Investigating the evolution and formation of coastlines and the response to sea-level rise

Abstract

To understand how waves and sea level shape sandy shoreline profiles, I use existing energetics-based equations of cross-shore sediment flux to describe shoreface evolution and equilibrium profiles, 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, with depth increasing as temporal scale increases. To assess which wave events are most important in shaping the shoreface in terms of occurrence and severity, I calculate the characteristic effective wave conditions for both cross-shore and alongshore shoreline evolution. Extreme events are formative in the cross-shore shoreface evolution, while alongshore shoreline 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.To understand how offshore wave climate and underlying geometry of a carbonate reef platform shapes evolution of atolls, 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 width and water depth depending on the offshore wave climate and characteristics of the available sediment. Formation of a sub-aerial landmass, like a motu, can be initiated by a change in offshore wave climate (like a storm), which can create a nucleation site from mobilization and deposition of coarse sediment on the reef flat. Once a motu is present, the shoreline should prograde until reaching a critical reef-flat width. Our conceptual model of reefflat evolution and motu formation is governed by understanding the hydrodynamics of the system and subsequent response of sediment transport. 4 Chapter 1 -IntroductionClimate change has many diverse and severe impacts on our planet, including accelerated rates of sea-level rise. Over the past twentieth century, eustatic sea level rose around 1.5 -2.0 mm/yr (IPCC, 2013; Cazenave et al., 2014; Hay et al., 2015), and predicted rates of eustatic sealevel could far exceed 5 mm/yr in by the end of coming century (IPCC, 2013;Horton et al., 2014;Kopp et al., 2014). Understanding the response of coastal systems to increased rates of sea-level rise is important; 10% of the world's population lives in the Low Elevation CoastalZone (IPCC, 2014; Wong et al., 2014). My research has focused on understanding the processes that shape different coastal systems and how these systems evolve. I am interested in the response of coastal sandy and carbonate sedimentary systems to changes in sea level and predicting how future climate might impact coastal evolution.Geomorphology is the study of landscape evolution through time: by understanding the processes that shape the landscape, it is possible to predict how a landscape may respond to changing forces. Changes in sediment flux can be a primary driver of coast...