Downstream hydrosedimentary impacts of dam flushing : experimental and numerical modelling
(2024)
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- Authors
- Supervisors
- Soares-Frazão, Sandra
- Abstract
- For centuries, dams have been used to manage water resources for production and protection against climatic hazards. However, the construction and operation of dams have significant socioecological impacts, both upstream and downstream. Sediment continuity is disrupted, thereby altering the biogeochemical and morphological characteristics of the downstream reach. Upstream, this disruption manifests itself mainly through reservoir sedimentation, which gradually reduces its capacity. To ensure the functional stability of a dam and its reservoir, it is necessary to mitigate sedimentation through, for instance, dam flushing operations. Although these have proved effective, they can also have harmful ecological consequences, particularly through morphological changes and overly intense concentrations sent downstream. However, the downstream impacts of dam flushing have not been explored to the same degree as reservoir sedimentation or flushing efficiency by the scientific community. This thesis therefore aims to foster scientific knowledge about the downstream hydromorphodynamics of dam flushing. The first part of this thesis dealt with an experimental study of dam flushing under controlled laboratory conditions. To achieve this, a Laser Profilometry Technique (LPT) was developed to study morphological changes during transient flows. By focusing on some profiles that are representative of the various morphodynamical processes, this cheap, flexible, and easy-to-use technique makes it possible to accurately monitor the development of the bed's morphological characteristics. This technique was applied to dam flushing experiments with a variable initial depth of the sediment deposit upstream of the dam. The hydromorphodynamical processes specific to the pressure and drawdown flushing phases were observed in the immediate vicinity of the dam. Water levels, morphological changes, and concentrations were measured. In particular, a new calibration method for calculating concentrations using Ultrasonic Velocity Profilers (UVPs) was developed. The second part of this thesis aimed to develop a numerical tool suitable for simulating highly laden transient flows such as dam flushes. It appears the use of simple models, which consider that sediment transport occurs exclusively at the bed level, is not appropriate for studying these flows. A Two-Phase/Two-Layer (2P2L) model was developed in order to adequately manage the inertia of the liquid and solid phases, while considering the vertical distribution of concentration. However, this complex model is based on many parameters for which closure expressions exist, except for two interparticle friction coefficients. Usually, such unclosed coefficients are tuned by trial and error, based on calibration data that must be available for the considered application. To eliminate the need to calibrate these coefficients each time the 2P2L model is used, a methodology was developed to enable users to develop their own closure expressions. This methodology is based on machine learning, which uses the results obtained by applying the 2P2L model to a numerical experiment for which an analytical solution exists. By implementing the closure expressions in the 2P2L model, good results were obtained for highly laden transient flows, without going through a time-consuming trial and error calibration. Finally, a promising first application of the 2P2L model to real flushes performed on a French river paved the way for future prospective studies investigating the downstream hydrosedimentary impacts of dam flushing.
- Affiliations
Citations
Meurice, R. (2024). Downstream hydrosedimentary impacts of dam flushing : experimental and numerical modelling. https://hdl.handle.net/2078.5/217732