A hydrophone was used to measure the intensity of transported bedload. In a past study, we proposed a method of analysis for use in field monitoring using sound pressure data obtained by a hydrophone and showed that bedload sediment discharge can be calculated quantitatively in flume experiments. In our design, the sampling frequency needed to be several times greater than the circumferential direction frequency. We therefore used a sampling frequency of approximately 20 times the value, in order to ensure a fair design. However, the large volume of sampling data is a disadvantage of our method. Therefore, in this study, the optimum sampling frequency and sampling time for a reduction in data volume were considered using field]monitoring data at Harukidaini sabo dam in the Haruki River within the Fujigawa River System. If the sampling frequency was around four times the circumferential direction frequency, the accuracy of the analysis remained nearly constant; however, for less values of sampling frequency, the analysis results were entirely different. Therefore, a sampling frequency of around four times the circumferential direction frequency provides an adequate amount of data. We also found that the analysis accuracy remained nearly steady if the sampling time was changed from 0.5 to 5 s. However, since the issue of sampling time depends on the temporal stability of the bedload, the setting of sampling time requires consideration in other observing sites. This is because the temporal stability of bedloads is affected by the terrain conditions and upstream sediment supply conditions.

Key wordsFbedload measurement, hydrophone, sound pressure, sampling frequency, sampling time