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Dispersion and Tidal Dynamics of Channel-Shoal Estuaries

  • Author(s): Holleman, Christopher Dean
  • Advisor(s): Stacey, Mark T
  • et al.
Abstract

Estuaries, and the varied ecosystems they support, are affected by human action in many ways. One of the fundamental environmental questions pertaining to estuaries is how material is mixed into ambient waters and transported within and beyond the estuary. Characterizing transport and mixing is essential for tackling local environmental questions, such as gauging the water-quality impacts of a wastewater outfall, or how dredging may alter estuarine circulation. In many estuaries the dominant physical dispersion mechanism is shear dispersion. A fundamental and idealized approach to the scalar transport problem is undertaken, in which canonical shear dispersion regimes are augmented with a new, intermediate regime. This mode of shear dispersion, in which the longitudinal variance of the plume increase with the square of time, is likely to occur in channel-shoal basins where bathymetric transitions can create sheared velocity profiles. A key tool in the engineer's arsenal for pursuing studies of scalar transport is the numerical model, but care must be taken to separate physical mixing effects from numerical artifacts. Unstructured models are particularly well-suited to estuarine problems as the grid can be adapted to complex local topography, but the numerical errors of these models can be difficult to characterize with standard methods. An in-depth error analysis reveals a strong dependence of numerical diffusion on the orientation of the grid relative to the flow. Flexible grid generation methods allow for optimizing the grid in light of this dependence, and can decrease across-flow numerical diffusion by a factor of two.

The most far-reaching impact of our actions, though, must certainly be global climate change. A period of rising sea level is being ushered in by climate change, and the final research-oriented chapter seeks to further our understanding of how rising sea levels will affect basin-scale tidal dynamics. Numerical and analytic approaches show that as basins get deeper tidal amplification becomes more effective and tidal range increases. The crux of the analysis, though, is the inclusion of inundation within these scenarios. Inundated areas dissipate incident tidal energy, countering the added amplification due to basins becoming deeper. The net effect, in the case of San Francisco Bay, is that tidal amplification under sea level rise actually decreases, such that a particular rise in mean higher high water in the

coastal ocean is predicted to raise mean higher high water within the bay by a lesser amount.

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