Drowning of Developed Coastal Barriers: Investigating the Threshold Rates of Sea-Level Rise of the Geologic-Human System

Abstract Coastal barriers, narrow sandy islands or spits found along many of the worlds coasts, face an uncertain future over the next century. Rates of sea-level rise already exceed those experienced over the last several thousand years and are predicted to be substantially greater by the end of the century, offering the potential that barriers could drown. Natural barriers respond to sea-level rise through a process of landward migration - during storms, overwash and breaching transfer sediment to the back of a barrier. Human activities intended to preserve coastal infrastructure, such as building seawalls, constructing groynes, nourishing beaches, stabilizing inlets, or armoring updrift headlands, have accumulated to the point where a new landform, unprecedented in earth?s history, has emerged: the "developed barrier". Typically, engineering activities on developed barriers prevent or counteract overwash. Two key questions guide this research: 1) Are there threshold rates of sea-level rise beyond which coastal barriers will drown completely? and 2) Will human responses to sea-level rise mitigate or exacerbate the potential for losses of developed barriers across entire systems? A multidisciplinary team of geologists, numerical modelers, and economists will engage in an integrated field and modeling study to examine the behavior and predict the fate of barrier systems. Focused geologic investigations will investigate how barriers responded to higher rates of sea-level rise in the past (rates that match those predicted for later this century). Numerical models will study the evolution of developed barrier systems at decadal to centennial time scales by coupling natural sediment transport processes and human alterations of the coast. Different human protection strategies will be explored, comparing coordinated versus uncoordinated responses and considering whether well-informed versus poorly informed responses result in different barrier futures. The most important question will be whether human activities intended to preserve infrastructure and beach recreation may make wholesale collapse, or "drowning" of barrier systems more likely. The drowning of barrier beaches due to sea-level rise is an issue of global importance. The same wave processes that created these barriers will cause them to change dramatically as sea levels rise. Barriers will do more than flood passively (like a bath tub); storms, waves, and tides can reshape entire coastlines. Barriers contain a beach, a resource that is particularly valued, and house significant amounts of infrastructure, including homes, resort complexes, hotels, and roadways. Accordingly, humans respond to coastal changes by bulldozing beaches, pumping sand, and armoring the coast. What are the long-term effects of these activities on the naturally occurring processes on the coast? This research aims to provide guidelines on how the global system of barriers will respond to changes in sea level and to changes in mitigation strategies, with a goal of identifying optimal societal responses to reduce the resulting potential economic losses. This research will involve communication with stakeholders and coastal managers to obtain their inputs to help formulate and assess decision-making strategies for model simulations and to disseminate the study findings. This program will also provide interdisciplinary training for students, who are the next generation of coastal researchers, on the diverse aspects of coastal processes.

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