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Measurement of sediment transport in mountain torrents is important to elucidate the nature of the process. Many obstacles have prevented long-term continuous measurement of natural river basins with a substantial scale, so only a few sporadic observations have been collected over short durations. Direct sampling, therefore, has recently been augmented by more indirect but stable methods (hereafter ‘indirect methods’) including a hydrophone pipe–microphone acoustic sediment-discharge measuring system (hereafter ‘hydrophone system’). Hydrophone systems count the times (hereafter ‘pulses’) at which bedload sediments strike a steel pipe–microphone acoustic sensor. A statistical analytical method can be used to calibrate and estimate bedload amounts sampled by a sediment pit with a linear combination of pulses and flow discharges. Sediment discharges exhibit striking variability in orders of magnitude during flood events. However, researchers have not explicitly taken into account the volatile nature of sediment transport phenomena, as most quantitative studies have applied unitary analytical forms. In this study, three hydrophone systems were installed in 100and 200-km-scale river basins together with sediment trap pits. During floods, most sediment (approximately 80%) occurred within the first one-quarter of sediment sequences reordered in a descending manner. Therefore, sediment–hydraulic quantities were dissected into two sets: the upper quarter and the lower three-quarters. Frequency distributions of sediment–hydraulic quantities for each set were compared closely to differentiate the two sets based on a set of cutoffs using only direct methods. Bimodal analytical fitting forms were drawn for each set. The results revealed that at the time scale of flood events, the quantitative accuracy of sediment discharge estimates can be improved by dissecting sediment discharge phenomena into periods of concentrated and of minor occurrences, incorporating sediment discharge variation during the duration of floods.

Bimodal analysis of sediment transport in mountain torrents using hydrophones and sediment traps