Study on the cross-sectional variability of bedload discharges in a mountain stream

Abstract

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.