Prediction of Sediment Yield by Shallow Landslide Based on Landslide Periodicity

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The purpose of this study is to develop a new method for prediction of sediment Yield by shallow landslide based on landslide periodicity. The main research areas are Mt. Shibisan, Yaku-Aina island, Mt. lichifusa-yama, Kagoshima-shi(Shirasu)and Aso-Nekodake, Southern Kyushu, Japan. Landslide scars were made out by interpretation of aerophotographs and field survey and topsoil thickness on the slopes was wmured using a cone-tipped metal stick in the research sites.1. Many landslide sears with different ages distribute at the 0-order basins in each research area according to the landslide chronological sap. Although the greater part of scars has been formed on different slope site each other, in the volcanic research areas the newer and older scars overlap on a same slope site during less than 100 years.2. The topsoil on the steep slope has a critical thickness for shallow landsliding beyond which the tpsoil can not grow up largely. The critical-thickness depends on geological and topographical setting of the slopes.3. The frequency of landslide in the forested basins depends on the landslide periodicity as well as rainfall if other factors are same.4. The shallow landslide potential in a forested basin is expressed by probability density function of the topsoil thickness. The probability function is time dependence. If many shallow landslides occur under a heavy rain, the density of topsoil thickness more than critical one is reduced and as a result of that the landslide potential falls off. On the contrary if the landslide does not occur during a long period, the density of topsoil thickness more than critical one increases and as a result of that the landslide potential rises.