Experimental study on the development process of torrent bed material scouring-induced debris flow by using image information.

We have been conducting hydrological observation for the behavior of subsurface flows to reveal the debris flow generation mechanism in the Nishinokaito torrent, Mount Fujiwara, Mie prefecture, Japan. Subsurface flows and a lot of sediment spouted from several holes created in the torrent bed material during heavy rainfall prior to debris flow. Serial camera-footage analysis revealed torrent bed material scouring occurred due to subsurface flows spouting. However, little is known about the development process of torrent bed material 　scouring-induced debris flow, especially the process of leading to the equilibrium sediment concentration. I n this study, we conducted hydraulic flume experiments on the process of sediment concentration leading to the equilibrium in debris flow front. Sediment concentration is estimated by using luminance value of image information in the condition of uniform light environment, uniform particle size of sand and adjusting the color of sand and water. Average sediment concentration of debris flow front increase as debris flow travel distance increase until average sediment concentration reach equilibrium. Sediment concentration near the river bed is to be close to the equilibrium even in the beginning of the debris flow development. As debris flow travel distance increase, as sediment concentration in the upper flow front is to be close to the equilibrium. It is thought that torrent bed material scouring don't occur because sediment concentration of debris flow front near the river bed is to be close to the equilibrium even in the beginning of debris flow development and average sediment concentration of debris flow front increases due to sediment supply scoured by the subsequent sediment flow .