Alexei Sentchev

University of the Littoral Opal Coast | ULCO · Département de Physique

Coastal water dynamics along the Vietnam Middle-South Coastal region (VMSC), part of the South China Sea, is highly complex with large spatio-temporal variability whose drivers are not yet well understood. For the first time, high-resolution surface current data from high-frequency radar (HFR) measurements were obtained in this region during the ea...

Transport and dispersion processes in the ocean are crucial, as they determine the lifetime and fate of biological and chemical quantities drifting with ocean currents. Due to the complexity of the coastal ocean environment, numerical circulation models have difficulties to accurately simulate highly turbulent flows and dispersion processes, especi...

Transport and dispersion processes in the ocean are crucial, as they determine the lifetime and fate of biological and chemical quantities drifting with ocean currents. Due to the complexity of the coastal ocean environment, numerical circulation models have difficulties to accurately simulate highly turbulent flows and dispersion processes, especi...

A method of assessing the mean eddy viscosity profile (EVP) in the sea surface boundary layer (SBL) under variable wind conditions is proposed. Performance of the method is tested using observations by an ADCP-equipped platform in the coastal environment of the northwestern Mediterranean Sea under variable (3–12 m s ⁻¹ ) wind conditions. EVP retrie...

This study focused on the detection of mesoscale meteorological phenomena, such as the nocturnal low-level jet (NLLJ) and sea breeze (SB), using automatic deterministic detection wavelet technique algorithms (HWTT and SWT) and the machine learning recurrent neural network (RNN) algorithm. The developed algorithms were applied for detection of NLLJ...

Particle pair statistics from synthetic drifter trajectories reconstructed from realistic, high-resolution numerical simulations (SYMPHONIE model) and HF radar velocity measurements are used to investigate the dispersion properties in the Gulf of Tonkin (GoT). This study takes an approach based on two-particles statistics providing the relative dis...

Reynolds stress anisotropy is estimated from the stress spheroids, based on 20 Hz ultrasonic anemometer measurements, performed in the coastal area of northern France, over a 1.5-year long period. Size and shape variation (i.e., prolate, oblate, disk, rod, etc.) of stress spheroids are used for the characterization of energy redistribution by turbu...

The nocturnal low-level jet (LLJ) and orographic (gravity) waves play an important role in the generation of turbulence and pollutant dispersion and can affect the energy production by wind turbines. Additionally, gravity waves have an influence on the local mixing and turbulence within the surface layer and the vertical flux of mass into the lower...

Two weeks of high-frequency radar measurements collected at the Alderney Race are compared with the results of a three-dimensional fully coupled wave–current model. Spatial current measurements are rare in this site, otherwise well investigated through modelling. Thus, the radar measurements offer a unique opportunity to examine the spatial reliabi...

The Alderney Race is assumed to have the largest tidal-stream energy potential in the north-western European coastal seas. Interaction of the powerful tidal stream with strong wind, high waves and irregular bathymetry creates hydrodynamic conditions of extreme complexity, with high levels of turbulence. A comprehensive dataset has been created to i...

The Alderney Race, located northwest of the Cotentin Peninsula (France), is a site with high tidal-stream energy potential. Circulation through the Alderney Race is complex, with current speed exceeding 3 m s ⁻¹ at neap tide. Towed acoustic Doppler current profiler (ADCP) measurements and static point velocity measurements were performed in July 20...

The Introduction presents motivations, significance and some key points of the research activities performed in the Alderney Race. This article is part of the theme issue ‘New insights on tidal dynamics and tidal energy harvesting in the Alderney Race’.

Sea breeze (SB) phenomena may strongly influence air quality and lead to important effects on human health. In order to study the impact of SB dynamics on the properties and toxicity of aerosols, an atmospheric mobile unit was deployed during a field campaign performed in an urbanized and industrialized coastal area in Northern France. This unit co...

An experiment was performed to study the power production by a Darrieus type turbine of the Dutch company Water2Energy in a tidal estuary. Advanced instrumentation packages, including mechanical sensors, acoustic Doppler current profiler (ADCP), and velocimeter (ADV), were implemented to measure the tidal current velocities in the approaching flow,...

Tidal circulation and tidal stream resource in Alderney Race (Raz Blanchard) were assessed by using a towed acoustic Doppler current profiler (ADCP) system and tidal modeling. Optimal Interpolation (OI) was applied to process the underway velocity measurements recorded at neap tide flood and ebb flow. The interpolation technique allows reconstructi...

There are now over 350 high frequency radar (HF radar) stations operating on the coast of 37 countries around the world that allow the mapping of ocean surface current. However, observation from HF radars are often interrupted (loss of data) in both space and time due to signal inference, backscatters, ocean state.Therefore, in this study, we will...

The tidal circulation in Alderney Race (Raz Blanchard) is assessed by using a towed Acoustic Doppler Current Profiler (ADCP) system and modelling. The study site is assumed to have the largest tidal stream potential in France. Optimal Interpolation (OI) was applied to process the underway velocity measurements. The interpolation technique allows re...

Wave impact on tidal energy conversion

The strongest ocean currents occur in coastal regions and have tidal origin. In such regions, high current speeds are typically the result of topographic flow amplification. Despite their sparsity, these sites are optimal for installation of the tidal energy conversion devices, and, therefore, require special techniques for monitoring local current...

A vertical axis tidal turbine (VATT) of the Dutch company "Water2Energy", mounted on a surface platform , was tested during several weeks in real sea conditions in a tidal estuary-the Sea Scheldt, Belgium. Velocity measurements were performed to estimate the major turbulent properties of the tidal flow at the experimental site, to evaluate the tida...

The present study demonstrates the time-depth variation of mean current, turbulence intensities, Reynolds stresses and power densities at a 18-m depth tidal energy site in the Eastern English Channel. The variables were derived from two-week of high-frequency ADCP measurements, which covered calm and storm periods. Currents exhibits quasi semi-diur...

Ocean surface boundary layer dynamics off the southern coast of France in the NW Mediterranean is investigated by using velocity observations by high-frequency (HF) radars, surface drifting buoys and a downward-looking drifting acoustic Doppler current profiler (ADCP). The analysis confirms that velocities measured by HF radars correspond to those...

A series of tidal turbine tests were conducted in a tidal estuary (the Sea Scheldt, Belgium). Two in-stream vertical axis tidal turbines were tested in real conditions during several weeks in winter 2014 and late summer 2015. Tidal current velocity variations were continuously recorded by one ADCP, operating at 1 Hz and two ADV operating at 16 and...

A methodology of tidal flow resource assessment in the eastern English Channel is presented. The study is performed using surface velocity time series recorded by VHF radars and ADCP velocity measurements. The most energetic area is located west of Cape Gris Nez with the peak velocity of 2.5 m/s, mean velocity of 1 m/s, and spring tide average velo...

A methodology of tidal flow resource assessment in the Dover Strait is presented. The resource assessment is performed using surface velocity time series recorded by Very High Frequency radars (VHFR) and ADCP velocity measurements. Following the EMEC guideline, the major parameters of tidal flow conventionally used for tidal energy site screening a...

The tidal circulation in the semi-enclosed Boulogne harbour (eastern English Channel) is measured during the various stages of the tidal cycle with a low-cost towed Acoustic Doppler Current Profiler (ADCP) system for the first time. The system is equipped with an interpolation algorithm which allows reconstructing space-time evolution of the veloci...

A methodology of assessing the hydrokinetic resources and tidal flow potential at a near-shore site in the Iroise Sea is presented. More than a year-long surface current velocity time series recorded by high frequency radars (HFR) were used for site screening. We focused on the analysis of tidal current pattern around the Ushant Island which is a p...

A year-long surface current velocity time series recorded by High Frequency radars (HFR) are used for assessing the hydrokinetic resources at a near-shore site in the Iroise sea. A combination of remotely sensed surface velocities and ADCP velocity profiles recorded in radar coverage zone allowed assessing the tidal current variability and power de...

The impact of tidal current, waves, and turbulence on particles re-suspension over the sea bottom is studied through eulerian high frequency measurements of velocity and particle size distribution (PSD) during 5 tidal cycles (65 hours) in a coastal environment of the eastern English Channel. High frequency variability of PSD is observed along with...

The MARS-3D model in conjunction with the particle tracking module Ichthyop is used to study circulation and tracer dynamics under a variety of forcing conditions in the eastern English Channel, and in the Boulogne-sur-Mer harbour (referred to hereafter as BLH). Results of hydrodynamic modelling are validated against the tidal gauge data, VHF radar...

Tidal current structure and turbulent quantities within a tidal bottom boundary layer (BBL) have been examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument was deployed on the seafloor, off the north-eastern French coast in the eastern English Channel over 12 tidal cycles and covered the period of the transition...

Current structure and turbulent quantities within a tidal bottom boundary layer (BBL) have been examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument was deployed on the seafloor, off the north-eastern French coast in the eastern English Channel over 12 tidal cycles and covered the period of the transition from m...

In recent years, monitoring nearshore surface currents became an important application of the high-frequency radar (HFR) technology. The Doppler shifts of backscat- Tered radio signals from surface waves provide the surface velocity component in the direction of a HFR beam. These radial velocities observed by multiple (usually two) Radars have to b...

Currents effects on waves have led to many developments in numerical wave modeling over the past two decades, from numerical choices to parameterizations. The performance of numerical models in conditions with strong currents is reviewed here, and observed strong effects of opposed currents and modulations of wave heights by tidal currents in sever...

Wind and wave effects on tidal current structure and turbulence throughout the water column are examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument has been deployed on the seafloor of 18-m mean depth, off the north-eastern French coast in the eastern English Channel, over 12 tidal cycles, and covered the perio...

Time series observation of particle size distribution (LISST-100x type C)simultaneously with currentvelocity measurements have been recorded in the coastal waters of the English Channel (France). The objective of the present study is to consider the dynamics of each particle class size, in relation with velocity dynamics, and to characterize the ch...

Time series observation of particle size distribution (LISST-100x type C)simultaneously with currentvelocity measurements have been recorded in the coastal waters of the English Channel (France). The objective of the present study is to consider the dynamics of each particle class size, in relation with velocity dynamics, and to characterize the ch...

This study describes physical processes (mainly the turbulence and re-suspension of particles due to turbulence) which control the micro scale variability of the bio-optical properties in highly turbid coastal waters. Time series analyses of different bio-optical and physical properties (temperature, salinity) have been performed from a boat in coa...

Correlation functions associated with the inverse covariances represented by polynomials of the homogeneous diffusion operator D are obtained analytically for an arbitrary polynomial of D, constrained by the positive-definiteness condition of the covariance operator. Copyright © 2012 Royal Meteorological Society

Wind and wave effects on tidal current structure and turbulence throughout the water column are examined using an upward-looking acoustic Doppler current profiler (ADCP). The instrument has been deployed on the seafloor of 20-m depth, off the North-Eastern French coast in the Eastern English Channel over 12 tidal cycles and covered the period of th...

The data from two high-frequency radars (HFR) operating in the Iroise Sea are re-processed by applying an improved version of the direction finding algorithm, removing wave-induced surface currents and the variational interpolation on a regular grid. Combining these processing techniques allowed reconstruction of the surface currents at a level of...

A technique for reconstruction of the 2d surface velocity field from radar observations is proposed. The method consecutively employs two processing techniques: At the first stage raw radial velocity data are subject to EOF analysis, which enables to fill gaps in observations and provides estimates of the noise level and integral parameters charact...

A high�frequency (1.2 MHz) four�beam Acoustic Doppler Current Profiler (ADCP) moored on the sea bottom was used for the direct measurements of the turbulence parameters in the shallow (20 m) coastal zone of the eastern English Channel. The measurements were as long as four tidal cycles during the period of the spring tide development. The measureme...

Abstract—A high�frequency (1.2 MHz) four-beam Acoustic Doppler Current Profiler (ADCP) moored on the sea bottom was used for the direct measurements of the turbulence parameters in the shallow (20 m) coastal zone of the eastern English Channel. The measurements were as long as four tidal cycles during the period of the spring tide development. The...

The use of high frequency radar (HFR) systems for near-real-time coastal ocean monitoring necessities that short time scale motions of the radar-derived velocities are better understood. While the ocean radar systems are able to describe coastal flow patterns with unprecedented details, the data they produce are often too sparse or gappy for applic...

A high-frequency (1.2 MHz) four-beam Acoustic Doppler Current Profiler (ADCP) moored on the seabed has been used direct measurement turbulence in a shallow coastal zone of the eastern English Channel over four tidal cycles at period of development of the spring tide. As measurements and estimations have shown, the Reynolds stresses were found to va...

High-frequency radars measure projections of surface velocity vectors on the directions of the radar beams. A variational method for reconstruction of the 2d velocity field from such observations is proposed. The interpolation problem is regularized by penalizing high-frequency variability of the surface vorticity and divergence fields. Twin-data e...

Two very high-frequency radars (VHFRs), operating in the southern Channel Isles region (English Channel) in February–March 2003, provided a continuous 27-day long dataset of surface currents at 2 km resolution over an area extending approximately 20 km offshore. The tidal range in the region of study is one of the highest in the world and the coast...

Using a combined model that couples a three-dimensional ocean circulation model, a model for tidal currents, and a model for particle transport, the structure of the velocity field of the tidal current and the transport of particles migrating over the vertical were studied in the zone of the influence of the riverine runoff in the eastern part of t...

Two very high-frequency radars (VHFR) operating on the Opal coast of eastern English Channel provided a nearly continuous 35-day long dataset of surface currents over a 500km2 area at 0.6–1.8km resolution. Argo drifter tracking and CTD soundings complemented the VHFR observations, which extended approximately 25km offshore. The radar data resolve t...

The Princeton Ocean Model coupled with a particle-tracking module is used to assess the effects of physical forcing and vertical migratory behaviour on the transport and dispersion of larvae in the region of freshwater influence (ROFI) of the eastern English Channel (EEC). Results of numerical modelling are compared with the observed concentrations...

High frequency (HF) radar surface velocities and coastal tidal gauge data are assimilated into a two-dimensional (2D) finite element spectral in time model to describe the Stokes drift of the major tidal constituents in the English Channel. The numerical model was fit to observations by optimizing the unknown sea surface height at the open boundari...

A random-walk Lagrangian particle-tracking model is applied to study horizontal mixing processes in the ROFI system of the eastern English Channel. A particle-tracking model is incorporated within the three-dimensional hydrodynamic model, POM, which is run under realistic tidal forcing and freshwater input. Our computations based on both the Lagran...

We study how the combination of tides and freshwater buoyancy affects the marine organisms accumulation and horizontal transport in the ROFI system of the eastern English Channel. The Princeton Ocean Model coupled with a particle-tracking module is used to study the migration of fish eggs and larvae under different forcing conditions. Results of mo...

A comprehensive model based on the three-dimensional hydrodynamic model (Princeton Ocean Model), barotropic tide model, and particle transport model was developed and used for a detailed description of the structure of the geophysical fields in the strait, which are formed under the influence of the interaction between the tidal currents, riverine...

Concentrations of fish larvae observed in spring 1995 are reviewed. The results show an accumulation of larvae in the vicinity of the French coast and also an alongshore migration. We apply the Princeton Ocean Model coupled with a particle tracking model to study the migration of fish eggs and larvae under different forcing conditions. Results are...

The M2 tidal component of the flow in the Dover Straits is reconstructed using a natural combination of two independent data sources: HF Ocean Surface Current Radar (HF OSCR) system and coastal tidal measurements. The method used is the variational data assimilation technique into a quasi-linearized finite element tidal model. The model uses triang...

The major difficulty faced by numerical models of tidal dynamics in the limited-area shelf basins concerns the treatment of open boundaries. We show that effective control of the open boundary conditions in a depth-averaged numerical model can be achieved by assimilation of data from coastal tide gauges in the interior of the basin, distant from it...

METHOD High-frequency radars measure projections of the surface velocity vectors on the directions r of the radar beams. We consider an interpolation method for reconstruction of the 2d velocity field v(x,y) from such observations. The technique is based on constrained minimization of the cost function Here v * are the observed radial velocities, σ...