Estimation of temporal changes of debris flows and hydraulic model tests of channel works with multi-drop structures

Abstract

A debris-flow disaster took place in the Nashizawa creek in Kiso River catchment, Nagano Prefecture, Japan on 9th July 2014 due to intensive rainfall from the typhoon Neoguri. There were few data available for estimation of debris-flow discharge in the catchment. A CCTV camera and a high sensitivity seismometer were installed along Nashizawa creek prior to the debris flows. Surges of debris flows were recorded by CCTV camera, though the camera was eventually destroyed by those surges during debris flows. The shape of debris-flow surges was estimated using the discharge analysed by CCTV camera data and fitting curve of relations between debris-flow discharge and acceleration of vibration. Peak discharge was extrapolated from peak acceleration of the vibration, and was estimated about 756 m3/s. At the Nashizawa creek, channel works with multi-drop structures were planned for construction after the debris-flow disaster. Some research was conducted experimentally for planning and design of channel works such as multi-drop structures. This research pointed out several problems for multi-drop structures in supercritical flow such as scouring and flow depth increases due to shock waves. We investigated characteristics of shockwaves and countermeasures using multi-drop structures in a steep channel with supercritical flow experimentally to design the channel works. We found that stair-type dissipaters are effective for mitigating shockwaves from multi-drop structures.