Thierry Fretaud’s research while affiliated with Bibliothèque Nationale de France and other places

Hydraulic structures such as dams have a direct or indirect influence on the hydro-sedimentary functioning of rivers. They can impact the sediment continuity of the river and can create a sediment imbalance with zones of sediment accumulation upstream and a lack of sediment downstream from the dam. Upstream deposits are a major issue for managers as they can impact the operation and/oraffect the safety of the structure and induce extra-flood hazards. Operators therefore seek to determine the physical characteristics of these deposits to determine their remobilization potential.Usually, these sediment deposits are described using bathymetric data, sonar images or samples (dredging, coring). For this study, new measurements from two geophysical methods (acoustic and electromagnetic) were used in addition to these current monitoring methods. The contribution of geophysical techniques is undeniable to improve the characterization of these deposits. These measurements make it possible to: i) define the internal structures of the sediments, ii) determine the spatial distribution of these structures over the entire development, iii) evaluate the volumes deposited when little bathymetric data is available on the reservoir. The coupling of these methods thus makes it possible to reconstruct the evolution and the structuring events of the sedimentary deposits. This approach provides complementary and important elements for managers in terms of exploitation and optimization of sedimentary deposits management scenarios.

Sand dynamics is an important issue in harnessed gravel-bed rivers where sand deposits can locally impact river management for ecological or safety purposes. However, sand flux is very complex to evaluate continuously in such rivers because of the strong spatial and temporal variability of the sand concentration throughout a river cross-section and along the river, and also because of the supply-limited aspect of sand. Sand transport capacity formulas are not applicable for such rivers. This paper introduces some methods easy to apply and based on the concentration index, i.e. assuming a possible relationship between the sand concentration at a specific position of a river cross-section and the section averaged concentration. These methods that use regular pump samplings and turbidity measurements are applied on the Lower-Isère, France, downstream of a series of dams during a flushing event. During this 2 week-event, we estimated a sand flux between 1.3 and 1.7 Mt depending on the method and site used. The concentration index method appeared robust and so very useful for a continuous evaluation of sand fluxes but the index concentration must be measured at a location where the turbulence intensity is high enough so that sand suspension can be observed and it must validated with regular sand gaugings. Eventually, we showed that the sand supply allowed the system to reach its sand transport capacity for approximately 3 days after a delay of 2 days after dam openings.

Hydraulic structures such as dams have a direct or indirect influence on the hydro-sedimentary functioning of rivers. They can impact the sediment continuity of the river and can create a sediment imbalance with zones of sediment accumulation upstream and a lack of sediment downstream from the dam. Sediment deposits upstream dams are a major issue for managers as they can impact the operation and/or affect the safety of the structure and increase the flood risk. Operators therefore seek to determine the physical characteristics of these deposits to determine their remobilization potential. Usually, these sediment deposits are described using bathymetric data, sonar images or sedimentary information (dredging, coring). For this study, new measurements from two geophysical methods (acoustic and electromagnetic) were used in addition to these current monitoring methods. In addition, this method can be essential when historical data is not available. The contribution of geophysical techniques is undeniable to improve the characterization of these deposits. These measurements make it possible to: i) define the internal structures of the sediments, ii) determine the spatial distribution of these structures over the entire development, iii) evaluate the volumes deposited when little bathymetric data is available on the reservoir. The coupling of these methods thus makes it possible to reconstruct the evolution and the structuring events of the sedimentary deposits. This approach provides complementary and important information for stakeholders in terms of dam exploitation and management of sedimentary deposits.

Measurement of suspended sediment is an important issue for river management. Suspended particles can be mineral or organic and of natural or artificial origin. They have different sizes that are often described by a median diameter. To quantify the concentrations of suspended particles or their composition, samples are taken at one or more points of the cross section. These samples can be either instantaneous or averaged over time, at a single point, or integrated over depth. The choice of sampling technique should be adapted to the size of the river, the purpose of the study and the available samplers. This article presents some characteristics of the samplers currently used in the Rhône watershed and raises the question of the harmonisation of practices for suspended solids sampling in rivers. This harmonisation is necessary in order to be able to compare and discuss the calculated suspended sediment fluxes in order to understand and manage them.

Suspended Particulate Matter (SPM) measurements are a very important challenge of operational flow monitoring. The ANR project MESURE led to the development of a compact dual-frequency ABS prototype tested on a river. Following this research project, a compact commercial version was developed by Ubertone, composed of a hyperband acoustic module, and of a battery-wifi-logger module. In this paper, we present the deployment of this UB-SediFlow during sediment managing operations. The UB-Sediflow was installed on a floating board. In parallel, another team collected SPM reference samples to qualify UB-SediFlow. Post- processing analysis over a large frequency range gave quality data and this campaign showed an easy deployable instrument allowing real time data visualization.