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theses

One of the main characteristics of the Anthropocene in estuaries is the modification of basin morphology through the creation of navigational channels. Although the benefits of these channels are evident from an economic perspective, the associated response of the wave climate, tidal flows, salinity intrusion, and sediment dynamics is scarcely studied. Since estuaries can be classified in several categories depending on parameter spaces dictated by hydrodynamic and morphological features, it is key to assess how the barotropic, baroclinic, and sediment dynamics respond to channel modifications in multiple urbanized systems. This dissertation focuses on the impact of channel deepening on waves, tides, and sediment transport in urbanized estuaries. The study region here is the Delaware Estuary, which has been dredged for over a century to ensure navigation into the ports of Wilmington, Philadelphia, and Trenton.

First, we explore the impact of locally generated wind waves on the momentum budget and subtidal exchange in the bay. We use a numerical model to diagnose the role of wind waves on surface drag, momentum budget, and residual circulation in the estuary. Model results reveal that wave induced forces (Stokes-Coriolis, breaking, and vortex forces) did not significantly add to the mean momentum budget during a typical storm. However, when we accounted for (i) the spatially variable wave height and age in the wind stress formulation, and (ii) the wave-induced Stokes drift, we found that the subtidal bay-ocean exchange increased by about 30%. We also highlight that wind and wave direction are also critical for the magnitude of the depth-integrated exchange. Part of this study on waves included an adjustment of the wave model to prevent whitecapping wave dissipation from creating breaking forces since that contribution is already included in the wind stress. Results from this part are generalizable to young seas in estuaries where the wave field is modulated by topography.

Second, we examine how historical channel deepening altered barotropic dynamics in the estuary and tidal river. Model results with historical and modern bathymetry reveal a doubling in tidal range near the head of the tides, consistent with a reduction in hydraulic drag in the shipping channel and relatively unchanged width convergence. Tidal current amplitude along the channel doubled in some areas and were strongly modulated by undulations in channel topography. Channel deepening also increased the tidal phase speed, with implications for the arrival time of high water in the system, especially in the tidal river where high water arrives about an hour earlier now than in the mid 1800s. In terms of wave dynamics, the tidal wave became more progressive after deepening and tidal energy fluxes increased. We also found that the tidal amplification caused by a doubling in channel depth is similar to the projected change in tides under 1 m of sea level rise and shoreline hardening reported recently by other authors.

The last part is a modeling study on the response of turbidity and sediment fluxes (pumping and mean advection) after channel deepening. Since sediment dynamics are closely tied to density-driven circulation, first we examined the baroclinic response to channel deepening under mean river discharge. The model revealed that the salt intrusion increased by a factor of 1.3 and that the magnitude of the exchange flow increased only locally in the lower bay by no more than 25%. Areas of enhanced sediment trapping were located in the saline reaches of each modeled scenario, and at lateral bathymetric transitions from channels (main and secondary) to shoals due to the influence of salinity fronts. Channel deepening led to the landward migration of these trapping zones, consistent with the increase in salt intrusion. The mean advection of sediment closely mimics the residual circulation patterns in both the stratified and fresh segments of the system, while pumping fluxes were strongly landward in both scenarios due to the flood dominance caused by tidal distortion.

ETD_10207

Ph.D.

Includes bibliographical references

doi:10.7282/t3-3ss3-kc28

ETD doctoral

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