Annualmeeting:2017 CSDMS meeting-035


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Atoll Morphometrics: Why do atolls and reef islands look the way they do?

Alejandra Ortiz, NCSU Raleigh North Carolina, United States. 2501 Stinson Drive

[[Image:|300px|right|link=File:]]Despite the essential role sub-aerial reef islands on atolls play as home to terrestrial ecosystems and human infrastructure, the morphologic processes and environmental forcings responsible for their formation and maintenance remain poorly understood. Given that predicted sea-level rise by the end of this century is at least half a meter (Horton et al., 2014), it is important to understand how atolls and their reef islands will respond to accelerated sea-level rise for island nations where the highest elevation may be less than 5 meters (Webb and Kench, 2010). Atolls are oceanic reef systems consisting of a shallow reef platform encircling a lagoon containing multiple islets around the reef edge (Carter et al., 1994). Atolls come in a variety of shapes from circular to rectangular and size from 5 to 50 km width of the inner lagoon (Fig. 1a and 1b). I want to understand why atolls vary in their morphology and whether wave climate is the primary driver of atoll morphology. Previous work has highlighted the importance of wave energy on reef morphology and atoll morphology (Stoddart, 1965; Kench et al., 2006). Around a given atoll, the morphology of the reef islands may change significantly from small individual islets or larger continuous islets that are more suitable for human habitation (Fig. 1c and 1d). I will create a global dataset of atoll morphometrics to compare to external forcing, e.g. comparing reef width to the mean wave climate. Using Google Earth Engine, a cloud-based geospatial analysis platform to collate Landsat imagery, I can measure a range of morphometrics including atoll size and shape, reef flat width, reef island size and shape, and distribution of reef islands around an atoll. I will compare these morphometrics to global waves simulated by WaveWatch3. By compiling a global dataset of atoll morphometrics, I am able to better understand the impact of wave climate on atoll morphology and long-term evolution.

References: Carter, R.W.G., Woodroffe, C.D.D., McLean, R.F., and Woodroffe, C.D.D., 1994, Coral Atolls, in Carter, R.W.G. and Woodroffe, C.D. eds., Coastal evolution: Late Quaternary shoreline morphodynamics, Cambridge University Press, Cambridge, p. 267–302.

Horton, B.P., Rahmstorf, S., Engelhart, S.E., and Kemp, A.C., 2014, Expert assessment of sea-level rise by AD 2100 and AD 2300: Quaternary Science Reviews, v. 84, p. 1–6, doi: 10.1016/j.quascirev.2013.11.002.

Kench, P.S., Brander, R.W., Parnell, K.E., and McLean, R.F., 2006, Wave energy gradients across a Maldivian atoll: Implications for island geomorphology: Geomorphology, v. 81.

Stoddart, D.R., 1965, The shape of atolls: Marine Geology, v. 3.

Webb, A.P., and Kench, P.S., 2010, The dynamic response of reef islands to sea-level rise: Evidence from multi-decadal analysis of island change in the Central Pacific: Global and Planetary Change, v. 72.

Figure 1. Aerial imagery of a) Rangiroa
Atoll and b) Hikueru Atoll in French Polynesia
with varying reef islands (motu) morphology
on the c) NW coast and the d) south coast.