Model and method to predict the turbulent kinetic energy induced by tidal currents, application to the wave-induced turbulence

A prediction model for the turbulent kinetic energy (TKE) induced by tidal-currents is proposed as a function of the barotropic velocity only, along with a robust method evaluating the different parameters involved using Acoustic Doppler Current Profiler (ADCP) measurements from Alderney Race. We find that the model is able to reproduce correctly the TKE profiles with coefficients of correlation on average higher than 0.90 and normalised root-mean-square errors (NRMSE) less than 14%. Different profiles are also tested for the mean velocity, no satisfactory prediction model is found but we are able to have decent estimates of the velocity shear and friction velocity. Two applications are then carried out. First the turbulent budget terms are estimated and discussed. We identify the turbulent production and dissipation of TKE as the most important mechanisms, then we discuss the validity of several theoretical results derived for isotropic turbulence for this application. A strong departure for the estimation of the turbulent dissipation is notably found and explained by the turbulent anisotropy. At last the prediction model for the TKE is used to infer the wave-induced TKE. We show the importance of removing the tidal component, waves can have a strong influence down to mid-depth

Propriétés hydrodynamiques du mascaret et de son influence sur la dynamique sédimentaire. Une approche couplée en canal et in situ (estuaire de la Sée, Baie du Mont Saint Michel)

The scientific aim of this thesis is to characterize the influence of tidal bore on hydrodynamic, suspended sediment and sedimentary deposits. A double approach has been developed, coupling a study of tidal bores in the Mont-Saint-Michel Bay (MSM) and physical simulation in flume. This work highlights variations of turbulent and viscous stresses associated with significant deceleration phases after the passage of the bore. Our study has remedied the scientific lock to perform measurements in turbulent and highly concentrated flow thanks to a methodological study and to the development of a new sampling tool. Direct measurements in the water column were used to quantify sediment transport crossing the tidal bore and to propose a new model of suspended sediment concentration (SSC) evolution (short and long term) in an estuary submitted to a tidal bore. The model displays a peak of SSC below the front of the bore and a zone of maximum turbidity, never described previously in the bay of MSM. The transport capacity of the tidal bore is confirmed by the wide range of MES grain-size (up to fine sands). The processes involved in erosion and deformation of sedimentary deposits are described (liquefaction and settlement) and highlighted by the results on hydrodynamic processes.L'objectif scientifique de cette thèse est de caractériser l'influence du mascaret sur l'écoulement, sur les sédiments en suspension et sur les dépôts sédimentaires. Une double approche a été menée couplant une étude du mascaret dans la baie du Mont-Saint-Michel (MSM) et des modélisations en canal. Les résultats mettent en évidence des variations de contraintes turbulentes et visqueuses associées à des phases de décélérations importantes après le passage du mascaret. Grâce à des développements méthodologiques et instrumentaux (calibrations, prélèvements), notre étude a pallié au problème de mesures dans des écoulements chargés et turbulents. Des mesures directes dans la colonne d'eau ont permis de quantifier le transport sédimentaire au passage du mascaret et de proposer un nouveau modèle d'évolution des concentrations en matières en suspension (MES) dans un estuaire soumis à un mascaret (à court et long terme). Ce modèle met en évidence un pic de MES au passage du front et une zone de turbidité maximum encore jamais décrite dans la baie du MSM. La capacité de transport de cet écoulement est confirmée par la gamme étendue de la granulométrie des MES (jusqu'à des sables fins). Les processus impliqués dans l'érosion et la déformation des dépôts sédimentaires sont décrits (liquéfaction et tassement) et expliqués à l'aide des résultats acquis sur l'hydrodynamique du phénomène

Sedimentary signatures of tidal bores: a brief synthesis

(IF 1.73; Q3)International audienceThis article aims at presenting a brief synthesis of sedimentary signatures assigned to tidal bore dynamics and impacts. According to the few studies published until now on tidal bore-induced facies within inner estuarine tidal channel infilling successions, only two major signatures can be reported: (1) soft sediment deformations (SSDs) due to overpressure linked to sudden water level elevation, high shear stress and vertical velocity acceleration below the tidal bore front and secondary waves; SSDs may be present throughout the channel infill succession, with the general exception of the uppermost part; tidal bore-induced SSDs have been described only in modern facies; (2) tidal bore couplets (TBCs) formed by an erosional surface overlain by massive sand drapes, related to the reworking of the sediment bottom during tidal bore passage; TBCs were first described in the ancient record. Studies in modern estuaries demonstrate that TBCs evolve towards tidal bore sequences from the tidal channel bottom (subtidal to low intertidal facies) to tidal channel bank (low to mid intertidal facies). In mid to upper intertidal facies, the occurrence of thicker-than-average tidal rhythmites, reflecting higher-than-average suspended sediment concentrations, are also considered as a possible signature of tidal bore dynamics

Suspended sediment concentration in relation to the passage of a tidal bore (See River estuary, Mont Saint Michel Bay, NW France)

International audienc

Processes involved in the formation of soft sediment deformations related to tidal bore passage.

International audienc

Complex current variability in a tide-dominated environment and interplay with extreme wind and wave events (Raz Blanchard, NW France)

International audienc

Morphodynamic of a sandy-muddy macrotidal estuarine beach under contrasted energy conditions (Vilaine estuary, France)

International audienceEstuarine and bay beaches are important areas for human activities. These beaches are variably affected by tides, waves, and winds that can commonly generate marked topographic and sedimentological contrasts. Betahon beach (South Brittany, France) is an intermediate-type beach exhibiting a low-tide terrace linked to a steeply sloping gravelly-sandy upper foreshore, and separated by a shore-parallel bluff from a mudflat on the lower foreshore. The beach exhibits linear ridge and runnel (R-R) bedforms perpendicular to the shoreline. Seasonal monitoring of the beach shows mudflat accretion by fluid mud deposition and erosion of R-R bedforms. A core obtained from the mudflat shows alternations of mud and sand. In order to understand the cross-shore dynamics of the beach, topographic surveys and wave and current monitoring were carried out during two contrasting energy conditions. Bed return flows occurred during high-energy events, inducing an infill of runnels by non-cohesive fine sediments and coarser sediments from the reflective upper beach. During low-energy conditions, a longshore flow channel was identified between the shore-parallel low-tide terrace bluff and the mudflat. Throughout the tide, onshore currents prevailed over the mudflat, inducing the filling of runnels and the base of the bluff with fluid mud

Influence of the tidal bore on sediment transport in the Mont-Saint-Michel estuary, NW France

The Mont Saint Michel estuary is a megatidal setting (tidal range up to 14 m). It is characterized by a strong tidal asymmetry during spring tides, with the flood stage much shorter and quicker than the ebb, reaching commonly a velocity of 2m/s into the estuarine channels. In estuaries with tidal ranges greater than 6 m, the difference of elevation between the rising tide and the river creates a discontinuity of velocity and pressure, called tidal bore (or "mascaret" in French). Visually, a tidal bore can be described as a wave or series of waves propagating upstream. This study takes place into a national project "ANR Mascaret". Part of the field work we performed recently on the tidal bores that propagate into the Mt St Michel estuary, aims in studying the impact of the bore fluid dynamics on sediment transport. This is an important issue for a better understanding of the complex fluid-sediment interactions and for the operation of restoration of the Mont-Saint-Michel's maritime characte

Underway velocity measurements in Alderney Race: toward a 3D representation of tidal motions

International audienceAlderney Race is a highly energetic tidal site located northwest of the Cotentin Peninsular (France). Circulation through Alderney Race is complex, with current speed exceeding 3 m/s at neap tide. Underway ADCP measurements and static point velocity measurements were performed in July 2018 focusing on assessment of circulation and vertical structure of tidal currents. Transect surveys revealed peculiar features of local dynamics such as change in location of the tidal jet on ebb and flood flow. Spatial expanse of the tidal jet was quantified and regions with largely sheared or nearly homogeneous velocity distributions were identified on the cross-sections. Velocity profiles acquired along the cross- sections were accurately characterized using a power law. Spatial variability of the power law exponent was found to correlate with the tidal conditions. The largest variation in profile shape was observed in the northern sector and assumed to be generated by the current interaction with a bathymetric constriction. The velocity profiles were found to vary from highly sheared on flood flow to nearly homogeneous on ebb flow, with corresponding range of power law exponent α variation from 6 to 14. Over a relatively smooth bathymetry, the velocity profile shape was properly approximated using the 1/7 power law with a range of variation of α from 6.5 to 8, with respect to the tidal conditions. To our knowledge, this is the largest field survey done using towed ADCP and the results could represent a significant advance in tidal site characterization and provide Advanced information to turbine developers

Model and method to predict the turbulent kinetic energy induced by tidal currents, application to the wave-induced turbulence

A prediction model for the turbulent kinetic energy (TKE) induced by tidal-currents is proposed as a function of the barotropic velocity only, along with a robust method evaluating the different parameters involved using Acoustic Doppler Current Profiler (ADCP) measurements from Alderney Race. We find that the model is able to reproduce correctly the TKE profiles with coefficients of correlation on average higher than 0.90 and normalised root-mean-square errors (NRMSE) less than 14%. Different profiles are also tested for the mean velocity, no satisfactory prediction model is found but we are able to have decent estimates of the velocity shear and friction velocity. Two applications are then carried out. First the turbulent budget terms are estimated and discussed. We identify the turbulent production and dissipation of TKE as the most important mechanisms, then we discuss the validity of several theoretical results derived for isotropic turbulence for this application. A strong departure for the estimation of the turbulent dissipation is notably found and explained by the turbulent anisotropy. At last the prediction model for the TKE is used to infer the wave-induced TKE. We show the importance of removing the tidal component, waves can have a strong influence down to mid-depth