Hiroyuki Yoshimatsu

Landslide Dams Formed by the 2004 Mid-Niigata Prefecture Earthquake in Japan

It was a remarkable aspect that a tremendous number of landslides were triggered by the Mid-Niigata Prefecture Earthqauke. Because the epicenter was located at a depth of 13 km just in the landslide prone area of central part of the Niigata Prefecture, severe damage was caused by the earthqauke-induced landslides. Furtehermore many landslide dams were formed mainly in the watershed of the Imogawa-River by the displaced soil mass of the earthquake-induced landlslides. Some large landslide dams should pose a great threat of flood and debris flow in case of dam collapse to the settlemnt of the Ryuko-District in the downstream area of the watershed. It was urgently needed to arrange the emergency operations to avoid the destructive collapse of the major landslide dams. This contribution illustrates overview of landslide dams and emergency operations against dam collapse.

Prediction of long term fluctuation in ground water level of landslide in a snowy district

It is well reported that landslides occur frequently during a period that a ground water level rises due to the heavy rain and snow melting. To perform a highly precise level of risk evaluation of these landslides the estimation method of ground water level fluctuation is necessary to develop including it for the snow melting period. Therefore, we examined the estimation method of water level fluctuation in consideration of the infiltration effect of snow melting water into under ground in snow melting period. The estimation method suggested the Ψ function method that calculates the maximum correlation in the observation values of ground water level and the estimated values analyzing effective snow melting amount by GA algorithm method. The presented method confirms the validity and precision through a comparison between the actual field observation data and estimation results.

Characters of Groundwater Fluctuation in the Fractured Zone Type Landslides

The authors proposed an estimation model of groundwater level which is able to be used when the basal groundwater level is already clarified and duration of precipitation is longer than ti. This model definition is as follows:γ= (κ·i·β/l) ·Δh+α where t1 is concentration time of groundwater from top of the catchment area to the observation point, γ is the precipitation density, κ is coefficient of permeability, i is hydraulic gradient, β is effective porosity, l is length of the catchment area, Δh is groundwater ascent volume, α is invalid precipitation density.Moreover, for the purpose of making to be sure the propriety of this model, we examined this model through the Taninouchi landslide, Kochi prefecture and Kuchisakamoto landslide, Shizuoka prefecture.