We investigated the trends in the number of persons killed or missing by sediment disasters such as landslides and debris flows in the last fifty years in Japan. Since sediment disasters initiated by earthquakes and volcanic activity occurred intermittently and the deaths and missings caused by them were much less than caused by rainfall-triggered sediment disasters, it is appropriate to discuss trends in the number of persons killed or missing by heavy-rainfall related sediment disasters compared with trends in rainfall-triggered disasters including flood disasters. The number of persons killed or missing by both sediment disasters and heavy-rainfall related disasters has apparently been decreasing. The number of persons killed or missing by heavy-rainfall related disasters calculated as a ten-year running average exceeded 1000 persons/year in the 1950 s, 300 persons/year in the late 1960 s, and 100 persons/year in the 1990 s. Furthermore, the ratio of heavy-rainfall related sediment disaster victims to the heavy-rainfall related disaster victims has decreased from 2/3 in the 1970 s to 2/5 after the 1980 s.
In order to investigate the relationship between the landform characteristics and the surface runoff generation process devastated mountains, field observations were conducted in two experimental basins, Dahou experimental basin in the southern part of China, and Jakujo Rachidani experimental basin in the central part of Japan. The following findings resulted from those observations. In Dahou experimental basin ; 1) The saturated hydraulic conductivity is very small, and the regolith layers are thick with great soil storage capacities. 2) The time lags between the peak of runoff and their corresponding peaks of rainfall are very short. 3) As total precipitation increases, the direct runoff percentage does not usually. 4) The pressure heads on the soil layer nearest to the ground surface are greatest. These results suggest that, in Dohou basin, most important flow pathway during storm is Horton overland flow. This flow is generated because the surface soil layer consists of lowly permeable soil. In Jakujo Rachidani experimental basin of Japan, Horton overland flow is not generated, because the surface soil layer consists of highly permeable soil. As the results, there is a certain amount of time lag between the peaks of runoff and the peaks of rainfall, and the direct runoff percentages in the Jakujo Rachidani experimental basin are less than that in Dahou experimental basin.
Recently the slit sabo dam has become the mainstream in the construction of various types of sabo dams because it dose not usually trap and retain, but rather releases sediment and earth nutrients that the down stream area needs. The slit sabo dams control a large quantity of sediment which is liable to bring about disasters when storms occur. In recession periods, the slit sabo dams, however, could release more sediment than expected. Appropriately designed slit sabo dams at river bends would be able to increase checked sediment to give stable sediment control. A hydraulic model experiment was carried out to see how the location and structure of a slit sabo dam constructed in a bend of Joganji River contributes to sediment control. Through the experiments, it was made clear that a slit sabo dam constructed at the downstream end of a bend with a slit prepared on the inner side of the sabo dam is more effective for sediment control than that with a slit on the outer side.
On July 10, 1997, a debris flow took place at the Harihara River, in Izumi City. A little before the disaster, an abnormal loud noise was heard by residents. People who felt uneasy about the river from the noise went to the river side to check it. But, as the water stage was not so high, they stopped worrying and returned home to sleep. After one hour or a while, a debris flow attacked the residential area and killed 21 persons. In this paper, this premonitory phenomenon is investigated by means of hearing survey, hydraulic computation and runoff analysis.
The environmental issue has put a new spin on Sabo facility planning at all level including soundscape. In this point of view, the soundscape of fishway of Sabo facilities is investigated for several types with power spectrum analysis and time fluctuation analysis of power. According to recent soundscape studies, the 1/ff power spectra and the big time fluctuation are one of important indexes of blunt sound. In this study, the soundscapes of some Sabo facilities and of natural torrent are shown for the comparison with the fishways. The guide wall type and slope type have bland 1/f sound, but former has less time fluctuation like other concrete pool type fishways, although it is as the same as one of natural torrents or falls. Consequently slope type fishways constructed with stones have the blunt soundscape both in the time fluctuation and in the spectrum characteristic.