山地河川における掃流砂量の横断方向での変動特性に関する一考察

抄録

Sediment transport measurements in mountain torrents are pivotal to comprehend the nature of the process. Long-term continuous measurement in natural river basins with a substantial scale has been circumvented by many obstacles. Direct sampling, therefore, has been augmented by more indirect but stable methods (hereafter “indirect method”) in recent years such as a hydrophone pipe-microphone acoustic sediment discharge measuring system (hereafter “hydrophone system”). A statistical analytical method based on the hydrophone system has been developed, which enables us to calibrate and estimate bedload amounts sampled by a sediment pit within a limited width of river sections. Observation of sediment transport phenomena, however, has not been fully expanded across a river cross section. Crosssectional variability of bedload discharges has to be understood to select appropriate observation points in a section and to estimate full-width sediment discharges out of those at a unit-width.
A bedload discharge observation facility consisting of a pair of sediment trap pits, together with a water level gauge, has been installed in a 100-km² scale river basin, in order to make pair observations at the right and the central point in a cross section. 12 small-to-medium size flood cases were observed and analyzed in order to quantitatively understand the nature of bedload discharges at each point section and their mutual relationship in a sequential time series as well as in aggregated sums over each case. Intensities of bedload discharges underwent fluctuation of an order of magnitude at a different timing for each point over the observed cases. As a result, relative intensities between the two observation points, expressed by their differences, changed from positive to negative across zero, which gave a fluctuation cyclic period of from about 20 to 40 minutes. Further observation, if combined with the indirect method of hydrophone systems, which have a shorter time constant as an observation system, is likely to clarify cross-sectional variability of bedload discharges in a more reliable manner.