Hydrophone has been used to measure bedload transport intensity by counting the number of pulse made by hitting of sand and gravels to a steel pipe. However, this method has some problems, for example, when the sediment rate is high, the sound level is continuously high, and thus, the number of pulses decreases or becomes zero. This problem can be eliminated by recording and integrating the sound pressure. Results of flume experiments carried out using glass beads have confirmed the existence of a relationship between the sediment rate and sound pressure. When the number of beads is small, the sound pressure is proportional to the sediment rate. As the number of beads increases, the detection rate, which is the ratio of the experimental value of the sound pressure to the anticipated value calculated with the proportional relation, decreases due to the influence of the interference of sound waves. These results agree well with those of a numerical simulation carried out using the wave data of the individual collisions. These results suggest that the transformed data whose sediment rate is equal to the integral multiple of that of the original data can be calculated by dividing and superposing the original data. In this study, we propose a method involving the use of the ratio between the detection rate corresponding to the transformed data and original data to calculate the detection rate and number of beads. We thus compute the sediment rate and average grain diameter.

Key wordsFbedload measurement, hydrophone, sound pressure, interference of sound waves