Modelling reef hydrodynamics and sediment mobility under sea level rise in atoll reef island systems

Holly K East,Eddie P Beetham,Yiqing Liang,Chris T Perry,Paul S Kench

doi:10.1016/j.gloplacha.2020.103196

Abstract

Abstract Low-lying coral reef islands will be significantly impacted by future sea-level rise (SLR). It is generally expected that SLR will destabilise reef islands because increasing reef submergence allows larger waves, and therefore greater energy transmission, across reef flats. However, the potential impacts of SLR on reef flat sediment transport and sediment delivery to island shorelines are poorly understood. Here, we use the currents of removal approach (coupling two-dimensional wave modelling with settling velocity data from 186 benthic sediment samples) to model shifts in both reef hydrodynamics and benthic sediment transport under different scenarios of mean reef submergence (MRS = +0 m, +0.5 m, +1 m) at two atoll rim reef sites in the Maldives. Under contemporary conditions (MRS = +0 m), we found that benthic sediment transport is likely occurring, consistent with active reef-to-island sediment connectivity. Under conditions of increased MRS, shifts in wave velocities, and in turn sediment potential mobility, were both non-linear and non-uniform. Significant between-site differences were found in the magnitude of projected shifts in sediment mobility under scenarios of increased MRS, which implies that morphological responses to increased MRS are likely to be diverse, even over local scales. Under increased MRS, the largest increases in sediment mobility were projected on the inner reef flat, whereas lagoonal zones remained as sinks for sediment deposition. We thus hypothesize that while reef islands will persist as sedimentary landforms under projected rates of MRS, lagoonward reef island migration is likely to occur. Findings have implications for predicting the future adaptive capacity of atoll nations. Our results highlight the need for national-scale vulnerability assessments to incorporate (1) potential increases in island mobility; (2) the importance of allowing natural sediment transport processes to occur (unhindered by human constructions); and (3) intra-regional diversity in reef system geomorphic responses to sea level rise.

Highlights

Low-lying coral reef islands are frequently considered to be among the most vulnerable landforms to climate change and associated sea level rise (SLR; IPCC, 2019)
We suggest it is more appropriate to employ mean reef submergence (MRS) as it is the difference between vertical reef accretion and SLR that is the key control on across-reef wave energy regimes (Quataert et al, 2015)
Under Vmean benthic sediment transport occurred from the oceanward reef crest (20.4 ± 13.7%) into the remainder of the oceanward environment (PM= ~10%), through inter-island passages, and into the lagoonward environment where sediment transport occurred in the lee of the inter277 island passages

Summary

Introduction

Low-lying coral reef islands are frequently considered to be among the most vulnerable landforms to climate change and associated sea level rise (SLR; IPCC, 2019). Increases in flooding and wave inundation events have been projected to render atoll nations uninhabitable by the end of the century (Quataert et al, 2015; Storlazzi et al, 2015, 2018) Given their vulnerability, reef islands have received increasing attention from geomorphic (Webb and Kench, 2010; Kench et al, 2015; Duvat et al, 2017; Kench et al, 2018) and hydrodynamic (Quataert et al, 2015; Storlazzi et al, 2015, 2018; Beetham et al, 2017) research in recent years. Sediment transport processes are key controls on reef island maintenance and morphological stability, but there is very limited understanding of both contemporary process regimes and how these processes may change under 57 future SLR scenarios

Methods

Results

Discussion

Conclusion