Author(s): Nobuhide Koi; Yasunori Muto; Takao Tamura
Linked Author(s): Yasunori Muto
Keywords: Forest dilapidations; Precipitation; Sediment yield
Abstract: Due to forest dilapidationsin recent years, river basinslocated in such conditions encounter not only the increases of sediment yield and inflow during the normal period but the rise of collapsing risk during the floodingtime. Thus how a degree of forest dilapidation affects the sediment yield and inflow should be clarified. In this study, aiming to derive a sound relation between rainfall and sediment discharge, in the Nishidani River Basin, a tributary of the Tonda River, which is a class B river in Wakayama Prefecture, Japan. The measurementperiod wasfrom June 2014 until December2015. Local rainfall and river water levelswere measuredby settinga rain gauge and severalhydro gauges. Sediment discharge in the river was measuredby a pit and river bed was measured at the beginning and the end of the measurement period, then river bed change in the section was calculated. From theobserved precipitations, water levels measured at thepit and sedimentationin the pit, the water level and sedimentation almost correlate withtheprecipitation, exceptthe flood event bythe Typhoon1411HALONGin which the pit filled up by sediment. The riverbedchange indicatesthe development of the channel, but not so muchchangedthe riverbed topography. In additionwe obtaineda simple correlation functionbetween the sedimentation and the accumulated rainfall for each event from a regression analysis. As a summary ofthe observations, we could not examine relationship betweenthe quantity of underground water and sediment yield. Thus we decided to install several hydro gauges in new investigation drill holes, and to performriver bed measurementsfor after every flood events, and still continue the observations.