The concentrations of Cs-137 and Pb-210ex in fluvial sediment and soil were analyzed to determine the sources of fine sediment collected at 9 tributaries located in Nagasawa reservoir catchment, Shikoku Island, Japan. The major sedi-ment source was identified as subsoil derived from landslide scar or channel bank, but sediment production from forest floor was also identified from the existence of Cs-137 and Pb-210ex/Cs-137 ratio. It was indicated that sediment production caused by past landslide dominated in subcatchments with high percentage of landslide area, but sediment production from forest area was also estimated in subcatchments with small landslide area. The surface soil of land-slide scars were characterized by high concentration of Pb-210ex. The concentrations of Pb-210ex in sediment collected in Tebako-dani catchment increased with downstream distance. This implied that residence time of sediment could be estimated by comparison with direct deposition rate of Pb-210ex. It was concluded that the sources of fine sediment could be determined by analyzing the concentrations of Cs-137 and Pb-210ex in channel bed.
The estimation of grain size distribution has arisen as the subject of bed load measurements using hydrophones. Al-though field bed load measurements using hydrophones in some rivers have already begun, its application to hydraulic model experiments is still in its infancy. In this study, we investigated the estimation of the grain size distribution of mixed sand through a systematic experiment executed in a linear rectangular channel. We further show an exemplary hydraulic model experiment using a hydraulic model equipped with a hydrophone at both the cross-section of the downstream end of the model and the cross-section of the planned slit sabo dam installed in the middle of the model segment. The relationship between the non-dimensional grain size distribution and the sensitivity of the channel in this hydraulic measurement system for recording pulse numbers indicated good agreement, provided that the sensitivity of the channel for recording the number of pulses is ideal for the objective mixed sand. The applicable range of the channel sensitivity is, however, narrow, thereby lacking in general application. Therefore, in order to estimate the grain size distribution of mixed sand, it is necessary to adjust the channel sensitivity. From the hydraulic model experiment using hydrophones for measuring bed load, the reason that the peak of bed load sediment occurs at the downstream end of the hydraulic model could be explained by the changes of the number of pulses at the slit sabo dam. The result of our hydraulic model experiment suggests that a rational explanation of the phenomena of bed material transport is made possible by applying hydrophones to the hydraulic model experiments.
Some landslide disasters are triggered by the seismic force in the Fukuoka Seihou - Oki earthquake on March 20, 2005, although they didn't have antecedent precipitation. The earthquake whose epicenter was located in the offshore of the city of Fukuoka had the magnitude of Mjma 7.0, and Mw 6.4-6.6, maximum acceleration of 489 gal in downtown Fukuoka. Almost failures occurred in this seismic event are shallow slides on the Granite steep slopes and collapses of embankment under the houses. There can be found some prominent cracks on slopes due to the displacement of slopes. These phenomena give us the data that provide conspicuous important information to understand the mechanism of slope instability as well as the susceptibility of failures in the earthquake. Hence, the overview of the landslides is given in this paper.