Morphodynamics of shallow coastal bays
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This thesis investigates the processes governing the hydro-morphodynamics of shallow coastal bays, through field investigations and numerical modeling. Particular attention is devoted to understand the coupling between three morphological units: salt marshes, tidal flats and tidal channels. This study demonstrates that sea level rise and sediment supply have a significant and complex control on the morphological evolution of coastal bays. Erosive processes in the lagoons ofVirginia Coast Reserve (VA) are explored through a detailed model for tidal currents and the dynamics of wind waves. The model reveals that both wave-induced erosion of the marsh boundary and tidal flat bed erosion increase with sea level rise. Both positive and negative feedbacks between wave energy at the boundaries and bottom shear stresses are predicted, depending on the fate of the sediments eroded from the salt marsh boundaries. A lD morphological model for the coupled evolution of marshes and tidal flats shows that both an increase in sea level and a decrease in sediment supply enhance marsh boundary erosion, suggesting that this erosion mechanism is a leading cause of marsh deterioration. In addtion, the model predicts that the scarp between salt marsh and tidal flat is a distinctive feature of marsh retreat, suggesting the use of this geometry as an indicator for ongoing erosion. Field investigations in Willapa Bay (WA) shed light on the transport of water and sediments in a channel-tidal flat complex. During calm weather suspended sediment concentration is higher in the channel than on the tidal flat, leading to a sediment flux toward the latter, while the opposite flux occurs during stormy weather. A "two-points" dynamical model shows that the morphology of a costal bay stems from a dynamical equilibrium between currents in the channels and wind waves on the tidal flats. Because multiple equilibria are present, abrupt morphological changes can be triggered by small changes in wind regime, sea level or sediment supply.
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