Author(s): Lorenzo Boisson; Celine Berni; Sebastien Proust; Benoit Camenen

Linked Author(s): Sébastien Proust, Benoît Camenen

Keywords: Sediment suspension; Deposits; Turbulent structures; Overbank flows

Abstract: Fines sediments represent a significant issue in Alpine rivers since it can raise both physical and ecological impacts. Indeed, large sedimentation in reservoir affects hydropower dam efficiency; also clogging and deposits overs a gravel matrix reduce the habitat quality. The objective of this work is to better understand fine sediment dynamics over gravel bars through laboratory experiments. Open-channel flows were investigated in an 18 m long and 2 m wide laboratory flume with an asymmetrical compound cross-section. The latter is composed of a rectangular Main Channel (MC) with hydraulically-smooth bed and sidewalls (made of glass) and one adjacent Flood Plain (FP, corresponding to a gravel bar) with a rough bed-surface (dense artificial grass simulating a meadow). At the flume entrance, a flow rate Q is injected in the MC larger than the capacity of the channel, resulting in a transverse flow from the MC to the FP. Two Q-values were tested (80 and 105 L/s) to change the magnitude of the FP water depth to MC water depth ratio (termed relative depth), i.e. the magnitude of overbank flow. First, water level profiles and flow structure (time-averaged velocity, turbulence statistics, and large-scale Coherent Structures (CSs)) were studied for a clear water case. Water level was measured using ultrasonic sound probe and velocity fluctuations were measured using an acoustic Doppler Velocimeter with a side-looking probe. Characteristic length-scales (in the streamwise and spanwise directions) of the CSs were measured based on two-point velocity measurements using two ADV probes. Second, similar experiments were made with water loaded with fine sediments (40 μm median diameter), the upstream concentration being close to 1.5 g/L, along with the transfer and deposits of fine sediments over the FP. The comparison of two test cases with same flow rate Q, one with clear water, the other with fine sediments, firstly enabled to assess the main factors controlling the transfer and deposits of fine sediments of: (i) the time-averaged transverse flow (from MC to FP); and (ii) quasi-two-dimensional CSs that form at the interfacial region between MC and FP. The second aim of the study was to analyse the retroactive effects of sediment deposition on the hydrodynamics of overbank flows. We analysed in particular the effects of the deposits on the streamwise changes in water level, on the spanwise profiles of the time-averaged velocity and turbulence statistics, and on the streamwise and spanwise length-scales of the CSs. The results show there is an interaction between fine sediment dynamics and flow characteristics.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022703

Year: 2022