Bedload monitoring in a mountain stream: Method for improving the accuracy of the calibration relationship between acoustic pulses and bedload discharge

Abstract

Surrogate measurements using Swiss geophone plates and Japanese acoustic pipes/plates are effective for monitoring bedload transport in mountainous streams. They are also becoming important tools for obtaining sediment discharge measurements at the outlets of sediment bypass tunnels, to monitor bedload discharge through such tunnels. They have been installed at the bypass tunnel of the Solis dam in Switzerland and the Koshibu dam in Japan. To obtain the practical amount of bedload discharge from surrogate measurements, determining a calibration relationship between geophone impulses/ acoustic pulses and bedload is necessary. We have installed acoustic pipes and a slot sampler in the Ashiarai-dani supercritical flume, located in the Hodaka mountain range, Japan, and have been conducting bedload monitoring for more than a decade. Recently, we developed a new method of analyzing data obtained with a set of horizontal and vertical acoustic pipes, to overcome the disadvantages of bedload measurements obtained using acoustic pipes only: (1) bedload particles saltating over the pipe remain undetected, and (2) saturation problems of acoustic pulses under high-discharge conditions. A vertical pipe was installed on the wall of the flume and the horizontal pipe was installed on the flume bed. We propose a ratio (Rhv) between pulses detected by these sensors, and applied this ratio to calibrate the contemporaneous pulses detected with a pipe located immediately upstream of a bedload slot sampler. Indeed the Rhv-corrected pulses correlated well with the bedload discharge obtained with the slot sampler. We conclude that bedload monitoring using concomitant vertical and horizontal pipes can be used to calibrate a centrally located pipe, thereby representing bedload discharges more accurately than those based on a single acoustic pipe: this monitoring method may be useful at sediment bypass tunnels.