Hydrodynamic Drivers and Morphological Responses on Small Coral Islands—The Thoondu Spit on Fuvahmulah, the Maldives

Abstract

Assessing the resilience of islands toward altered ocean climate pressures and providing robust adaptation measures requires an understanding of the interaction between morphological processes and the underlying hydrodynamic drivers. In this sense, this study presents changing sediment volumes on various temporal scales for the fringing reef island Fuvahmulah. Based on three field campaigns, conducted over 2 years, aerial imagery provides information on marine aggregates of the island's beaches. In addition, high resolution climate reanalysis data serves as input into an empirical and a numerical approach. Together, both approaches describe the driving processes behind volumetric seasonal and interannual changes: On the one hand, the empirical method quantifies sediment transport rates for calcareous sediments over the whole time span of the data set by considering wind and swell waves from multiple directions. On the other hand, the numerical method gives insights into the complexity of currents induced by dominant wave components. Combining these methods facilitates hindcasting and predicting morphological changes under varying wave climate, assessing sediment pathways over the whole reef, and describing the seasonal and interannual evolution of the sand spit Thoondu. As a result, this study reveals sediment distribution on different spatio-temporal scales and elucidates their significance in the design of conventional and alternative low-regret coastal adaptation.

Structure-from-Motion on shallow reefs and beaches: potential and limitations of consumer-grade drones to reconstruct topography and bathymetry

“…The inter-tidal reef has three longshore sections: On the right side, inhabitants of the island have cut a channel into the reef. The channel provided safe landing for boats before the seaport opened in 2002 (Ratter et al 2019;David and Schlurmann 2020). This area is constantly submerged and has the highest water depth of the study site.…”

“…In turn, they provide several ecosystem services amounting to nearly 200,000 US dollars per acre per year (Costanza et al 2014). Natural hazards and anthropogenic pressures affect small islands' coastlines and the underwater marine environment surrounding them (Kench and Brander 2006;Duvat and Magnan 2019;David and Schlurmann 2020). In the face of climate change, the increase in ocean temperatures is forecasted to lead to more extensive coral bleaching (Skirving et al 2019), causing increased coral mortality (Hédouin et al 2020).…”

“…Survey grade MBES cover areas with a sensor covering a lateral area of 120 À 150 to record bathymetry with a vertical resolution of \1% of the water depth (Colbo et al 2014). However, surveygrade echo sounders depend on vessel draft and are usually not suitable for shallow waters (\4-5 m depth, Leon et al 2015;David and Schlurmann 2020). Consumer-grade SBESs-for example used by fishers-have a smaller immersion depth but recorded water depths vary between centimeter to lower decimeter scale compared to groundtruth water depths (Bandini et al 2018).…”

“…This study stems from a project dealing with sea level rise and associated impacts on small islands (Ratter et al 2019;David and Schlurmann 2020). The project focuses on obtaining field measurements with aerial surveys to evaluate sediment transport as natural sea level rise adaptation on the Maldivian reef island of Fuvahmulah.…”

“…While on-land (dry beach) areas were the main objective of the project, this work analyzes data collected on reef flats on Fuvahmulah. Error metrics for varying water depths on reef flats are currently not available (David and Schlurmann 2020). The purpose of this study is therefore to assess the quality of reconstructed underwater topographies (bathymetries) from a SfM-MVS algorithm for different water levels and thus to utilize this information in future studies.…”