Hydrophones have been used to measure bedload transport intensity by counting the number of pulses generated when bedload sediments such as sand or gravel strike a steel pipe. However, this method has disadvantages; when sediment rate is high, contact with the pipe is continuous, and therefore, readings rarely drop below a count of one or zero. In contrast to this method, we proposed an analysis method using sound pressure data obtained with a hydrophone in a past study. Flume experiments show that bedload sediment discharge can be calculated quantitatively when using this method. In this study, the applicability of the method was analyzed using field observations. The method was first tested and then modified in field experiments at the Bouzudaira sabo dam in the Yotagiri River within the Tenryu River System. It was determined that a relational expression between the number of grains and the detection rate could be computed for conditions with grain sizes of mixed diameter. To compute the number of grains, the geometric mean of values calculated with multiple division numbers using sound wave data can be used for a high]accuracy calculation during sound wave interference analysis. Field observations using the modified method and direct measurements made by the observation facility were conducted at the same point for the 12th and 15th typhoons in 2011. It was confirmed that our method has an analytical limit, but this limit is considered to be negligibly low. Analysis results of bedload transport intensity were in accordance with field observations results. However, when the bedload transport intensity was high, analysis results of the grain diameter were larger than field observations results. These results indicate that the increase of flow velocity and the decrease in the rate of impact are balanced.

Key wordsFbedload measurement, hydrophone, sound pressure, field observation