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  • Estuarine circulation  (2)
  • Sediment transport  (2)
  • American Geophysical Union  (4)
  • American Chemical Society (ACS)
  • 1
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    Flood dispersal and deposition by near-bed gravitational sediment flows and oceanographic transport : a numerical modeling study of the Eel River shelf, northern California (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2005. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 110 (2005): C09025, doi:10.1029/2004JC002727.
    Description: A large flood of the Eel River, northern California, created a thick sediment deposit between water depths of 50 and 70 m in January 1997. The freshwater plume, however, confined sediment delivery to water depths shallower than 30 m. Mechanisms proposed to explain the apparent cross-shelf transport include dispersal by oceanographic currents, resuspension by energetic waves, and gravitationally forced transport of a thin layer of fluidized mud. Field observations indicate that these processes were all active but cannot determine their relative significance or whether these mechanisms alone explain the location, size, and timing of deposition. Approximately 30% of the sediment delivered by the Eel River is accounted for in the midshelf mud bed and inner shelf, but the fate of the remaining 70% is uncertain. A three-dimensional, hydrodynamic model was used to examine potential mechanisms of sediment transport on the Eel River shelf. The model includes suspended sediment transport and was modified to account for a thin, near-bed layer of fluidized mud. It was used to simulate flood dispersal on the Eel River shelf, to compare the relative importance of transport within the near-bed fluid mud layer to suspended sediment transport, and to evaluate sediment budgets for floods. Settling properties of fine-grained sediment, both within the flood plume and the fluid mud layer, critically impact depositional patterns. To a lesser degree, wind-driven ocean currents influence the volume of sediment that escapes the shelf, and wave magnitude affects the cross-shelf location of flood deposits. Though dilute suspension accounts for a large fraction of total flux, cross-shelf transport by gravitational forcing appears necessary to produce a midshelf mud deposit similar in volume, location, and timing to those seen offshore of the Eel River.
    Description: The Office of Naval Research’s Coastal Geoscience Program supported this through program N0014-01-1-008.
    Keywords: Flood sediment dispersal ; Northern California shelf ; Sediment transport
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 2
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    Rapid sediment deposition and fine-scale strata formation in the Hudson estuary (2010)
    American Geophysical Union
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): F02004, doi:10.1029/2003JF000096.
    Description: A 9 month time series of tripod-mounted optical and acoustic measurements of sediment concentration and bed elevation was used to examine depositional processes in relationship to hydrodynamic variables in the Hudson River estuary. A series of cores was also taken directly under and adjacent to the acoustic measurements to examine the relation between the depositional processes and the resulting fine-scale stratigraphy. The measurements reveal that deposition occurs as a result of sediment flux convergence behind a salinity front and that the accumulation rates are sufficient to deposit up to 25 cm of new high-porosity sediment in a single ebb-tidal phase. Subsequent dewatering and erosion reduces the thickness of the initial deposit to several centimeters. These depositional events were only observed on spring tides. Ten depositional events during two spring tidal cycles produced a seasonal deposit of 18 cm, consistent with estimates of seasonal deposition from cores. A proxy for near-bed suspended grain size variations was estimated from the combined acoustic and optical measurements, implying that the erosional processes resuspend only the finer-grained sediments, thus leaving behind silt and very fine grained sand beds. The thickness of the deposited homogenous clayey silt beds, and the vertical separation between beds interlaminated with silt and very fine sand, are roughly consistent with the acoustic measurements of changes in bed elevations during deposition and erosion. The variability in individual bed thickness is the result of variations of processes over an individual tidal cycle and is not a product of variations over the spring neap fortnightly timescale.
    Description: The authors would like to acknowledge the Hudson River Foundation, who provided funding for this work under grant 009/00A.
    Keywords: Sediment transport ; Estuarine processes ; Fluid mud
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 3
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    Response to channel deepening of the salinity intrusion, estuarine circulation, and stratification in an urbanized estuary (2019)
    American Geophysical Union
    Details
    Publication Date: 2022-10-20
    Description: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124(7), (2019): 4784-4802, doi: 10.1029/2019JC015006.
    Description: Modifications for navigation since the late 1800s have increased channel depth (H) in the lower Hudson River estuary by 10–30%, and at the mouth the depth has more than doubled. Observations along the lower estuary show that both salinity and stratification have increased over the past century. Model results comparing predredging bathymetry from the 1860s with modern conditions indicate an increase in the salinity intrusion of about 30%, which is roughly consistent with the H5/3 scaling expected from theory for salt flux dominated by steady exchange. While modifications including a recent deepening project have been concentrated near the mouth, the changes increase salinity and threaten drinking water supplies more than 100 km landward. The deepening has not changed the responses to river discharge (Qr) of the salinity intrusion (~Qr−1/3) or mean stratification (Qr2/3). Surprisingly, the increase in salinity intrusion with channel deepening results in almost no change in the estuarine circulation. This contrasts sharply with local scaling based on local dynamics of an H2 dependence, but it is consistent with a steady state salt balance that allows scaling of the estuarine circulation based on external forcing factors and is independent of depth. In contrast, the observed and modeled increases in stratification are opposite of expectations from the steady state balance, which could be due to reduction in mixing with loss of shallow subtidal regions. Overall, the mean shift in estuarine parameter space due to channel deepening has been modest compared with the monthly‐to‐seasonal variability due to tides and river discharge.
    Description: Funding was provided by NSF Coastal SEES (OCE 1325136). Data supporting this study are posted to Zenodo (https://doi.org/10.5281/zenodo.2551285) or are available by contacting the author.
    Description: 2019-12-07
    Keywords: Estuarine circulation ; Salinity intrusion ; Stratification ; Dredging ; Hudson River
    Repository Name: Woods Hole Open Access Server
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  • 4
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    Mechanisms of exchange flow in an estuary with a narrow, deep channel and wide, shallow shoals (2021)
    American Geophysical Union
    Details
    Publication Date: 2022-10-20
    Description: © The Author(s), 2020. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geyer, W. R., Ralston, D. K., & Chen, J. Mechanisms of exchange flow in an estuary with a narrow, deep channel and wide, shallow shoals. Journal of Geophysical Research: Oceans, 125(12), (2020): e2020JC016092, https://doi.org/10.1029/2020JC016092.
    Description: Delaware Bay is a large estuary with a deep, relatively narrow channel and wide, shallow banks, providing a clear example of a “channel‐shoal” estuary. This numerical modeling study addresses the exchange flow in this channel‐shoal estuary, specifically to examine how the lateral geometry affects the strength and mechanisms of exchange flow. We find that the exchange flow is exclusively confined to the channel region during spring tides, when stratification is weak, and it broadens laterally over the shoals during the more stratified neap tides but still occupies a small fraction of the total width of the estuary. Exchange flow is relatively weak during spring tides, resulting from oscillatory shear dispersion in the channel augmented by weak Eulerian exchange flow. During neap tides, stratification and shear increase markedly, resulting in a strong Eulerian residual shear flow driven mainly by the along‐estuary density gradient, with a net exchange flow roughly 5 times that of the spring tide. During both spring and neap tides, lateral salinity gradients generated by differential advection at the edge of the channel drive a tidally oscillating cross‐channel flow, which strongly influences the stratification, along‐estuary salt balance, and momentum balance. The lateral flow also causes the phase variation in salinity that results in oscillatory shear dispersion and is an advective momentum source contributing to the residual circulation. Whereas the shoals make a negligible direct contribution to the exchange flow, they have an indirect influence due to the salinity gradients between the channel and the shoal.
    Description: The ideas in this paper were influenced by discussions with Robert Chant. Funding was provided by National Science Foundation grants OCE‐1325136, OCE‐1634490, and OCE‐1736539.
    Description: 2021-04-29
    Keywords: Estuarine circulation ; Tidal dispersion ; Lateral circulation
    Repository Name: Woods Hole Open Access Server
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