The combined measurement of velocity and acoustic turbidity allows the study of sediment transport on site.

River flow management (navigation, energy production, biodiversity…) needs a good understanding of sediment transport processes. Ubertone's UB-Flow F156 profiler allows the user to accurately observe the velocity profile near the stream bed. This instantaneous profile gives access to the shear stress as well as the turbulent intensity contributing to sediment transport. Moreover, the combined measurement of the acoustic turbidity profile allows to follow, in real-time, the evolution of the suspended sediment concentration (SSC).

 

Measurements have been conducted with our UB-Flow F156 on the Couesnon River in the bay of Mont Saint-Michel. The river is flushed on a daily basis in order to remove the sediments around the rock. Ubertone's device has been installed to observe the velocity profile near the bed of the Couesnon. The results we obtained at the beginning of the process are displayed in the graph below. These graphs show the combined evolution of the velocity and the acoustic turbidity which is proportional to the sediment's concentration.

Analysis of the data obtained by UB-Flow F156 on the Couesnon river at the beginning of the flush

These graphs display the data analysis in four different quadrants :

2a - velocity over time. Coherent structures can be observed in this graph.

2b- acoustic turbidity through time. The river-bed appears as a peak in which the position changes with the bed form. Here we can observe the progressive increase of the concentration due to the suspension of the particles.

2c - mean velocity +/- standard deviation. The standard deviation is directly linked to the turbulence.

2d - velocity and acoustic turbidity over time in a cell at 50 mm over the bed. The instantaneous variations are due to turbulence. This figure shows how the sediments are suspended beyond a velocity threshold.

La gestion des cours d'eau (navigation, production d'énergie, biodiversité) nécessite une bonne compréhension des processus de transport solide en rivière. L'utilisation du profileur UB-Flow F156 permet d'observer finement le profil de vitesse près du fond. Ce profil instantané donne accès à la contrainte de cisaillement ainsi qu'à l'intensité turbulente contribuant au transport solide. Par ailleurs, la mesure conjointe du profil de turbidité acoustique permet de suivre en temps réel l'évolution de la concentration de sédiments en suspension.

Des mesures ont été réalisées avec l'UB-Flow F156 sur la rivière du Couesnon en baie du Mont Saint Michel (photographie ci-contre). Cette rivière subit quotidiennement des chasses destinées à désensabler le mont afin de lui rendre son caractère insulaire. L'appareil a été installé de manière à observer le profil près du fond. L'illustration ci-dessous présente les résultats obtenus au début de la chasse. On y distingue nettement l'évolution conjointe de la vitesse et de la turbidité acoustique (proportionnelle à la concentration).