Field Measurements of Debris Flows on Mount Unzendake, 1993

Between June and August, 1993 field observations and measurements of debris flows have been carried out at two locations on Mount Unzendake. One of the instrumented study sites was a downstream reach of the Mizunashi River, which is11.6m wide and have an average slope of about 2.9°. The other was a downstream reach of the Nakao River, which is 7.2m wide and have an average slope of about 1.4°. Radio current-meter and ultrasonic water level gauge were used to obtain surface velocity, depth and discharge for the several flows. The value of nondimensional surface velocity us/u* was found 5. Peak discharge was found Q=195m3/s at the Mizunashi River and Q=40m3/s at the Nakao River. Spectra of velocity and water-level variance shows maximum at f≈10-2Hz.