On bedload measurement performances of high-resolution acoustic (ACVP) and conductivity (CCP) profilers

Guillaume Fromant,David Hurther,Thibaut Revil-Baudard,Ryan S Mieras,Julien Chauchat,Jack A Puleo,A Paquier,N Rivière

doi:10.1051/e3sconf/20184004007

Abstract

Intense sediment transport experiments were performed in a gravity driven open-channel flow with two sizes of non-spherical acrylic particles of diameter dp=1.0mm and dp=3.0 mm, and a maximum packing volumetric concentration of 0.55 m3/m3. Two flow conditions were adapted to ensure the same sediment transport regime for particle sizes as sheet flows (Shields numbers above unity) with an equi-repartition of the total net sediment transport rate between the suspended load and the bedload (Suspension number around unity). An acoustic scattering-based system, the Acoustic Concentration and Velocity Profiler (ACVP) and electric conductivity probes, the Conductivity Concentration Profiler (CCP) with two different vertical resolutions of 1mm (CCP1mm) and 2 mm (CCP2mm),were used to measure time-resolved and averaged concentration profiles across the bed-load and suspension layers. A detailed comparative analysis of concentration and sheet flow layer thickness measurements obtained with the two systems across both the suspension and the bedload layers is presented. The capabilities and limitations of the two flow measurement technologies are outlined. Average sediment concentration profiles were overestimated by 10% with the ACVP in the dense sheet layer when ⟨φ(z)⟩ ≳ 0.35, and by 100% with the CCP in the more diluted region when ⟨φ(z)⟩ ≲ 0.015 and ⟨φ(z)⟩ ≲ 0.20 for CCP1mmand CCP2mm, respectively. Good agreement is found elsewhere between the three systems in terms of average and time-resolved concentration as well as bed level position and sheet flow layer thickness.

Highlights

Over the past two decades, considerable research efforts have been dedicated to the understanding and modeling of sediment transport processes under energetic flow forcing conditions for which bedload moves as a sheet flow
This paper reports on detailed measurements of particle concentration and velocity profiles under gravity-driven sheet flow currents involving two different measurement systems: an Acoustic Concentration and Velocity Profiler (ACVP) [9] and Conductivity Concentration Profilers (CCP) [10] with two vertical resolutions (1 mm versus 2 mm)
The main objective of this study is to evaluate the performances and limitations of the above-mentioned measurement techniques, the ACVP and the CCPs, in measuring time-resolved and average sediment concentration profiles over the entire sediment transport layer

Summary

Introduction

Over the past two decades, considerable research efforts have been dedicated to the understanding and modeling of sediment transport processes under energetic flow forcing conditions for which bedload moves as a sheet flow In this context, it is widely accepted that the geophysical community suffers from the lack of high-resolution flow measurement tools adapted to the study of sediment transport physics in energetic environmental flows [1], [2]. The limited measurement capabilities have limited advances in process-based modeling of sediment transport in contrast to empirical models commonly implemented in numerical engineering codes that are applied for long-term predictions of morphological evolution. Such predictions are known to be subject to large uncertainties and errors which is a main research concern in the context of adaptation to abrupt climate changes. Recently have advanced acoustic and conductivity techniques been developed [9], [10] and used to profile such complex particle flows at sufficiently high temporal (OO(0.1 s)) and spatial (OO(0.001 m)) rates to resolve the small turbulent flow scales involved in the benthic boundary layer sediment transport processes

Objectives

Results

Conclusion