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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.

Reconstructing the topography of shallow underwater environments using Structure-from-Motion—Multi View Stereo (SfM-MVS) techniques applied to aerial imagery from Unmanned Aerial Vehicles (UAVs) is challenging, as it involves nonlinear distortions caused by water refraction. This study presents an experiment with aerial photographs collected with a consumer-grade UAV on the shallow-water reef of Fuvahmulah, the Maldives. Under conditions of rising tide, we surveyed the same portion of the reef in ten successive flights. For each flight, we used SfM-MVS to reconstruct the Digital Elevation Model (DEM) of the reef and used the flight at low tide (where the reef is almost entirely dry) to compare the performance of DEM reconstruction under increasing water levels. Our results show that differences with the reference DEM increase with increasing depth, but are substantially larger if no underwater ground control points are taken into account in the processing. Correcting our imagery with algorithms that account for refraction did not improve the overall accuracy of reconstruction. We conclude that reconstructing shallow-water reefs (less than 1 m depth) with consumer-grade UAVs and SfM-MVS is possible, but its precision is limited and strongly correlated with water depth. In our case, the best results are achieved when ground control points were placed underwater and no refraction correction is used.

Climate change and subsequent processes triggered by climate change demand novel assessments and protection schemes in coastal environments, as frequency and intensity of extreme events as well as mean sea water levels are expected to rise. Most often, conventional coastal engineering approaches are solely built for protection purposes, but often come with negative side-effects to the coastal environment and communities. During the last decade, new concepts in coastal engineering have started emerging. Several technical measures with an ecosystem-based design have been developed and, in some places, already implemented over the last decade. These low-regret measures, for instance green belts, coir fibers and porous submerged structures, reveal their full potential as stand-alone coastal protection or when used in combination with each other. They are believed-and in some cases documented-to be a better alternative or potential complement to conventional "hard" coastal engineering protection. Concrete examples are taken from the densely populated coastal area of Jakarta Utara (North Jakarta) and the National Capital Integrated Coastal Development (NCICD), showing benefits and further opportunities, but also challenges for applied low-regret coastal protection measures and ecosystem-based disaster risk reduction. An assessment of the application potential of three "soft" protection measures is given and discussed.