There have been many previous experiments on sediment transport process in mountainous rivers. To measure the sediment discharge in a river, it is hard to catch and weigh transported sediment directly and consecutively. Therefore many indirectly measuring methods have been discussed and proposed. Especially, hydrophone fixed on river bed, and method using acoustic signal of hydrophone with sediment particles collision are widely used. However, specific and versatile method to analyze sediment discharge has not been developed. In order to develop them, it is necessary to make clear the relationship between amplitude of the hydrophone acoustic signal and the momentum of a sediment particle. The authors conducted a series of experiments on acoustic characteristics of sediment particles collisions with a hydrophone. Experiments results are as follows. (1) Amplitude of circumferential mode is linear to almost0.7powered of the momentum of moving particle, and amplitude of radial mode is linear to almost2.1powered of the momentum of moving particle. Therefore when only one amplification step is available, circumferential mode would be better. (2)Momentum of moving particle strongly receives influence of the movement speed of the particle. Moreover, the proportionality relation of particle diameter and amplitude is reversed above a certain domain. (3) The particle size distribution width measurable with one hydrophone was approximately13times. (4) Time duration of reverberant sound indicates that detection limit particle number was31sec]1. Detection rate was high when number of collision particle is smaller than31sec]1, and detection rate decreased rapidly when number of collision particle is larger. Therefore, it is necessary to consider the particle diameter when setting hydrophone pipe length. (5) Applying several hydrophones with different characteristic(e.g. fixation method, pipe thickness), wide particle size distribution width will be measurable. Furthermore, this method can cancel the relation of particle diameter and amplitude reverse.

Key wordsFhydrophone, measurement of sediment discharge, natural resonance modes