Sediment transport process in mountain streams has been studied chiefly in the development of theoretical as well as experimental bed load equations with difficult field direct sampling. Attempts have been made to utilize more indirect but stable monitoring methods (hereafter gindirect method") in recently years such as a hydrophone sediment discharge measuring system (hereafter ghydrophone system") which requires a basic analysis of its sensitivity to stream flow properties to be used for bed load estimation and analysis. Hydrophone systems count the times that bed load sediments strike the acoustic sensor of the system. Hydrophone systems make use of correlation among acoustic, flow discharge, and sediment discharge aspects of hydro]sediment phenomenon. Groups of transported bed load more than a certain velocity give kinetic energy as they clash to hydrophone acoustic sensors, which gives rise to acoustic energy in the systems. The acoustic energy is converted to electric signals upon some amplification. Electric signals more than a certain threshold are then recorded as pulses. Recorded pulses are therefore considered to be affected by grain and kinetic properties of stream flow. The functional dependency of hydrophone pulses on stream flow warrants careful examination when bed load discharge is estimated based on observed pulses.
In this study, the sensitivity of hydrophone pulses to grain and kinetic properties was examined utilizing bed load direct sampling and flow discharge observation conducted concomitantly in 100 and 200km2]scale river basins. Direct sampling of bed load enables us to observe not only discharge volume per unit period but also useful clues on bed load grain property. Numbers of grains in a unit bed load volume can be estimated from the grain property. Bed load detection rates were introduced methodically, which was nominally defined as the ratio of pulses to the grain numbers caught by direct sampling.
Examination of the detection rates indicates that hydrophone pulses respond to a grain group of more than 0.85 mm in diameter in observed cases. The detection rates were observed to be strongly varied in a certain range of flow discharge. The total number of pulses influenced by this sensitivity to flow was estimated to be not more than a few percent in studied cases. Thus the flow sensitivity of detection rates can be seen as limited in some cases. However, attention needs to be paid for this detection rate variability when sediment transport phenomenon is analyzed with hydrophone pulses.


Key wordsFsediment sampling, hydrophone, sensitivity, detection rate