied by lateral flow speed), thereby confining the fast settling sediment classes in the channel. Residual sediment transport is landward on the flats, because of settling lag, but is strongly seaward in the channel. The seaward transport mainly occurs during big ebbs and is controlled by a length scale ratio Ld/XWL, where Ld is a cross-shore advective distance (settling time multiplied by river outlet velocity), and XWL is the immersed cross-shore length of the intertidal zone. Sediment trapping requires Ld/XWL < 1, leading to more trapping for the faster settling classes. Sensitivity studies show that including stratification and reducing tidal range both favor sediment trapping, whereas varying channel geometries and asymmetry of tides has relatively small impacts. Implications of the modeling results on the south Skagit intertidal region are discussed.
  • Preprint
    The landward and seaward mechanisms of fine-sediment transport across intertidal flats in the shallow-water region—A numerical investigation
    ( 2011-03-15) Hsu, Tian-Jian ; Chen, Shih-Nan ; Ogston, Andrea S.
    This study investigates transport of fine sediment across idealized intertidal flats with emphasis on resolving processes at the tidal edge, which is defined as the very shallow region of the land-water interface. We first utilize a two-dimensional, vertical numerical model solving the non-hydrostatic Reynolds-averaged Navier-Stokes equations with a k-ε turbulence closure. The numerical model adopts the Volume of Fluid method to simulate the wetting and drying region of the intertidal flat. The model is demonstrated to be able to reproduce the classic theory of tidal-flat hydrodynamics of Friedrichs and Aubrey (1996) and to predict the turbidity at the tidal edge that is similar, qualitatively, to prior field observations. The Regional Ocean Modeling System (ROMS) is also utilized to simulate the same idealized tidal flat to evaluate its applicability in this environment. We demonstrate that when a small critical depth (hcrit =2 cm) in the wetting and drying scheme is adopted, ROMS is able to predict the main features of hydrodynamics and sediment-transport processes similar to that predicted by the RANS-VOF model. When driving the models with a symmetric tidal forcing, both models predict landward transport on the lower and upper flat and seaward transport in the subtidal region. When the very shallow region of the tidal edge is well resolved, both models predict an asymmetry of tidal velocity magnitude between the flood and the ebb that may encourage landward sediment transport on the flat. Further model simulation suggests that the predicted landward transport of sediment on the flat is mainly due to the settling-lag effect while the asymmetry of tidal velocity magnitude may add a lesser but non-negligible amount. When the bed erosion is limited by the availability of soft mud, the predicted transport direction becomes landward in both the subtidal region and on the flat. These results suggest that the tidal flow generally encourages landward transport while significant seaward transport may be caused by other mechanisms. Comparisons with field observations show similarities in the net landward transport on the flat and enhanced stresses and suspended-sediment concentrations near the very shallow region of the tidal edge. The field results also indicate significant transport of sediment occurs through the channels, as a function of three-dimensional processes, which are not incorporated in the present idealized modeling.
  • Article
    Estuarine exchange flow quantified with isohaline coordinates : contrasting long and short estuaries
    (American Meteorological Society, 2012-05) Chen, Shih-Nan ; Geyer, W. Rockwell ; Ralston, David K. ; Lerczak, James A.
    Isohaline 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. This tidal exchange is driven by tidal pumping due to the jet–sink flow at the mouth constriction, leading to a linear dependence of exchange flow on tidal volume flux. Finally, a tidal conversion parameter Qin/Qprism, measuring the fraction of tidal inflow Qprism that is converted into net exchange Qin, is proposed to characterize the exchange processes among different systems. It is found that the length scale ratio between tidal excursion and salinity intrusion provides a characteristic to distinguish estuarine regimes.
  • Article
    Cyclone-driven deep sea injection of freshwater and heat by hyperpycnal flow in the subtropics
    (American Geophysical Union, 2010-11-04) Kao, Shuh-Ji ; Dai, Minhan ; Selvaraj, K. ; Zhai, W. ; Cai, Pinghe ; Chen, Shih-Nan ; Yang, J. Y. T. ; Liu, J. T. ; Liu, C. C. ; Syvitski, James P. M.
    The western tropical Pacific gives birth to 23 tropical cyclones annually, bringing torrential rainfall to mountainous islands across Oceania resulting in a global sediment production hotspot, in which many rivers have great hyperpycnal potential. By using a temperature (T) and salinity (S) profiler, we observed anomalously warm, low salinity turbid water at 3000–3700 m depths in seas ∼180 km off southwestern Taiwan immediately after Typhoon Morakot in 2009. This 250m-thick bottom-hugging water occupies ∼2400 km2, and contains 0.15% freshwater, suggesting a remarkably high fraction (6–10%) of event rainfall from southwestern Taiwan. These characteristics indicate the turbid water originated from shallow coastal waters via hyperpycnal flow. Apparently, sediment produced from the land during tropical cyclones open an “express gate” to convey heat and freshwater vertically to the deep ocean basin subsequently warming the deep water from the bottom up.