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This paper reports on a combined experimental and numerical study dedicated to barrier reefs hydrodynamics. A network of pressure sensors and velocity profilers has been deployed for more than 2 months over the Ouano reef barrier, New Caledonia. The primary aim of the study is to assess the relevance of the classical depth‐averaged momentum balance in such a complex and poorly documented environment. The combined analysis of experimental and numerical measurements reveals a specific hydrodynamic behavior contrasting with sandy beaches and fringing reefs. The cross‐reef current induced by wave breaking over the barrier reef plays an important role in the momentum budget, in particular through friction processes. The hydrodynamic behavior over the barrier reef is thus characterized by the progressive transition from a nearly classical beach type behavior on the forereef, where the gradient of radiation stress is balanced by a barotropic pressure gradient associated to the wave setup, to an open‐channel type regime, dominated by frictional head loss. The reef top wave setup shows a clear depth dependency mainly attributed to the forereef curvature. During extreme wave events, the measurements tend to indicate a transition toward a critical hydraulic regime above the reef top. The numerical simulations, involving a non‐hydrostatic wave‐resolving model coupled to a K−ϵ turbulence model, highlight the vertical structure of the flow. Over the reef flat, a classical log‐layer profile is observed, in agreement with measurements, while above the forereef an anticlockwise circulation develops under the breaking zone. Key Points: The first complete evaluation of momentum balance over a reef barrier from measurements and numerical modelingReef hydrodynamics evolves from classical beach regime on the forereef to a friction‐driven flow over the reef flatWave resolving numerical simulations highlight the complexity of the vertical structure of the flow [ABSTRACT FROM AUTHOR]

Wave runup is known to depend on offshore wave conditions and coastal morphology. While most field studies on wave runup have focused on low‐to‐mild‐sloping sandy beaches, runup measurements on steep and irregular rocky cliff profiles are still very scarce. Here we investigate the physical processes controlling wave runup in such environments and the range of applicability of empirical runup formula. This study focuses on the steep rocky cliffs (0.1 < tanβ < 0.4) of Banneg Island, a small island located in the Molène archipelago, Brittany, France, occasionally flooded during extreme water level events. A statistical parameter for extreme runup is derived from the measurements of pressure sensors deployed in the intertidal zone. Deep water wave parameters are used to force a high‐resolution wave model, and nearshore wave parameters and high‐resolution topographic data are analyzed concurrently with runup time series in order to assess the dependence of the runup on hydrodynamic conditions and morphological parameters. The wave runup is shown to be strongly related to the square root of the offshore significant wave height times the offshore wavelength. The measurements also reveal the depth dependence of the runup, which is mainly attributed to the curvature of the foreshore profile. In comparison to empirical relation obtained for a mild‐sloping beach, the present data show a significant reduction in normalized wave runup, that is attributed to enhanced bottom friction over the rocky bottom. Plain Language Summary: When waves reach the shores, they travel up and down the beach before being reflected seaward. The maximum vertical excursion of the waterline relative to the still water level, called the wave runup, is a key parameter for the design of coastal structures and the prediction of overtopping volumes during storm events. Most runup studies in natural environments have focused on smooth and mild‐sloping sandy beaches, and empirical formula to predict the wave runup has been derived for these environments. Here we study the process of wave runup over steep rocky cliffs. Using pressure sensors deployed in the intertidal zone of Banneg Island—a small island located west of Brittany, France, exposed to very high waves—we measured runup events. Then we investigated the link between these runup data, offshore wave parameters, and cliff slopes. Our results reveal that the wave runup is linearly dependent to the square root of the offshore wave height times the offshore wavelength. In addition, a significant reduction of the runup is found compared to sandy environments, which is attributed to the frictional effect exerted by the rocky bottom on the flow. Key Points: Wave runup is measured over steep rocky cliffs with pressure sensorsWave runup shows a linear dependence to the square root of the offshore waveheight times the offshore wavelengthRegression analysis reveals a significantly reduced runup compared to sandy environments that is attributed to enhanced bottom friction [ABSTRACT FROM AUTHOR]

We use spin-up/spin-down laboratory experiments to study the neutrally stratified Ekman boundary layer. The experiments are performed in the 13 m diameter, 1 m deep Coriolis rotating tank of the LEGI in Grenoble, France. A global flow rotation is produced by an initial change in the tank rotation speed. It then slowly decays under the effect of Ekman friction, evolving from the turbulent state to the laminar state. It is checked that the Ekman layer itself remains in a quasi-steady state during this decay. The velocity is measured by Particle Imaging Velocimetry (PIV) at two scales: the global rotation in a horizontal plane, and the vertical profile inside the boundary layer, where the three velocity components are obtained by stereoscopic PIV. The friction law is obtained by relating the decay rate of the bulk velocity to the velocity itself. This method is justified by the fact that this bulk velocity is independent of height beyond the top of the boundary layer (a few cm), as expected from the Taylor-Proudman theorem for rotating fluids. The local measurements inside the boundary layer provide profiles of the mean velocity and Reynolds stress components, in particular the cross-isobar angle between the interior and near surface velocities. In the laminar regime, good agreement is obtained with the classical Ekman's theory, which validates the method. In the turbulent regime, the results are found consistent with the classical Atmospheric Boundary Layer (ABL) model based on the von Karman logarithmic layer. Our experiments therefore indicate that this theory, in principle valid for very large Reynolds numbers, is already relevant close to the transitional regimes. A fit of the empirical coefficients A and B appearing in this theory yields A = 3.3 and B = 3.0. Extrapolating the results to the atmospheric case gives a friction velocity u* about 12% higher than the traditional fit for the ABL. We may safely deduce that for the oceanic bottom boundary layer, corresponding to lower Reynolds numbers than the atmosphere, our result provides a correct estimate within 10%. The previous laboratory results of Caldwell et al. ['A laboratory study of the turbulent Ekman layer,' Geophys. Fluid Dyn. 3, 125-160 (1972)] provided frictions velocities about 20% higher than in our experiments, and slightly higher cross-isobar angles. We attribute this difference to the higher vortical Rossby number Rot in those experiments, and maybe also to roughness effects. We take into account the effect of this vortical Rossby number within the framework of the Ekman layer (Rot → 0) by replacing the tank rotation rate by the fluid rotation rate. [ABSTRACT FROM AUTHOR]