Information on the time and location of debris flow initiation is essential for planning preventive measures and evacuation systems. To predict the time and location of debris flow initiation, we applied a method that combines the kinematic wave method and the index proposed by Takahashi (1978). The method was tested with data from the Boyong River at Merapi Volcano, Indonesia, and the Nojiri River at Sakurajima Volcano, Japan. A new diagram was also developed to show temporal and spatial variations in Takahashi's index in relation to temporal variation in rainfall intensity and discharge rates, and spatial variation in the slope gradient, catchment area, width of channel, and depth of surface flow along the main channel. Four specific debris-flow occurrences were analyzed. For the Boyong River, the proposed method correctly predicted debris flow initiation at rainfall intensity peaks for a stretch of river lying 2, 150-3, 679m from the summit, with slope gradients between 14° and 17°. For the Nojiri River, the method correctly predicted debris flow initiation at rainfall intensity peaks for stretches of river 520-735m and 1, 422-1, 777m from the summit, with slope gradients between 10.2° and 17.9°.
During a heavy downpour, runoff fluctuates rapidly in the upstream and a lot of sediments, woody debris and other suspended load are transported and deposited. Therefore it is important to prevent sediment from accumulating in the fishways. To find an appropriate solution to this problem, a suitable fishway known as the protruding boulder type fishway was designed. The authors investigated the characteristics of this type of fishway through a hydraulic model experiment and the fish release test. The purpose of the hydraulic model experiment was to verify whether various shapes of boulder could reduce the flow velocity while the essence of the fish release test was to observe the behavior of fishes around the boulders. The experiment revealed the following findings: 1) There was a suitable shape for boulders that could decelerate flow. In the case where the upstream side of a boulder is flat, the velocity of upstream flow reduces. But the downstream flow becomes turbulent in the case where the tail reach of the boulder is flat. 2) As it is difficult for fish to find a route of ascent, boulders should be closely arranged laterally for the upstream flow to dam up across the entire channel. 3) In the case of dam-up flow, turbulent flow arising from the boulder shape further makes fish ascent more difficult. To maintain sanctuaries for fishes, it is necessary that the longitudinal distances between the boulders are widened and lateral distances narrowed. It is also important that boulders do not have the shape which disturbs downstream flow.
In view of the increasing flood and sediment disasters from the streams in Siwalik region of Nepal, it is important to examine both the historical changes and present geomorphological characteristics of the stream systems. With the objective of analyzing stream course change characteristics and identifying governing factors, we carried out the study in three streams of eastern Siwalik region using sequential aerial photographs covering period from 1964 to 2003. Overlay of rectified photographs indicates that forest cover in the headwaters of the streams remained more or less unaffected over the recent times. The stream reaches are divided into four geomorphologic units: hill, terrace, floodplain and alluvial fan. Overlay of stream planform indicates that there is distinct variation in their development patterns in different stream reaches. It is revealed that the stream channels in the terrace reach are characterized by gradual widening. Similarly in the floodplain reach, gradual widening and course shifting are predominant. The streams are characterized by quick generation of runoff flow and high sediment content, which could be responsible for the frequent course changes. Application of structural countermeasures against bank erosion in terrace and floodplain area and conservation works such as gully control in the Hills are suggested.
Two strong typhoons in July and August 2004 caused the record heavy rainfall in Taiwan. Several floods and sediment disasters occurred in middle Taiwan, and 71 deaths and missings were recorded. Total damage was less than the former damage by the typhoons in 2001, however, the cumulative rainfall marked the maximum record at 2, 115mm, was bigger than typhoon Herb in 1996.
The earthquake of M6.8 occurred at 17:56 on Oct. 23, 2004 in mid Niigata prefecture, and seismic intensity 7 was recorded in Kawaguchi-cho. After that, the aftershock activity has continued so actively that several aftershocks of seismic intensity 6+ were observed, and they caused heavy damage in various places. The main shock appeared right near Yamakoshi-mura, and seismic intensity at this district was 6+. This area is the foremost area of frequent landslides in Japan, and by this earthquake many slope failures and landslides occured on the wide range of Higashiyama hill with center at Yamakoshi-mura. Slope failures and landslides appeared at 1, 662 points and the total of sediment yield is estimated to reach 7×107m3 as a result of the analysis of aerial photographs. There are ten large-scale landslides that have sediment yield of more than 106m3 individually, and five of them concentrate along the River Imo. Moreover, landslide dams were formed at more than 30 points along the River Imo, and urgent measures have been taken.
Most landslides move slowly and across short distances. Some landslides however, travel long distances. They are called long traveling landslides.The latter type cause large and widespread damage. This study overviews long travelling landslides in Japan. Spatial distribution and some characteristics of their movements are reported. The ratio (Tr) of the horizontal travelling distances to the horizontal length of the original slope is defined as a new parameter. The movement is classified as: full fluidized (Tr≥0.9), partly fluidized (0.3≤Tr<0.9) and non fluidized (Tr<0.3). Tr is influenced by not only the volume of the landslide, but also by other factors.
The further development of landslide prediction models requires field understanding about soil pore pressure distribution and water table dynamics in steep hillslopes. However, soil pore water pressure dynamics in steep hillslopes is poorly understood, since most of previous observation has been conducted at relatively gentle hillslopes or in a twodimensional line such as vally. Here we present new data about pore water pressure distribution in a steep zero-order hollow in Tanakami Mountains. Forty tensiometers in twenty nests were installed with recording pressure transducers. Pore pressures were recorded at 10-minute intervals throughout the observation period. Tensiometers showed that there were small difference in the timing and magnitude of pore pressure change at 20cm depth during medium size storm, regardless of slope position. While, the temporal pore pressure changes at the soil-bedrock interface were related to the upslope drainage area and the soil depth. At peak rainfall intensity, pore pressures at soil-bedrock interface were mainly influenced by soil depth, indicating that the dominance of downward flux within soil layer. While, after the rainfall peak, pore pressures was more related to upslope drainage area, suggesting that subsurface lateral flow gave impacts on pore pressure distribution.
The data of slope failures is important to make Sabo plan and watershed management. The data is generally acquired by interpreting aerial photographs, but there are several difficulties to acquire accurate data due to the interpreters' ability and transcription error. Remote sensing technology is available to cover them. In consideration of the performance limitation of remote sensing, the auto-detect system for slope failures area has been studied with aerial digital orthophotographs. The system is composed of five processes of (1) gradating the digital orthophotograph into two tones, (2) polygonizing the gradated objects, (3) removing the image noise caused by reflection of broad leaves etc. (4) extracting liner-shaped objects such as river beds and roads and (5) checking these incline. Applying this system to a drainage area produced an adequate result.