An Observational Research on Hydrological Data and Sediment Transport in the Nshidani and Kumanogawadanigawa River Basin and Mountain Watershed

Due to forest dilapidations in recent years, river basins located 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 flooding time. For example, in the Tonda River, which is a class B river in Wakayama Prefecture, Japan, river structures like a fishway lost their function because of river bed degradation. On the other hand, by the heavy rainfall “Kii Peninsula flooding” caused by Typhoon 1112 TALAS, river channel blockage occurred by mountain slope collapse at Takijiri, Tanabe city. In this study, aiming to derive a sound relation between rainfall and sediment discharge, we carried out an observational research on hydrological data including sediment transport in the Nishidani and Kumanogawadanigawa River Basin, a tributary of the Tonda River.

The measurement period was from June 2014 until November 2017 in the Nshidani River Basin. Local rainfall was measured by setting a rain gauge at around the centre of the basin. Several hydro gauges were set to measure ground water levels in investigation drill holes. Sediment discharge in the river was measured by a pit set near the downstream end of the basin. Furthermore river bed geometry was measured at the beginning and the end of the measurement period every year, upstream from the pit. In addition, from June 2016 through November 2017, we performed similar measurement in the Kumanogawadanigawa River Basin near the Nishidani River Basin as the comparison area where the vegetation condition is different.

From the observed precipitations, water levels measured at the pit and sedimentation in the pit, the water level and sedimentation almost correlate with the precipitation, except the flood event by the Typhoon 1411 HALONG in which the pit filled up by sediment. In addition we performed the comparison with other basins which were available to quantity of sedimentation data inside the country.

As a summary of the observations, in most basins, we could show an exponential function in the relationship between the accumulated rainfall and the quantity of sedimentation. Moreover we could find that it has the threshold in rainfall and quantity of sedimentation changes big in it.

Furthermore, we examined influence that the difference by the tree species or a difference of the forest floor vegetation based on the management of the forest gave to the sedimentation. As a result, we could show possibility to grasp the influence quantitatively.