2019 CSDMS meeting-110

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Potential impacts of excavation pits on atoll reef islands

Alejandra Ortiz, North Carolina State University Raleigh North Carolina, United States. aortiz4@ncsu.edu
Carter Rucker, North Carolina State University Raleigh North Carolina, United States. carucker@ncsu.edu
Johnathan Woodruff, North Carolina State University Raleigh North Carolina, United States. jlwoodr3@ncsu.edu

The shorelines of atoll reef islands (also called motu, sandy cays, or islets) frequently are the only available landforms in an atoll system, e.g. Kwajalein Atoll encompasses over 2174 km² but only 16 km2 of that area is emergent land, and thus understanding the drivers of coastal landscape evolution is vital. In particular, atolls are highly vulnerable to several threats of climate change from accelerated rates of sea level rise (causing flooding or potential drowning) to ocean acidification (decreasing coral reef resiliency) to ocean warming (causing coral bleaching). However, we lack a thorough understanding of the potential drivers of landscape change in these systems. In addition atolls can be exposed to high energy wave climates, however, the carbonate reef platform that encircles the inner lagoon of an atoll, commonly filters much of the incident wave energy. This reef platform or reef flat is typically shallow (1-2 m below MSL) with a near constant depth across the reef-flat width; the reef-flat widths range from a 100s meters to over a kilometer on different atolls. Both numerical modeling and field observations have found that these shallow reef-flats are key for driving wave breaking at the ocean edge of the reef flat offshore of the atoll reef islands and decreasing wave energy at the shoreline. As demand for construction materials increases, these carbonate reef platforms have been excavated, with large pits ranging from 10-80 m in width and average depths of 4 m. This study seeks to understand how the presence of excavation pits on reef flats change the wave energy at the shoreline. Utilizing 1D XBeach model, we investigate the impact of varying excavation pit geometry on the shoreline wave energy. We found that the presence of an excavation pit increases wave energy at the shoreline.