Abstract. The Bay of Bengal is a well-known breeding ground to some of the deadliest cyclones in history. Despite recent advancements, the complex morphology and hydrodynamics of this large delta and the associated modelling complexity impede accurate storm surge forecasting in this highly vulnerable region. Here we present a proof of concept of a physically consistent and computationally efficient storm surge forecasting system tractable in real time with limited resources. With a state-of-the-art wave-coupled hydrodynamic numerical modelling system, we forecast the recent Supercyclone Amphan in real time. From the available observations, we assessed the quality of our modelling framework. We affirmed the evidence of the key ingredients needed for an efficient, real-time surge and inundation forecast along this active and complex coastal region. This article shows the proof of the maturity of our framework for operational implementation, which can particularly improve the quality of localized forecast for effective decision-making over the Bengal delta shorelines as well as over other similar cyclone-prone regions.

Short waves are of key importance for nearshore dynamics, particularly under storms, where they contribute to extreme water levels and drive large morphological changes. Therefore, it is crucial to model accurately the propagation and dissipation of storm waves in the nearshore area. In this paper, field observations collected in contrasted environments and conditions are combined with predictions from a third-generation spectral wave model to evaluate four formulations of wave energy dissipation by depth-induced breaking.The results reveal a substantial over-dissipation of incident wave energy occurring over the continental shelf, resulting in a negative bias on significant wave height reaching up to 50%. To overcome this problem, a breaking coefficient dependent of the local bottom slope is introduced within depth-induced breaking models in order to account for the varying degrees of saturation naturally found in breaking and broken waves. This approach strongly reduces the negative bias observed in the shoreface compared to default parameterizations, yielding significant improvements in the prediction of storm waves. Among the implications of this study, our new parameterization of the breaking coefficient results in systematically increased predictions of the wave setup near the shoreline compared to the default parameterization. This increase reaches a factor 2 for gently sloping beaches.

This study explores the spatial distribution and the driving mechanisms of the wave‐induced cross‐shore flow within the shoreface and surf zone of a dissipative beach. Unpublished results from a field campaign carried out in early 2021 under storm wave conditions are presented and compared with the predictions from a state‐of‐the‐art phase‐averaged three‐dimensional circulation modeling system based on the vortex force formalism. Under storm wave conditions, the cross‐shore flow is dominated by a strong seaward‐directed current in the lower part of the water column. The largest current velocities of this return current are located in the surf zone, where the dissipation by depth‐induced breaking is most intense, but offshore‐directed velocities up to 0.25 m/s are observed as far as 4 km from the shoreline (≃12 m‐depth). Numerical experiments further highlight the key control exerted by non‐conservative wave forces and wave‐enhanced mixing on the cross‐shore flow across a transition zone, where depth‐induced breaking, whitecapping, and bottom friction all significantly contribute to the wave energy dissipation. Under storm conditions, this transition zone extended almost 6 km offshore and the cross‐shore Lagrangian circulation shows a strong seaward‐directed jet in the lower part of the water column, whose intensity progressively decreases offshore. In contrast, the surf zone edge appears clearly delimited under fair weather conditions and the seaward‐directed current is weakened by a near bottom shoreward‐directed current associated with wave bottom streaming in the shoaling region, such that the clockwise Lagrangian overturning circulation is constrained by an additional anti‐clockwise overturning cell at the surf zone edge.

Abstract. The Bay of Bengal is a well-known breeding ground to some of the deadliest cyclones in history. Despite recent advancements, the complex morphology and hydrodynamics of this large delta and the associated modelling computational costs impede the storm surge forecasting in this highly vulnerable region. Here we present a proof of concept of a physically consistent and computationally efficient storm surge forecasting system tractable in real-time with limited resources. With a state-of-the-art wave-coupled hydrodynamic numerical modelling system, we forecast the recent super cyclone Amphan in real-time. From the available observations, we assessed the quality of our modelling framework. We affirmed the evidence of the key ingredients needed for an efficient, real-time surge and inundation forecast along this active and complex coastal region. This article shows the proof of the maturity of our framework for operational implementation, which can particularly improve the quality of localized forecast for effective decision-making.

While wave processes on shore platforms have been recently advanced by a number of field‐based studies, few attention has been paid to the role of bed roughness on wave dissipation and wave setup dynamics in these environments. This study reports on a new field experiment conducted under storm wave conditions on a gently sloping shore platform which was instrumented from 10 m water depth up to the shoreline. Data analyses are complemented with numerical simulations performed with a 3D fully coupled modeling system using a vortex force formalism to represent the effects of short waves on the mean circulation. An accurate representation of wave dissipation by both depth‐induced breaking and bottom friction is found essential to reproduce the transformation of short waves across the platform and the resulting wave setup. Wave energy dissipation by bottom friction is dominant in the subtidal part of the platform and contributes to about 40% of the total wave energy dissipation. The enhanced wave bottom friction on the platform decreases the wave height before breaking, which reduces the contribution of wave forces to the wave setup compared to a smooth bottom (mechanism 1). Conversely, an idealized analysis of the cross‐shore momentum balance reveals that the wave‐induced circulation increases the wave setup, this process being enhanced on a rough bottom (mechanism 2). The contribution of mechanism 2 increases with the bottom slope, accounting for up to 26% of the wave setup for a 1:20 sloping shore platform, and overcoming mechanism 1.