Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SW Spain

An application of the grain size trend analysis (GSTA) is used in an exploratory approach to characterize sediment transport on Camposoto beach (Cádiz, SW Spain). In May 2009 the mesotidal beach showed a well-developed swash bar on the upper foreshore, which was associated with fair-weather conditions prevailing just before and during the field survey. The results were tested by means of an autocorrelation statistical test (index I of Moran). Two sedimentological trends were recognized, i.e. development towards finer, better sorted and more negatively skewed sediment (FB–), and towards finer, better sorted and less negatively or more positively skewed sediment (FB+). Both vector fields were compared with results obtained from more classical approaches (sand tracers, microtopography and current measurements). This revealed that both trends can be considered as realistic, the FB+ trend being identified for the first time in a beach environment. The data demonstrate that, on the well-developed swash bar, sediment transported onshore becomes both finer and better sorted towards the coast. On the lower foreshore, which exhibits a steeper slope produced by breaking waves, the higherenergy processes winnow out finer particles and thereby produce negatively skewed grain-size distributions. The upper foreshore, which has a flatter and smoother slope, is controlled by lower-energy swash-backwash and overwash processes. As a result, the skewness of the grain-size distributions evolves towards less negative or more positive values. The skewness parameter appears to be distributed as a function of the beach slope and, thus, reflects variations in hydrodynamic energy. This has novel implications for coastal management

Reply to J.J. Muñoz-Perez et al. Comments on “Confirmation of beach accretion by grain-size trend analysis: Camposoto beach, Cádiz, SWSpain” by E. Poizot et al. (2013) Geo-Marine Letters 33(4)

In a novel finding for a beach environment, Poizot et al. (2013) identified an FB+ trend (sediments becoming finer, better sorted and more positively skewed upshore) on a well-developed swash bar on the upper foreshore of the Camposoto beach of Cádiz in SW Spain. In their Discussion of that paper, Muñoz-Perez et al. (2014) provide some supporting arguments and also report grain-size, beach profile and other data from nearby beaches which differ from those of Poizot and colleagues for Camposoto beach, pointing out that a trend observed on one beach may not apply to a neighbouring beach. However, even though the absolute values differ, the overall trends actually do show the same general behaviour. In our Reply to their comments, we also address some difficulties in comparing granulometric datasets generated by different analytical techniques

Swell impact on reef sedimentary processes: A case study of the La Reunion fringing reef

Two surface-sediment sampling campaigns were carried out in November and December 2003, before and after a strong swell event, in the back-reef area of a microtidal fringing reef on the western coast of La Reunion, Indian Ocean. The spatial distributions of the mean grain size, sorting and skewness parameters are determined, and grain-size trend analysis is performed to estimate the main sediment transport pathways in the reef. The results of this analysis are compared with hydrodynamic records obtained in the same reef area during fair weather conditions and during swell events. Sediment dynamics inferred from the hydrodynamic records show that significant sediment erosion and transport occur only during swell events and under strongly agitated sea states. Under normal wave conditions, there is a potential for onshore sediment transport from the reef-flat to the back-reef, but this transport is episodic and occurs principally during high-tide stages. Sediment transport trends revealed by the grain-size trend analysis method show onshore and alongshore low-energy transport processes that are in agreement with the hydrodynamic records. The grain-size trend analysis method also provides evidence of an offshore high-energy transport trend that could be interpreted as a real physical process associated with return flow from the shore to the reef. The impact of swell on the reef sediment dynamics is clearly demonstrated by onshore and alongshore transport. Considering different combinations of the vector transport trends computed through the grain-size trend analysis approach, more realistic and pertinent results can be obtained by applying an exclusive OR operation (XOR case) on the vectors. The main results presented here highlight a trend towards the accumulation of carbonate sands in the back-reef area of the fringing reef. These sediments can only be resuspended during extreme events such as storms or tropical cyclones

Impact of the blockage ratio on the transport of sediment in the presence of a hydrokinetic turbine: Numerical modeling of the interaction sediment and turbine

International audienceInteractions between hydrokinetic turbines and near scale and far scale bed sediment particles are considered a critical area of assessment; however, a limited number of research studies have been published to address this issue. The current paper explores the influence of the blockage ratio on the morphology evolution downstream from a hydrokinetic turbine. A modeling framework is derived to predict the significant transport induced by a turbine installed on the erodible fluvial sandy bed surface, such as the Eulerian multi-phase model for the sediment and the Blade Element Momentum Theory (BEMT) for the turbine, using the open source platform OpenFOAM. Two configurations of different blockage ratios are considered. The configurations show differences in the bed morphology and the sediment transport. It has been shown for both configurations that the scour capabilities are enhanced below the turbine due to the acceleration of the flow and increasing local shear stress on the sediment

High resolution characteristics of turbulence tied of a fish farm structure in a tidal environment

International audienceCharacterization of tidal flow variations and turbulence levels surrounding aquaculture infrastructures contribute to optimal fish production management and to a good environmental quality assessment. Such flow characterization was performed in Cherbourg Roadstead, France. An acoustic Doppler current profiler was used to measure flow velocities and a vertical microstructure profiler to assess turbulence levels and length scales in the vicinity of a commercial fish farm. Flow measurement results show that fish nets increase local turbulence level, impacting the flow and vorticity produced by the nets, contributing to more scouring of the ocean floor. A wavelet analysis allows to evaluate vortex sizes and where they are localized along the water column. Flow characterization shows these vortices are produced in the vicinity of fish nets and in their wake. Larger and more energetic turbulent structures are produced below aquaculture cages, which extend into the wake and onto the ocean floor

Numerical investigation of the local impact of hydrokinetic turbine on sediment transport—Comparison between two actuator models

International audienceHydrokinetic turbines interact with the dynamics of the sedimentary bottom at small and large scale. Despite the interest that the study of these interactions deserves, little research has been published in the field. In this paper, we investigate by numerical simulation the interaction between modeled hydrokinetic turbines and bed load under clear-water scour conditions. The mixture of water and sediment is accounted for by an Eulerian multiphase model developed in the open source platform OpenFOAM. The turbine blades are parameterized by two models: the Blade Element Method (BEM) and the Actuator Disc Theory (AD). A good agreement with measurements is obtained in the near wake. The effects of the two different turbine models on the bedload sediment transport and on the wake characteristics are then examined. The local impact of the turbine modeled by BEM is more pronounced on the bed than the one modeled by the AD. So, the BEM modeling is preferable to the AD one for local studies of the impact of a turbine on the bed morphology. However, combining BEM with the two-phase fluid-sediment is time consuming and make difficult the use of this method to study a farm of several stream turbines. In such a case the use of AD is still remains recommended for now. Moreover, more studies must be done at real scale as the turbine rotational speed is relatively low and could induced less impact of the swirl on the bottom shear stress

Les gradients géochimiques : une nouvelle approche géostatistique d'interprétation du transport sédimentaire

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Les gradients géochimiques : une nouvelle approche géostatistique d'interprétation du transport sédimentaire

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Caractérisation des sources et de la dynamique sédimentaires d'une zone intertidale (Baie de Tatihou – France)

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MSTA: a GIS tool for proxy analysis

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