Worldwide, sand dunes and hard coastal structures help to minimize loss of lives and property from storm impact and flooding along or behind coastlines. Both sand dunes and hard coastal structures have their benefits and shortfalls in terms of protective capacity, cost, flexibility, and impact to coastal systems. Combining these two inherently different coastal risk reduction measures into a single hybrid system can preserve some of the individual benefits of each while creating an Engineering-with-Nature™ system that fulfills the requirements of high levels of protection, adaptability to future challenges related to climate change, sustainability, and pleasing natural aesthetics. Although such hybrid systems have the potential to become viable alternatives to conventional coastal risk reduction schemes, there are still many unknowns related to the interaction between the soft and hard structural components and their effectiveness in storm surge mitigation and flood prevention that require targeted research efforts to create acceptable design guidelines. Specifically, the combination of hard and soft alternatives into a single structure has not been studied in detail. Here, hybrid coastal risk reduction systems consisting of traditional hard structures (levees, revetments, and sea walls) covered by sand layers attempting to mimic dunes are investigated. It is emphasized that these sand covers can look like natural dunes, but they cannot evolve like natural dunes in the long term due to spatial constrictions along developed coasts and lack of natural sediment supply in eroding coastal systems. Just like any engineered beach and dune system, these hybrid structures require episodic maintenance nourishment, particularly after storm impact. The present overview covers design advances and issues related to both hard structures and engineered sand dunes for coastal risk reduction and investigates existing hybrid approaches. Future research goals to better understand hybrid coastal risk reduction systems and to create applicable design guidelines are discussed.@en

Meteotsunami waves can be triggered by atmospheric disturbances accompanying tropical cyclone rainbands (TCRs) before, during, and long after a tropical cyclone (TC) makes landfall. Due to a paucity of high-resolution field data along open coasts during TCs, relatively little is known about the atmospheric forcing that generate and resonantly amplify these ocean waves, nor their coastal impact. This study links high-resolution field measurements of sea level and air pressure from Hurricane Harvey (2017) with a numerical model to assess the potential for meteotsunami generation by sudden changes in air pressure accompanying TCRs. Previous studies, through the use of idealized models, have suggested that wind is the dominant forcing mechanism for TCR-induced meteotsunami with negligible contributions from air pressure. Our model simulations show that large air pressure perturbations (∼1–3 mbar) can generate meteotsunamis that are similar in period (∼20 min) and amplitude (∼0.2 m) to surf zone observations. The measured air pressure disturbances were often short in wavelength, which necessitates a numerical model with high temporal and spatial resolution to simulate meteotsunami triggered by this mechanism. Sensitivity analysis indicates that air pressure forcing can produce meteotsunami with amplitudes O(0.5 m) and large spatial extents, but model results are sensitive to atmospheric factors, including model uncertainties (length, forward translation speed, and trajectory of the air pressure disturbance), as well as oceanographic factors (storm surge). The present study provides observational and numerical evidence that suggest that air pressure perturbations likely play a larger role in meteotsunami generation by TCRs than previously identified.@en

Infragravity (IG) waves are expected to contribute significantly to coastal flooding and sediment transport during hurricane overwash, yet the dynamics of these low-frequency waves during hurricane impact remain poorly documented and understood. This paper utilizes hydrodynamic measurements collected during Hurricane Harvey (2017) across a low-lying barrier-island cut (Texas, U.S.A.) during sea-to-bay directed flow (i.e., overwash). IG waves were observed to propagate across the island for a period of five hours, superimposed on and depth modulated by very-low frequency storm-driven variability in water level (5.6 min to 2.8 h periods). These sea-level anomalies are hypothesized to be meteotsunami initiated by tropical cyclone rainbands. Estimates of IG energy flux show that IG energy was largely reduced across the island (79-86%) and the magnitude of energy loss was greatest for the lowest-frequency IG waves (<0.01 Hz). Using multitaper bispectral analysis, it is shown that, during overwash, nonlinear triad interactions on the sea-side of the barrier island result in energy transfer from the low-frequency IG peak to bound harmonics at high IG frequencies (>0.01 Hz). Assuming this pattern of nonlinear energy exchange persists across the wide and downward sloping barrier-island cut, it likely contributes to the observed frequency-dependence of cross-barrier IG energy losses during this relatively low surge event (<1 m)@en

Core-enhanced sand dunes are hybrid coastal defence structures that combine the storm surge protection offered by traditional hard coastal structures with the aesthetically pleasing appearance and wave energy dissipation potential of coastal sand dunes. Essentially, these are hard coastal structures covered under a sacrificial sand dune. In this study, ongoing conceptual design efforts of such structures for a potential Houston-Galveston region storm surge suppression system are presented along with results from physical model tests of three different core-enhanced dunes. These tests were carried out in a moveable-bed wave flume to determine the erosion evolution and technical feasibility of various core-enhanced dunes under irregular wave attack. Results were compared to a plain sand dune without core-enhancement. The three different cores under investigation were a clay levee, an armour stone revetment, and a T-wall. Findings showed that profile evolution of the core-enhanced dunes varied significantly based on core structure.@en