2016 Volume 69 Issue 3 Pages 4-14
Pipe hydrophone is an acoustic sensor that is widely used to measure sediment discharge in mountainous rivers in Japan. Monitoring data has been dramatically increased in the last decade. However, because the sound pressure of small colliding grain is smaller than that of the noise due to water flow, there is a lower limit of detectable grain diameter. Also because some of sediment grains do not collide against the pipe, the sediment discharge may be underestimated. Here we carried out the flume experiments on the same installation condition with general field measurements and measured the sound pressure of the noise due to water flow and that of the colliding grain. Then we analyzed a lower limit of detectable grain diameter and estimated the collision ratio by the same method Uchida et al. (2015) proposed. The results of this study are as follows. (1) On the experimental flume because the magnitude of the noise due to water flow was dependent on the noise near the water surface, the noise due to water flow became larger with the water depth became more shallow and the turbulence of the water surface became larger. (2) Based on the result, we estimated that on individual collision in case the diameter was 2 mm detecting probability was around 40% and in case the diameter was 3 mm detecting probability was more than 90%. On collective collision in case the diameter was 1 mm detecting probability was around 30% and in case the diameter was 2 mm detecting probability was almost 100% (3) In case dimensionless tractive force took a value within the range of 0.1≦τ ＊＜0.3 and the collision ratio ranged between 0.3 to 0.7.
