From 1991 to 1995, the Mt. Unzen volcano had many pyroclastic flows; the flow deposits widely and thickly covered the flank of the volcano and created a radical alteration of the hydrologic and erosion regime of the areas. As a result, debris flows have frequently occurred in the rivers on the flank of the volcano and the topography of the basins has much changed. In this paper, we analyzed the temporal variations of runoff processes of the debris flows, which followed the pyroclastic flows observed from 1995 to 2000 at the Mizunashi River at the Unzen volcano, based on field surveys, interpretation of aerial photographs and runoff analysis of debris flows. The area contributed to occurrence of Horton's overland flow declined with time as the gully erosion area increase. The rate of sediment production from gully erosion became a peak between September 1995 and August 1996 and then declined with time. Temporal variations of sediment yield by debris flows show the same tendency as those of sediment production by gully erosion. The temporal variations of the runoff process of debris flows following eruptions were examined by runoff model. The results showed that a roughness coefficient, rainfall loss, infiltration capacity and the area that contributed to the occurrence of Horton's overland flow are important factors for reproduction of the runoff processes of debris flows. Occurrence frequency and the magnitude of the debris flows have decreased since the end of volcanic activity because of recovery of the infiltration capacity, decrease of Horton's overland flow in frequency, and the magnitude of the decrease of the area contributing to occurrence of Horton's overland flow.
The Sediment Disasters Prevention Law enacted in April 2001, and establishment of warning and evacuation schemes at sediment-related-disaster prone areas is now an urgent necessity. Slope-specific risks are not taken into account in the conventional critical lines for failure occurrence, which are set based only on the relationship between rainfall and slope failure. It is therefore difficult to set critical lines precisely in this approach. To solve this problem, the authors propose a new approach to setting critical lines for failure occurrence by assigning risk to each slope using the rough set theory and taking the risk of each slope into account through data envelopment analysis. The effectiveness of the new approach is demonstrated through a comparison of precision with the conventional approach of linear critical lines.
The aim at this research is to develop a numerical simulation method that can predict the red sediment outflow from pineapple fields to river mouths in Okinawa river basins where red sediment outflow causes serious turbid water problems. The red sediment outflow model was developed under various conditions: expanding the ratio of field surface cover by pineapple leaf reduces the area of sediment yielding, erosion occurrence due to the shear force by the surface flow on the over-saturated surface of the pineapple field, and sediment movement as suspended load. This model was employed to develop a numerical simulation method based on a two-dimensional analysis for the riverbed variation, and can predict the red sediment outflow from the pineapple fields to rivers. Results of a numerical simulation of the Saan River catchment in Higashi Village accurately reproduced the red sediment hydrograph.
Alocal heavy rain hit Minamata City of Kumamoto Prefecture on 20th July in 2003. A large scale of disaster occurred in Hogawachi Atumari district and Fukagawa Shinyashiki district of Minamata City at about 4 in the morning(87mm/h of rain at 4)and it claimed the lives of 19 people. Both areas are composed of extremely weathering andesite in geology. The inclination of the collapsed slope of Hogawachi Atumari district is about 30° and that of Shinyashiki district is about 40°. The volume of the debris flow of the Atumari River was estimated in 1-1.5×105m3 and that of the slope failure of Shinyashiki district was estimated in 100-150m3 from the results of brief surveying or measurement of a laser profiler. The debris flow of the Atumari River flew straight down from the collapsed slope without damming up there. It is supposed that the debris flow was caused by the slope failure due to a rise of pore pressure of much seepage water into the weathering andesite layer with many cracks. The slope failure of Shinyashiki district which is called a surface failure was caused by the action of piping flow among the shallow talus accumulation layer. Sediment of the debris flow in the Atumari River consisted of 2 kinds of materials (muddy and gravelly one). It is said that the debris flow of the Atumari River ran for several ten minutes.
Debris flows occurred in Kyushu region due to localized rainfall which was caused by a seasonal rain front from 18th to 20th July, 2003, causing 20 dead person, 7 injured person and 63 damaged houses. Authors made investigations at Hougawachi-Atsumari district, Minamata city, Kumamoto Pref. and Sanjyou 1-choume, Dazaifu city, Fukuoka Pref., where were damaged by debris flows. In both cases, the debris flows were initiated by landslides. In order to estimate the volume of sediment discharge and velocity, authors surveyed geometry in both site. The volume of sediment discharge plunging into the village of Hougawachi-Atsumari district was estimated from about 98, 000m3 to 107, 000 m3, and the velocity of debris flow was estimated from about 2.9m/s to 23.5m/s. The volume of debris flow plunging into the village of Sanjyou 1 choume district was estimated about 12, 000m3. And the velocity of debris flow was estimated from about 4.2m/s to 14.3m/s.
The earthquake of magnitude 5.5 (12km deep of seismic center) hit at about 00:13 on July 26, 2003, was centered in the northern part of Miyagi. The recorded seismic intensity was a little less than 6 in Ono Naruse-cho, Yamoto Yamoto-cho, Miyagi prefecture. Furthermore the earthquake of magnitude 6.2 (12 km deep of seismic center) was almost centered at the same places at 07:13 on the same day. The seismic intensity of this earthquake was a little over 6 on Naruse-cho, Yamoto Yamoto-cho, Kibazuka Nango-cho. In addition, the earthquake of magnitude 5.3 (12km deep of seismic center) was centered at the almost same place at 16: 56 on the same day. By this earthquake, we recorded the seismic intensity a little less than 6 in Maeyachi Kanan-cho. In this way, there were three times earthquakes in a day, the seismic intensity with a little less than 6 and a little over 6. It is the first time in a history of internal seismometry in such a continuation earthquake. By this earthquake, 674 persons were injured (50 serious wounds/624 slight wounds). And the dwelling house damage were 1, 017 complete destruction, partial destruction 2, 245, 8, 078 one part damage (the August 29 present). We showed off terror of earthquake directly above its epicenter in the inland. Such damage concentrated on Miyagi prefecture. By an earthquake directly above its epicenter in this inland, sediment disaster occurred in each place in North Miyagi districts such as Kanan-cho, Yamoto-cho, Naruse-cho. The Japan Society of Erosion Control Engineering dispatched the disaster Investigating commission about slope collapse by the earthquake of 2003.7.26 North Miyagi in order to grasp the general conditions of sediment disasters by the earthquakes. We reorganize the disaster investigating commission, “Slope collapse investigation commission by the earthquake of the 2003.5.26 offshore of Miyagi prefecture” to “Slope collapse investigation commission by the earthquake of the 2003.5.26 offshore of Miyagi prefecture and the earthquake of 2003.7.26 North Miyagi prefecture”. This paper summarized the investigations related to the property to the earthquake damages, focused on slope failures in hilly areas.