Infragravity waves: From driving mechanisms to impacts

Infragravity (hereafter IG) waves are surface ocean waves with frequencies below those of wind-generated “short waves” (typically below 0.04 Hz). Here we focus on the most common type of IG waves, those induced by the presence of groups in incident short waves. Three related mechanisms explain their generation: (1) the development, shoaling and release of waves bound to the short-wave group envelopes (2) the modulation by these envelopes of the location where short waves break, and (3) the merging of bores (breaking wave front, resembling to a hydraulic jump) inside the surfzone. When reaching shallow water (O(1–10 m)), IG waves can transfer part of their energy back to higher frequencies, a process which is highly dependent on beach slope. On gently sloping beaches, IG waves can dissipate a substantial amount of energy through depth-limited breaking. When the bottom is very rough, such as in coral reef environments, a substantial amount of energy can be dissipated through bottom friction. IG wave energy that is not dissipated is reflected seaward, predominantly for the lowest IG frequencies and on steep bottom slopes. This reflection of the lowest IG frequencies can result in the development of standing (also known as stationary) waves. Reflected IG waves can be refractively trapped so that quasi-periodic along-shore patterns, also referred to as edge waves, can develop. IG waves have a large range of implications in the hydro-sedimentary dynamics of coastal zones. For example, they can modulate current velocities in rip channels and strongly influence cross-shore and longshore mixing. On sandy beaches, IG waves can strongly impact the water table and associated groundwater flows. On gently sloping beaches and especially under storm conditions, IG waves can dominate cross-shore sediment transport, generally promoting offshore transport inside the surfzone. Under storm conditions, IG waves can also induce overwash and eventually promote dune erosion and barrier breaching. In tidal inlets, IG waves can propagate into the back-barrier lagoon during the flood phase and induce large modulations of currents and sediment transport. Their effect appears to be smaller during the ebb phase, due to blocking by countercurrents, particularly in shallow systems. On coral and rocky reefs, IG waves can dominate over short-waves and control the hydro-sedimentary dynamics over the reef flat and in the lagoon. In harbors and semi-enclosed basins, free IG waves can be amplified by resonance and induce large seiches (resonant oscillations). Lastly, free IG waves that are generated in the nearshore can cross oceans and they can also explain the development of the Earth's “hum” (background free oscillations of the solid earth)

Optimal shape design of coastal structures minimizing coastal erosion

Coastal erosion is and will be an increasing major environmental issue. We apply shape optimization techniques to the design of coastal structures such as breakwaters, groins and other innovative shapes. Actually, we compute the solution of a boundary value problem describing the water waves scattered by the structure and modify accordingly its shape, in order to minimize a pre-defined cost function taking into account the strength (energy) of the waterwaves. The optimization procedure relies on a new global semi-deterministic algorithm, able to pursue beyond local minima

What on Earth?!

International audienc

Multi-decadal variations in delta shorelines and their relationship to river sediment supply: An assessment and review

International audienceThe inception, growth, and decline of numerous large and small river deltas on Earth have been strongly influenced by human population dynamics and interventions on catchments, notably deforestation and reforestation. Over the last half century, the effects of catchment conditions in determining fluvial sediment supply have been exacerbated or moderated by dams and reservoirs. The sediment balance of river deltas, crucial in terms of delta shoreline stability, advance or retreat, and subsidence, has, in turn, been affected by variations in fluvial sediment supply. The shoreline mobility and resulting subaerial coastal area changes of a selection of 54 of the world’s deltas was quantified over 30 years based on data culled from the literature and from satellite images. These changes were analyzed alongside fluvial sediment loads. Delta shoreline mobility in response to changing fluvial loads has been variable, reflecting the miscellaneous factors that influence the supply of sediment to deltas. 29 deltas are in overall erosion, 18 show shoreline advance, whereas seven do not show any significant change. The sediment loads received by 42 deltas diminished relative to values prior to 1970, by more than 50% for 28 of them. Ten deltas showed advance, some significantly, notwithstanding fluvial sediment load decreases exceeding 25%. Overall, with the exception of the Colorado (Tx) and the Indus, losses in subaerial coastal area have been rather low. It would appear that diminishing fluvial sediment supply, the driving force in deltaic equilibrium at a multi-decadal timescale, has not, thus far, had a significant negative impact on multi-decadal delta shoreline mobility. This is important in terms of gauging currently perceived delta vulnerability. Notwithstanding, a clear link exists between the mobility of delta shorelines and the reduction in fluvial sediment loads. Eroding deltas have been affected by a reduction that is twice as important as that of stable or advancing deltas since 1970. Dams currently in place will reduce, in the future, the sediment load to their deltas of 25 of the 54 rivers by more than 50% and 100% for 15 of them. It is important to envisage the supply of sediment to deltas less in terms of its direct role in generating accretion, and eventual delta shoreline advance, and more in terms of an agent of resilience. The reduction of fluvial sediment supply to deltas will negatively impact their resilience to other drivers in the future: anthropogenic, climate change, and sea-level rise. The variability of delta shoreline behavior in the face of changing fluvial sediment loads also calls for more in-depth studies of individual deltas in order to build up future management plans addressing vulnerability and loss of resilience to marine forcing, subsidence, and sea-level rise

Optimal shape design of defense structures for minimizing short wave impact

International audienceThe paper presents coastal structures design in the presence of waves by the minimization of a cost function. It aims to show that shape optimization can be efficiently applied to ocean engineering. This is an underlying guiding principle for the design of harbors or offshore breakwaters. We compute the solution of a specific simplified boundary value problem describing the short wave propagation toward a vertical sea cliff or vertical wall and modify accordingly the shape of defense structures in order to minimize a pre-defined cost function taking into account the strength (energy) of the water waves. The optimization procedure relies on a global semi-deterministic search algorithm able to escape from local minima

Méthode de couplage vague-morphodynamisme du littoral par principe de minimisation.

International audienc

A fuzzy inference system for wind-wave modeling

International audienceForecasting of sea-state characteristics, with warning time of a few hours, appears a necessity in Operational Oceanography. linking sophisticated marine monitoring systems with forecasting modeling tools. In this paper, instead of using conventional numerical models, a Takagi-Sugeno-rule-based Fuzzy Inference System (FIS) was developed aiming at forecasting wave parameters based on the wind speed and direction, and the lagged-wave characteristics. Initial and final antecedent fuzzy membership functions were identified using the subtractive clustering method. The model was applied on the wind and wave dataset recorded in years 2000-2006 by an oceanographic buoy deployed in the Aegean Sea. The model showed perfect fit for the training period (2000-2005; 12,274 data points), and expanded its hindcasting ability during 2006 (1044 data points), as the verification part of the series. Model results, for a lead time of 3 h, showed good agreement between the predicted and the observed significant wave height (RMSE=0.216) and zero-up-crossing period (RMSE=0.315). According to other model performance criteria, the fuzzy model slightly underpredicted both wave characteristics (the linear regression slope was 0.911 for wave height and 0.788 for wave period), and reduced its forecasting ability at higher prediction intervals (+6 to + 12 h). Overall, model results illustrated that the developed FIS could serve as a valuable tool for the operational prediction of wave characteristics in Northern Aegean Sea, through the utilization of the POSEIDON network

Un exemple de mise en place de solutions dites NBS/SSE : le projet CATCHSED

International audienc

Low complexity shape optimization & a posteriori high fidelity validation

International audienceIn this paper, shape optimization tools are applied to coastal engineering. A barred beach protection study is described, using a new type of water wave attenuator device, namely geotextile tube, to reduce the mobilization of sediments. A global optimization algorithm, able to pursue beyond local minima, is used to search for the optimum properties of the geotextile tube. The optimal configuration is then post-validated for generated currents

Progress in global optimization and shape design

In this paper, we reformulate global optimization problems in terms of boundary value problems. This allows us to introduce a new class of optimization algorithms. Indeed, many optimization methods can be seen as discretizations of initial value problems for differential equations or systems of differential equations. We apply a particular algorithm included in the former class to the shape optimization of coastal structure