Degree

Doctor of Philosophy (PhD)

Department

Department of Oceanography and Coastal Sciences

Document Type

Dissertation

Abstract

Tidal marshes are susceptible to rapidly converting to open water due to their low elevations within the tidal frame. A continuous supply of externally supplied mineral sediment and in situ organic material production is necessary to offset elevation losses from relative sea level rise and erosion. Complex feedbacks between hydrodynamics, the erosion, transport, and deposition of sediment, and physical modification of the landscape by the establishment of intertidal vegetation govern the spatial and temporal patterns of elevation response. This dissertation focuses on modeling how these dynamics drive marsh landscape evolution in the past, present, and into the future under changing natural and anthropogenic forcing conditions. Special attention is paid to microtidal marshes because their smaller tidal prism limits sediment delivery by tidal currents and squeezes habitable intertidal elevations into an especially narrow window. The first two studies (Chapters 2-3) examine the impact of a morphodynamic feedback between long-term marsh platform development and the frictional dissipation of the tidal signal penetrating the marsh. Large microtidal marsh platforms may experience gradual tidal restriction in their landward portions, ultimately triggering runaway marsh loss even under the very same forcing conditions in which they formed. Adding in accelerated relative sea level rise then results in abrupt marsh loss. The third study (Chapter 4) is a hindcast-forecast simulation of the evolution of a large microtidal interdistributary estuary receiving combined fluvial and nearshore sediment input, and is meant to serve as an analog for Barataria Basin, LA, USA. The hindcast extends 1400 yrs into the past, while the forecast spans 50-100 yrs into the future. The morphodynamic impact of wind waves, through marsh edge erosion and sediment resuspension, control past geomorphic development and future predictions of marsh extent. The inclusion or exclusion of waves from forecasts makes a bigger impact on 50 yr predictions of basin-wide marsh extent than even large engineered river sediment diversions. The final study (Chapter 5) assesses the relative impact of different marsh restoration actions involving the beneficial re-use of dredged sediment: new marsh creation, thin-layer placement, and a nearshore “channel-seeding” technique. For a given volume of sediment added, thin-layer placement is most effective at preserving total marsh area.

Date

4-20-2025

Committee Chair

Giulio Mariotti

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