The Temporal Response of the Length of a Partially Stratified Estuary to Changes in River Flow and Tidal Amplitude

James A. Lerczak College of Oceanic and Atmospheric Sciences, Oregon State University, Corvallis, Oregon

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W. Rockwell Geyer Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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David K. Ralston Woods Hole Oceanographic Institution, Woods Hole, Massachusetts

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Abstract

The temporal response of the length of a partially mixed estuary to changes in freshwater discharge Qf and tidal amplitude UT is studied using a 108-day time series collected along the length of the Hudson River estuary in the spring and summer of 2004 and a long-term (13.4 yr) record of Qf , UT, and near-surface salinity. When Qf was moderately high, the tidally averaged length of the estuary L5, here defined as the distance from the mouth to the up-estuary location where the vertically averaged salinity is 5 psu, fluctuated by more than 47 km over the spring–neap cycle, ranging from 28 to >75 km. During low flow periods, L5 varied very little over the spring–neap cycle and approached a steady length. The response is quantified and compared to predictions of a linearized model derived from the global estuarine salt balance. The model is forced by fluctuations in Qf and UT relative to average discharge Qo and tidal amplitude UTo and predicts the linear response time scale τ and the steady-state length Lo for average forcing. Two vertical mixing schemes are considered, in which 1) mixing is proportional to UT and 2) dependence of mixing on stratification is also parameterized. Based on least squares fits between L5 and estuary length predicted by the model, estimated τ varied by an order of magnitude from a period of high average discharge (Qo = 750 m3 s−1, τ = 4.2 days) to a period of low discharge (Qo = 170 m3 s−1, τ = 40.4 days). Over the range of observed discharge, LoQo−0.30±0.03, consistent with the theoretical scaling for an estuary whose landward salt flux is driven by vertical estuarine exchange circulation. Estimated τ was proportional to the discharge advection time scale (LoA/Qo, where A is the cross-sectional area of the estuary). However, τ was 3–4 times larger than the theoretical prediction. The model with stratification-dependent mixing predicted variations in L5 with higher skill than the model with mixing proportional to UT. This model provides insight into the time-dependent response of a partially stratified estuary to changes in forcing and explains the strong dependence of the amplitude of the spring–neap response on freshwater discharge. However, the utility of the linear model is limited because it assumes a uniform channel, and because the underlying dynamics are nonlinear, and the forcing Qf and UT can undergo large amplitude variations. River discharge, in particular, can vary by over an order of magnitude over time scales comparable to or shorter than the response time scale of the estuary.

Corresponding author address: James A. Lerczak, College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Administration Building, Corvallis, OR 97331-5503. Email: jlerczak@coas.oregonstate.edu

Abstract

The temporal response of the length of a partially mixed estuary to changes in freshwater discharge Qf and tidal amplitude UT is studied using a 108-day time series collected along the length of the Hudson River estuary in the spring and summer of 2004 and a long-term (13.4 yr) record of Qf , UT, and near-surface salinity. When Qf was moderately high, the tidally averaged length of the estuary L5, here defined as the distance from the mouth to the up-estuary location where the vertically averaged salinity is 5 psu, fluctuated by more than 47 km over the spring–neap cycle, ranging from 28 to >75 km. During low flow periods, L5 varied very little over the spring–neap cycle and approached a steady length. The response is quantified and compared to predictions of a linearized model derived from the global estuarine salt balance. The model is forced by fluctuations in Qf and UT relative to average discharge Qo and tidal amplitude UTo and predicts the linear response time scale τ and the steady-state length Lo for average forcing. Two vertical mixing schemes are considered, in which 1) mixing is proportional to UT and 2) dependence of mixing on stratification is also parameterized. Based on least squares fits between L5 and estuary length predicted by the model, estimated τ varied by an order of magnitude from a period of high average discharge (Qo = 750 m3 s−1, τ = 4.2 days) to a period of low discharge (Qo = 170 m3 s−1, τ = 40.4 days). Over the range of observed discharge, LoQo−0.30±0.03, consistent with the theoretical scaling for an estuary whose landward salt flux is driven by vertical estuarine exchange circulation. Estimated τ was proportional to the discharge advection time scale (LoA/Qo, where A is the cross-sectional area of the estuary). However, τ was 3–4 times larger than the theoretical prediction. The model with stratification-dependent mixing predicted variations in L5 with higher skill than the model with mixing proportional to UT. This model provides insight into the time-dependent response of a partially stratified estuary to changes in forcing and explains the strong dependence of the amplitude of the spring–neap response on freshwater discharge. However, the utility of the linear model is limited because it assumes a uniform channel, and because the underlying dynamics are nonlinear, and the forcing Qf and UT can undergo large amplitude variations. River discharge, in particular, can vary by over an order of magnitude over time scales comparable to or shorter than the response time scale of the estuary.

Corresponding author address: James A. Lerczak, College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Administration Building, Corvallis, OR 97331-5503. Email: jlerczak@coas.oregonstate.edu

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  • Abood, K. A., 1974: Circulation in the Hudson River estuary. Hudson River Colloquium, Vol. 250, O. A. Roels, Ed., New York Academy of Sciences, 38–111.

    • Search Google Scholar
    • Export Citation
  • Banas, N. S., B. M. Hickey, P. MacCready, and J. A. Newton, 2004: Dynamics of Willapa Bay, Washington: A highly unsteady partially mixed estuary. J. Phys. Oceanogr., 34 , 24132427.

    • Search Google Scholar
    • Export Citation
  • Bowen, M. M., and W. R. Geyer, 2003: Salt transport and the time-dependent salt balance of a partially stratified estuary. J. Geophys. Res., 108 , 3158. doi:10.1029/2001JC001231.

    • Search Google Scholar
    • Export Citation
  • Chatwin, P. C., 1976: Some remarks on the maintenance of the salinity distribution in estuaries. Estuarine Coastal Mar. Sci., 4 , 555566.

    • Search Google Scholar
    • Export Citation
  • Geyer, W. R., and R. Chant, 2006: The physical oceanography processes in the Hudson River estuary. The Hudson River Estuary, J. S. Levinton and J. R. Waldman, Eds., Cambridge University Press, 13–23.

    • Search Google Scholar
    • Export Citation
  • Geyer, W. R., J. H. Trowbridge, and M. M. Bowen, 2000: The dynamics of a partially mixed estuary. J. Phys. Oceanogr., 30 , 20352048.

  • Hansen, D. V., and M. Rattray Jr., 1965: Gravitational circulation in straits and estuaries. J. Mar. Res., 23 , 104122.

  • Harleman, D. R. F., and M. L. Thatcher, 1974: Longitudinal dispersion and unsteady salinity intrusion in estuaries. Houille Blanche, 1/2 , 2533.

    • Search Google Scholar
    • Export Citation
  • Hetland, R. D., and W. R. Geyer, 2004: An idealized study of the structure of long, partially mixed estuaries. J. Phys. Oceanogr., 34 , 26772691.

    • Search Google Scholar
    • Export Citation
  • Hunkins, K., 1981: Salt dispersion in the Hudson estuary. J. Phys. Oceanogr., 11 , 729738.

  • Kranenburg, C., 1986: A time scale for long-term salt intrusion in well-mixed estuaries. J. Phys. Oceanogr., 16 , 13291331.

  • Keulegan, G. H., 1966: The mechanism of an arrested salt wedge. Estuary and Coastline Hydrodynamics, A. T. Ippen, Ed., McGraw-Hill, 546–574.

    • Search Google Scholar
    • Export Citation
  • Lerczak, J. A., W. R. Geyer, and R. J. Chant, 2006: Mechanisms driving the time-dependent salt flux in a partially stratified estuary. J. Phys. Oceanogr., 36 , 22962311.

    • Search Google Scholar
    • Export Citation
  • MacCready, P., 2007: Estuarine adjustment. J. Phys. Oceanogr., 37 , 21332145.

  • Monismith, S. G., W. Kimmerer, J. R. Burau, and M. T. Stacey, 2002: Structure and flow-induced variability of the subtidal salinity field in northern San Francisco Bay. J. Phys. Oceanogr., 32 , 30033019.

    • Search Google Scholar
    • Export Citation
  • Ralston, D. K., W. R. Geyer, and J. A. Lerczak, 2008: Subtidal salinity and velocity in the Hudson River estuary: Observations and modeling. J. Phys. Oceanogr., 38 , 753770.

    • Search Google Scholar
    • Export Citation
  • Simpson, J. H., R. Vennell, and A. J. Souza, 2001: The salt fluxes in a tidally-energetic estuary. Estuarine Coastal Shelf Sci., 52 , 131142.

    • Search Google Scholar
    • Export Citation
  • Smith, R., 1996: Combined effects of buoyancy and tides upon longitudinal dispersion. Buoyancy Effects on Coastal and Estuarine Dynamics, D. G. Aubrey and C. T. Friedrichs, Eds., Amer. Geophys. Union, 319–329.

    • Search Google Scholar
    • Export Citation
  • Stacey, M. T., and D. K. Ralston, 2005: The scaling and structure of the estuarine bottom boundary layer. J. Phys. Oceanogr., 35 , 5571.

    • Search Google Scholar
    • Export Citation
  • Stacey, M. T., J. R. Burau, and S. G. Monismith, 2001: The creation of residual flows in a partially stratified estuary. J. Geophys. Res., 106 , 1701317038.

    • Search Google Scholar
    • Export Citation
  • Vallino, J. J., and C. S. Hopkinson, 1998: Estimation of dispersion and characteristic mixing times in Plum Island Sound estuary. Estuarine Coastal Shelf Sci., 46 , 333350.

    • Search Google Scholar
    • Export Citation
  • Warner, J. C., W. R. Geyer, and J. A. Lerczak, 2005: Numerical modeling of an estuary: A comprehensive skill assessment. J. Geophys. Res., 110 , C05001. doi:10.1029/2004JC002691.

    • Search Google Scholar
    • Export Citation
  • Wells, A. W., and J. R. Young, 1992: Long-term variability and predictability of Hudson River physical and chemical characteristics. Estuarine Research in the 1980s, C. L. Smith, Ed., State University of New York Press, 29–58.

    • Search Google Scholar
    • Export Citation
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