Acoustic wave propagation in rivers: an experimental study

Abstract

Abstract. This research has been conducted to develop the use of\npassive acoustic monitoring (PAM) in rivers, a surrogate method for bedload\nmonitoring. PAM consists in measuring the underwater noise naturally\ngenerated by bedload particles when impacting the river bed. Monitored\nbedload acoustic signals depend on bedload characteristics (e.g., grain size\ndistribution, fluxes) but are also affected by the environment in which the\nacoustic waves are propagated. This study focuses on the determination of\npropagation effects in rivers. An experimental approach has been conducted\nin several streams to estimate acoustic propagation laws in field\nconditions. It is found that acoustic waves are differently propagated\naccording to their frequency. As reported in other studies, acoustic waves\nare affected by the existence of a cutoff frequency in the kilohertz region. This cutoff frequency is inversely proportional to the water depth: larger water depth enables a better propagation of the acoustic waves at low frequency.\nAbove the cutoff frequency, attenuation coefficients are found to increase\nlinearly with frequency. The power of bedload sounds is more attenuated at\nhigher frequencies than at low frequencies, which means that, above the\ncutoff frequency, sounds of big particles are better propagated than sounds\nof small particles. Finally, it is observed that attenuation coefficients\nare variable within 2 orders of magnitude from one river to another.\nAttenuation coefficients are compared to several characteristics of the\nriver (e.g., bed slope, surface grain size). It is found that acoustic waves\nare better propagated in rivers characterized by smaller bed slopes. Bed\nroughness and the presence of air bubbles in the water column are suspected\nto constrain the attenuation of acoustic wave in rivers.