Many low‐elevation, coral reef‐lined, tropical coasts are vulnerable to the effects of climate change, sea level rise, and wave‐induced flooding. The considerable morphological diversity of these coasts and the variability of the hydrodynamic forcing that they are exposed to make predicting wave‐induced flooding a challenge. A process‐based wave‐resolving hydrodynamic model (XBeach Non‐Hydrostatic, “XBNH”) was used to create a large synthetic database for use in a “Bayesian Estimator for Wave Attack in Reef Environments” (BEWARE), relating incident hydrodynamics and coral reef geomorphology to coastal flooding hazards on reef‐lined coasts. Building on previous work, BEWARE improves system understanding of reef hydrodynamics by examining the intrinsic reef and extrinsic forcing factors controlling runup and flooding on reef‐lined coasts. The Bayesian estimator has high predictive skill for the XBNH model outputs that are flooding indicators, and was validated for a number of available field cases. It was found that, in order to accurately predict flooding hazards, water depth over the reef flat, incident wave conditions, and reef flat width are the most essential factors, whereas other factors such as beach slope and bed friction due to the presence or absence of corals are less important. BEWARE is a potentially powerful tool for use in early warning systems or risk assessment studies, and can be used to make projections about how wave‐induced flooding on coral reef‐lined coasts may change due to climate change.

Ameland inlet is centrally located in the chain of West Frisian Islands (the Netherlands). A globally unique dataset of detailed bathymetric charts starting in the early 19th century, and high-resolution digital data since 1986 allows for detailed investigations of the ebb-tidal delta morphodynamics and sediment bypassing over a wide range of scales. The ebb-tidal delta exerts a large influence on the updrift and downdrift shorelines, leading to periodic growth and decay (net erosion) of the updrift (Terschelling) island tip, while sequences of sediment bypassing result in shoal attachment to the downdrift coastline of Ameland. Distinct differences in location, shape and volume of the attachment shoals result from differences in sediment bypassing, which can be driven by morphodynamic interactions at the large scale of the inlet system (O(10 km)), and through interactions that originate at the smallest scale of individual shoal instabilities (O(0.1 km)). Such shoal instabilities would not be considered to affect the ebb-tidal delta and inlet dynamics as a whole, but as we have shown in this paper, they can trigger a new sediment bypassing cycle and result in complete relocation of channels and shoals. These subtle dynamics are difficult, if not impossible, to capture in existing general conceptual models and empirical relationships. These differences are, however, essential for understanding tidal inlet and channel morphodynamics and hence coastal management.

Wave-induced flooding is a major coastal hazard on tropical islands fronted by coral reefs. The variability of shape, size, and physical characteristics of the reefs across the globe make it difficult to obtain a parameterization of wave run-up, which is needed for risk assessments. Therefore, we developed the HyCReWW metamodel to predict wave run-up under a wide range of reef morphometric and offshore forcing characteristics. Due to the complexity and high dimensionality of the problem, we assumed an idealized one-dimensional reef profile, characterized by seven primary parameters. XBeach Non-Hydrostatic was chosen to create the synthetic dataset, and Radial Basis Functions implemented in MATLAB ® were chosen for interpolation. Results demonstrate the applicability of the metamodel to obtain fast and accurate results of wave run-up for a large range of intrinsic reef morphologic and extrinsic hydrodynamic forcing parameters, offering a useful tool for risk management and early warning systems.

Many coral reef-lined coasts are low-lying with elevations <4 m above mean sea level. Climate-change-driven sea-level rise, coral reef degradation, and changes in storm wave climate will lead to greater occurrence and impacts of wave-driven flooding. This poses a significant threat to their coastal communities. While greatly at risk, the complex hydrodynamics and bathymetry of reef-lined coasts make flood risk assessment and prediction costly and difficult. Here we use a large (>30,000) dataset of measured coral reef topobathymetric cross-shore profiles, statistics, machine learning, and numerical modeling to develop a set of representative cluster profiles (RCPs) that can be used to accurately represent the shoreline hydrodynamics of a large variety of coral reef-lined coasts around the globe. In two stages, the large dataset is reduced by clustering cross-shore profiles based on morphology and hydrodynamic response to typical wind and swell wave conditions. By representing a large variety of coral reef morphologies with a reduced number of RCPs, a computationally feasible number of numerical model simulations can be done to obtain wave runup estimates, including setup at the shoreline and swash separated into infragravity and sea-swell components, of the entire dataset. The predictive capability of the RCPs is tested against 5,000 profiles from the dataset. The wave runup is predicted with a mean error of 9.7-13.1%, depending on the number of cluster profiles used, ranging from 312 to 50. The RCPs identified here can be combined with probabilistic tools that can provide an enhanced prediction given a multivariate wave and water level climate and reef ecology state. Such a tool can be used for climate change impact assessments and studying the effectiveness of reef restoration projects, as well as for the provision of coastal flood predictions in a simplified (global) early warning system.

Coral reefs are effective natural coastal flood barriers that protect adjacent communities. Coral degradation compromises the coastal protection value of reefs while also reducing their other ecosystem services, making them a target for restoration. Here we provide a physics-based evaluation of how coral restoration can reduce coastal flooding for various types of reefs. Wave-driven flooding reduction is greatest for broader, shallower restorations on the upper fore reef and between the middle of the reef flat and the shoreline than for deeper locations on the fore reef or at the reef crest. These results indicate that to increase the coastal hazard risk reduction potential of reef restoration, more physically robust species of coral need to be outplanted to shallower, more energetic locations than more fragile, faster-growing species primarily being grown in coral nurseries. The optimization and quantification of coral reef restoration efforts to reduce coastal flooding may open hazard risk reduction funding for conservation purposes.

Sustainable management of barrier islands and tidal inlet systems requires a knowledge of sediment transport pathways throughout the system. This paper places in situ suspended sediment observations (obtained using a LISST) in context with seabed sediment samples and hydrodynamic measurements to identify such pathways. The results indicate two distinct populations of sediment in suspension on the ebb-tidal delta: locally resuspended fine sand and (largely flocculated) mud exported from the Wadden Sea on ebb tide. This reinforces the notion of the strong dependence of sediment pathways on particle size. Future work will combine additional lines of evidence to better distinguish suspended sand from sand-sized flocs and provide a more robust definition of these pathways.

Coastal aeolian sediment transport is influenced by supply-limiting factors caused by sediment sorting by grain size. Sorting processes can lead to coarsening of the bed surface and influence the formation of aeolian ripples. However, the influence sorting processes and bedforms might have on the magnitude of the transport is not fully understood. This study explores sorting processes and their influence on the magnitude and mode of aeolian transport by using sediment tracers. Sand was painted in different colors according to particle size and placed on a supratidal beach in Noordwijk, the Netherlands. Several experiments were conducted with varying wind

Quantifying and characterizing suspended sediment is essential to successful monitoring and management of estuaries and coastal environments. To quantify suspended sediment, optical and acoustic backscatter instruments are often used. Optical backscatter systems are more sensitive to mud particles (< 63µm) and flocs, whereas acoustic Accepted ArticleThis article is protected by copyright. All rights reserved.Confidential manuscript submitted to JGR-Oceans allows us to estimate whether there are more sand or mud particles floating through the water. The relationship between "seeing" and "hearing" can be described in a single number, the sediment composition index (SCI). We successfully tested this approach in laboratory experiments and then applied it to a site on the coast of the Netherlands. This approach gives us a new way to understand environments that are both sandy and muddy.