Investigating the Evolution and Formation of Coastlines and the Response to Sea-Level Rise

Alejandra Ortiz, Ph.D., 2015
Andrew Ashton, Advisor

I use existing energetics-based equations of cross-shore sediment flux to describe shoreface evolution, utilizing linear Airy wave theory instead of shallow-water wave assumptions. By calculating a depth-dependent characteristic diffusivity timescale, I develop a morphodynamic depth of shoreface closure for a given time envelope. I calculate the characteristic effective wave conditions for both cross-shore and alongshore shoreline evolution. Extreme events are formative in the cross-shore evolution, while alongshore evolution scales linearly with the mean wave climate. Bimodal distributions of weighted wave heights are indicative of a site impacted more frequently by tropical storms rather than extra-tropical storms.

I simulate the hydrodynamics of a simplified reef flat, using XBeach, a two-dimensional model of infragravity wave propagation. The reef flat self-organizes to a specific water depth depending on the offshore wave climate and sediment characteristics. Formation of a sub-aerial landmass, like a motu, can be initiated by a change in offshore wave climate, which can create a nucleation site. Once a motu is present, the shoreline should prograde until reaching a critical reef-flat width. Our conceptual model of reef-flat evolution and motu formation is governed by understanding the hydrodynamics of the system and subsequent response of sediment transport.