This study evaluated four functional models developed by the previous study (Kosugi et al., 2013), which calculate bedrock groundwater level (BGL) changes caused by rainwater infiltration by using antecedent precipitation indices (APIs). BGLs observed at 25 boreholes excavated at 5 different locations were used for the evaluation. Results showed that Pw2 model, which assumes power functions between BGLs and two types of APIs, was selected to be the best model for the 14 boreholes. Pw1 and Li1 models, which assume power and linear functions, respectively, and use a single API, were selected as the best model for 10 and 1 boreholes, respectively. Optimized half-life times (HLTs) of the APIs were generally longer than those used for shallow landslide and debris flow predictions. In general, the optimized parameters showed typical values for each location, suggesting that geological characteristics for each location had more dominant effects for BGL changes than characteristics of each point situated in the same geological settings. The proposed models can readily estimate BGLs anteceding storms and earthquakes. Moreover, they can be used for indicating the biggest BGL increases ever. Thus, the proposed models are effective for establishing evacuation systems against deep landslides and detecting vulnerable slopes.
In the Rokko Mountains, located in Southwest Honshu Island in Japan, the crush zone of the active faults are 1.5 kilometers in maximum width. The fault process zone in the surrounding slopes is 2 km in maximum width. Furthermore, the moderately-sloping upper mountains consist of rock mass with very little effect of faulting. Severely weathered granite has hardly developed in the crush zone as well as in the process zone. Instead, block failures and topples are the dominant slope failure modes developed within the weathered rocks where the fault system predominates displacement processes in the Rokko Mountains. Dominant fault patterns found in the Rokko Mountains are an active right-lateral fault and a subsidiary Riedel shear fractures propagating out of the main fault. Hence, vertical joint system parallel to the main fault has developed in the vicinity. In fact, the weathered rock mass becomes structurally unstable when steep slopes in the crush zone or process zone have a gradient of 35 degrees or more which is highly vulnerable to failure. In the crush zone and process zone, slopes with gradient of 35 degrees are widely distributed because the rock mass adjacent to the faulting is relatively weak. Accompanied by the abrupt uplifting on the northwestern side of the main fault, frequent downcutting of antecedent streams has formed steep slopes in this area. This is indicated by the rock mass near the faulting being structurally vulnerable and the joint system having densely developed which in turn creates favorable conditions for development of fault valley. In such slopes, weathered rock mass consisting the crush zone or the process zone is easily unstablized and difficult to form weathering crust while sediment production is potentially greater in weathered rock mass.
Geology is one of the important inherent factor of collapse generation. In this paper, we discuss the relationship between surface failure due to rainfall and geology by comparing actual collapse data with geological map data. Two sediment-related disasters in Miyazaki Prefecture in 2007 are chosen. The correlation between rainfall characteristics and collapse generation depends on rainfall index R'. As a result, many surface failures occurred on the Neogene system. These surface failures occurred mostly in igneous rock areas, according to lithofacies, regardless of slope inclination. In addition, collapse generation rates differ between slopes with large inclinations.
The large deep-seated landslide and landslide dam followed heavy rainfall during typhoon No. 12 in September 2011 in the Kii Mountain range. A large landslide dam resulted from deep-seated landslide in Akadani area, Gojo city, Nara pref., therefore, Kinki Regional Development Bureau, Ministry of Land Infrastructure, Transport and Tourism, began to construct the temporary overflow channel as a part of urgent countermeasures in September 16th, 2011. Although just before its completion, large landslide occurred again, in June 19th, 2012, by heavy rainfall during typhoon No. 4. The temporary overflow channel was covered with thick debris, therefore, we had to abandon the recovery of that, and to change the plan for permanent countermeasures following the urgent countermeasures. Before of this re-slide, debris discharge from the deep-seated landslide slope had occurred frequently even in the dry season, and it had affected the progress of the countermeasure works. By the results of the survey to clarify the mechanism of the frequency debris discharge, it is seemed that flowing water origin in much spring water, resulted from concentration of underground water to the slope, scoured the foot of the debris deposition on the slope, and it made remained deposition on the slope more unstable, caused frequency debris discharge. These results prove that survey for evaluation of slope stability should be done in advance of plan or design for countermeasures and it decrease reconsideration and reconstruction, enable to progress countermeasures rapidly as result. In this term, we clarified the points to evaluate stability of slope with characteristic of geology, topography, debris discharge, hydrology of the slide slope after occurrence of deep-seated landslide. And more we considered the way of the countermeasures for the landslide dam under the unstable deepseated landslide slope.
2 large landslides and many surface failures occurred due to the Typhoon No. 14, 2005 at the upstream of the Monobe River in Kochi prefecture. The first large landslides was the Nakao landslide which occurred on the Butsuzo tectonic line between the Shimanto belt and Chichibu composite belt and the second was the Nakanishidani landslide which located at the chichibu composite belt. The fractured sandstone layer slipped on the weathered mudstone layer in both landslide areas. These landslides occurred due to the rainfall which had a large total rainfall amount of 1203 mm and relative small maximum hourly rainfall intensity of 75 mm/h. It should be emphasized that the rainfall of more than 30 mm/h continued during 10 hours. The geomorphological characteristics of the slope on which 2 landslides occurred were examined based on the LiDAR data with 1 m mesh. The gentle slope with the knick line at the boundary of the slope was recognized above the slope of both landslides. The linear depression zone and the stairs-like slope with the combination of steep slope to gentle slope were found in the gentle slope on the ridge of the Nakao landslide. These geomorphological features were the sign of old deformation of the ridge and slopes. Active landslides were found near the ridge above the Nakanishidani landslide. The continuous knick line or the gentle slope band traversed at the middle of the side slope of the ridge of both landslides.
The hydrological process for the behavior of subsurface flows is not well understood in a torrent bed material during heavy rainfall. To elucidate this process, it is important to understand and predict the debris flow generation mechanism. Herein we report the results of subsurface flows spouted torrent bed materials as well as the occurrence and nonoccurrence conditions in the Nishinokaito torrent, Mount Fujiwara, Inabe-shi, Mie prefecture, Japan. Several pipe exits were discovered in a torrent bed material. They were emplaced in a sand and gravel layer between 1.0 and 1.5 m below the surface of the torrent bed material. The gravel supported the internal walls of the pipe exits. Subsurface flows spouted when the rainfall intensity exceeded 5 to 7 mm in 10 minutes and the soil water index exceeded 110 mm.
Recently, there have been many deep-seated landslides in Japan. The deep-seated landslides were a result of rising groundwater levels caused by heavy rainfall, deeply weathered rocks, and hydrogeomorphological formation of underground areas that are prone to the storage of groundwater. Some methods for site prediction of potential deepseated landslides were examined based on geomorphic, geological and hydrological surveys. This study aimed to develop a method for predicting the sites of potential deep-seated landslides by using mountain stream water. To this end, I measured the electric conductivity of mountain stream water in the upstream direction. The distribution map of the measured electric conductivity helped to predict the sites of potential deep-seated landslides.
Due to strong seasonal rain-front activity, heavy rainfall occurred in the Mid-Niigata region between July 29 and August 1, 2013. The total amount of rainfall reached approx. 400(mm) at Karuizawa observatory, Nagaoka City. From late Monday, July 29, to early Tuesday, July 30, rainfall-induced slope failures, debris flows, and floods occurred throughout the Tochio area of Nagaoka City. Early Thursday, August 1, rainfall-induced slope failures occurred throughout the coastal area of Izumozaki Town and Teradomari area of Nagaoka City, resulting in one fatality. Although total amounts of rainfall in 2004 and 2013 did not differ considerably, sediment movements such as slope failures, debris flows, and floods were far more frequent in 2004 in the belt-shaped area extending from Izumozaki Town to Tochio area. Sediment movements are thought to have been less frequent in 2013, because rainfall was distributed over two distinct periods, divided by approximately two rain-free days in between. On the other hand, as much as 400(mm) accumulated within only 24 hours on Tuesday, July 13, 2004.
A debris flow occurred on September 16, 2013, on Mount Iwaki, located in Aomori Prefecture, Japan. The debris flow occurred specifically in the Ushironagane Valley on Mount Iwaki, east of the Kurasuke Valley, where in 1975 a catastrophic debris flow took place. While this year’s debris flow did not cause as much damage, we have decided to survey the impacts of this debris flow. Field survey was conducted on September 20, four days following the debris flow.