I have a formation in geophysics (fluid mechanics, hydraulics, hydrology and geomorphology).
Modelling fundamental geomorphic processes has captured the attention of scientists for many decades. In particular, researchers and engineers have investigated flows above an erodible bed in order to improve bed load transport predictions in turbulent streams.
In Switzerland, this phenomenon is responsible for filling dams and reducing their profitability. This research theme is also fundamental for sustainable land use management since the shape of rivers are conditioned by the interactions between sediments and water flows.
Despite all these scientific efforts, these interactions are still an intricate problem. In most approaches, bed load flux predictions are given by empirical formulae giving one flux for one water discharge. These formulae do not comply with observations of large fluctuations in sediment flux under steady flow conditions in both mountain streams and laboratory flumes.
Many scientists have tried to understand the origins of these fluctuations. Some of them attempted to model this system from fundamental equations in order to detect the growth of instabilities. While this strategy has given clues on the origins of bed forms and their migration, they are still not able to predict the fluctuations in bed load transport.
Another strategy consists in considering that the effects of instabilities are too chaotic to be predicted and that the fluctuations have to be described as a function of probabilistic input, output, deposition and entrainment in a specific window. This statistical approach uses a stochastic model to represent the observable fluctuations. This model aims at giving the probability distribution function of bed load flux for different types of flows and beds.
Looking closer at this shallow-water/granular-bed system, this research proposal aims to test and improve the stochastic approach in the context of mountain rivers.
Experiments will be conducted 2-D flume, 2 to 5% inclined and with transport conditions close to the incipient motion. Under these conditions, the developed bed forms are anti-dunes for which the surface gravity waves are in phase with the bed profile. To extract information from this system, various measurements will be combined. Particle Tracking to follow the grain dynamics, Particle Image Velocimetry to follow the hydrodynamics, and live video treatment to record bed load flux will be used. We will focus on the dependence of the fluctuations in bed load transport flux and the anti-dune geometry with different parameters (water discharge, mean bed load rate, grain diameter, Froud number, Shields number).
Based on the collected data, the stochastic approach will be evaluated and corrections that account for the periodic fluctuations of bed load transport due to the migrating bed forms will be introduced in the model.
|Gauthier Rousseau, Angeliki Sklivaniti, Daniel Vito Papa, and Christophe Ancey
Talk presented at EGU 2017
|The role of the hyporheic flow on sediment transport processes : an experimental approach using particle image velocimetry|