Passive acoustic measurement of bedload grain size distribution using self-generated noise

Teodor Petrut,Thomas Geay,Sebastien Zanker,Philippe Belleudy,Cédric Gervaise

doi:10.5194/hess-22-767-2018

Abstract

Abstract. Monitoring sediment transport processes in rivers is of particular interest to engineers and scientists to assess the stability of rivers and hydraulic structures. Various methods for sediment transport process description were proposed using conventional or surrogate measurement techniques. This paper addresses the topic of the passive acoustic monitoring of bedload transport in rivers and especially the estimation of the bedload grain size distribution from self-generated noise. It discusses the feasibility of linking the acoustic signal spectrum shape to bedload grain sizes involved in elastic impacts with the river bed treated as a massive slab. Bedload grain size distribution is estimated by a regularized algebraic inversion scheme fed with the power spectrum density of river noise estimated from one hydrophone. The inversion methodology relies upon a physical model that predicts the acoustic field generated by the collision between rigid bodies. Here we proposed an analytic model of the acoustic energy spectrum generated by the impacts between a sphere and a slab. The proposed model computes the power spectral density of bedload noise using a linear system of analytic energy spectra weighted by the grain size distribution. The algebraic system of equations is then solved by least square optimization and solution regularization methods. The result of inversion leads directly to the estimation of the bedload grain size distribution. The inversion method was applied to real acoustic data from passive acoustics experiments realized on the Isère River, in France. The inversion of in situ measured spectra reveals good estimations of grain size distribution, fairly close to what was estimated by physical sampling instruments. These results illustrate the potential of the hydrophone technique to be used as a standalone method that could ensure high spatial and temporal resolution measurements for sediment transport in rivers.

Highlights

Sediment transport analyses in river catchments are one of the key activities stipulated by the European water framework directive (European Commission, 2001) and applied in French environmental policies
Bedload transport models are highly sensitive to incipient motion, which is directly related to river bed grain size distribution (GSD)
A new strategy has been presented for data processing on hydrophone measurements for monitoring the bedload GSD in a gravel river bed

Summary

Introduction

Sediment transport analyses in river catchments are one of the key activities stipulated by the European water framework directive (European Commission, 2001) and applied in French environmental policies. Bedload transport models are highly sensitive to incipient motion, which is directly related to river bed grain size distribution (GSD). Bedload GSD is linked to both surface and substrate GSD. In his paper, Parker (1990) constructed a two-size fraction transport model, assuming that the bedload GSD is identical to substrate GSD, for stable armored river beds, and becomes identical to surface GSD whenever the armor is destroyed. The development of surface-based and mixed-size transport models has received considerable attention (Heimann et al, 2015; Kuhnle, 1993; Parker, 1990; Recking, 2016; Wilcock and Kenworthy, 2002; Wilcock and McArdell, 1993). Knowing the bedload GSD solves the problem of initiation of motion and, en-

Results

Discussion

Conclusion