Modeling Motu Profile Response to Varying Wave and Storm Climate

The atolls of the Pacific Ocean are low-lying landforms (less than 5m in elevation), typically consisting of reef-building corals often mounted by subaerial islets, or motu, which encircle a central lagoon. These motu, perched atop old coral reefs, typically consist of sand and gravel, and are sometimes anchored by relict geologic features (highstand coral reefs). Despite the vital role these islets play as home to terrestrial ecosystems and human infrastructure, the morphologic processes responsible for their formation and maintenance remain poorly understood. For example, although extreme events are hypothesized as a formation mechanism, motu are found in regions where hurricanes or tropical cyclones rarely occur and across varying storm gradients and frequency tracks. Here we use hydrodynamic and event-based morphodynamic modeling to better understand the role of storm events on the formation and evolution of motu.

Using XBeach, a two-dimensional model of infragravity wave propagation and sediment transport, coupled with the coastal wave model, SWAN (Simulating WAves Nearshore), we simulate the hydrodynamic and morphodynamic impacts of storm events on the nearshore, beach, and backbarrier portions of atolls. We investigate the effect of different representative profile morphology, for example motu height or the distance from the reef edge to the motu, on storm response. We further test the effect of storm intensity and inundation scenarios (i.e. difference in elevated water levels of the lagoon and ocean) on storm hydrodynamics and morphologic change. Model scenarios are informed and compared to basin-wide analysis of the variation of atoll and motu characteristics, such as reef width, motu width, and motu spacing across the Pacific Ocean. Atoll morphologies and storm responses are affected by both geographic location and, locally, the shoreline orientation (compared to storm tracks). Combining these different model scenarios with measured morphometrics allows us to develop a better understanding of the response of atolls and motu to climatic forcings, such as varying wave climate, storm frequency and intensity, and increased sea levels.