Hydrological Observation of Subsurface Flows Spouting from Pipe Exits in Torrent Bed Material and its Triggering Rainfall Conditions of the Nishinokaito River in Mount Fujiwara, Mie Prefecture, Japan

Abstract

Several pipe exits were discovered in the torrent bed material in debris flow generation areas of the Nishinokaito River, Mount Fujiwara, Mie prefecture, Japan. These exits were in a sand and gravel layer between 1.0 and 1.5 m below the surface of the torrent bed material. Gravel supported the internal walls of the pipe exits. Subsurface flows spouted when the rainfall intensity exceeded 5 to 7 mm in 10 minutes and the soil water index exceeded about 110 mm. Due to hydrological processes, subsurface flows spouting at the new pipe exits during heavy rainfall may lead to debris flow generation. We studied the critical rainfall thresholds using the radial basis function network (RBFN) method.