Steep mountainous streams are often characterized by bedrock channel, which temporally causes sediment storage and scouring due to channel obstructions, such as bedrock projection and large rocks. In this paper, the influence of bedrock projection and large rocks on long-term riverbed changes with sediment transport was statistically examined in a mountainous gorge channel, Oyabu Creek, Kyushu, Japan. This study was conducted based on the 12-years field monitoring during 1995 and 2007 in a steep mountainous stream, and sediment deposit, scouring, and variation depth with mean bedrock projection area, bedrock relief (wetted perimeter / channel width) and the volume and number of large rocks are calculated from these data and field measurement in 2006. As a result, both of sediment deposit, scour and variation depth were increased with increasing in mean bedrock projection area and volume of large rocks rather than bedrock relief and the number of large rocks. Furthermore multiple correlation analysis of mean bedrock projection area and volume of large rocks with sediment variation depth show high correlation coefficient. Comparing changes in sediment storage depth between bedrock projection segment and non-projection segment implies that bedrock projection segment caused considerable sediment storage with alternatively deposit and scouring dominated periods within 3 years. These results suggest that mean bedrock projection area and the volume of boulders play an important role on sediment transport providing channel bed relief in a steep mountainous stream.

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Sediment transport process in mountain streams has been studied chiefly in the development of theoretical as well as experimental bed load equations with difficult field direct sampling. Attempts have been made to utilize more indirect but stable monitoring methods (hereafter “indirect method") in recently years such as a hydrophone sediment discharge measuring system (hereafter “hydrophone system") which requires a basic analysis of its sensitivity to stream flow properties to be used for bed load estimation and analysis. Hydrophone systems count the times that bed load sediments strike the acoustic sensor of the system. Hydrophone systems make use of correlation among acoustic, flow discharge,and sediment discharge aspects of hydro-sediment phenomenon. Groups of transported bed load more than a certain velocity give kinetic energy as they clash to hydrophone acoustic sensors, which gives rise to acoustic energy in the systems. The acoustic energy is converted to electric signals upon some amplification. Electric signals more than a certain threshold are then recorded as pulses. Recorded pulses are therefore considered to be affected by grain and kinetic properties of stream flow. The functional dependency of hydrophone pulses on stream flow warrants careful examination when bed load discharge is estimated based on observed pulses.In this study, the sensitivity of hydrophone pulses to grain and kinetic properties was examined utilizing bed load direct sampling and flow discharge observation conducted concomitantly in 100 and 200 km²-scale river basins. Direct sampling of bed load enables us to observe not only discharge volume per unit period but also useful clues on bed load grain property. Numbers of grains in a unit bed load volume can be estimated from the grain property. Bed load detection rates were introduced methodically, which was nominally defined as the ratio of pulses to the grain numbers caught by direct sampling.Examination of the detection rates indicates that hydrophone pulses respond to a grain group of more than 0.85 mm in diameter in observed cases. The detection rates were observed to be strongly varied in a certain range of flow discharge. The total number of pulses influenced by this sensitivity to flow was estimated to be not more than a few percent in studied cases. Thus the flow sensitivity of detection rates can be seen as limited in some cases. However, attention needs to be paid for this detection rate variability when sediment transport phenomenon is analyzed with hydrophone pulses.

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In Japan, there is a place with danger of sediment-related disaster very a lot. In these places, it must be done measures construction immediately. However, it is very difficult to decide priority of construction from a lot of object points. Contrary to expectation, there has not been much progress to date as far as the construction of countermeasures against sediment-related disasters is concerned. Under such a situation, it is important to evaluate the safety of sediment-related disaster dangerous places and the effects of construction measures at present for efficiency in construction projects.However, there is currently no unified analytical technique to address this issue and this poses a problem to each selfgoverning body. In this study, a critical line (CL) for determining the sediment-related disaster was set up in two situations - one before the construction measures and the other after the construction measures, and evaluated the safety of sediment-related disaster dangerous places and the effects of construction measures was examined by measuring the safe domain areas for those CLs. Because this method is the statistical technique that used an existing database, it is advantageous in that new research or analysis is unnecessary. It is thought that this evaluation result is useful for a future plan of construction measures very much.

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The Sabo plan did not consider the difference in the timing of flooding and sediment discharge at the confluence of a tributary and the main river. We postulated that the water surface and riverbed deformation at the confluence vary greatly with the timing of flooding and sediment discharge. We carried out basic flume experiments to show the influence of the timing of a flood at the confluence on riverbed deformation. In addition, we performed experiments in which the sediment discharge from the branch river was controlled by a slit-type Sabo dam. The trunk flume was 6.0 m long and 0.3 m wide with a 1/80 slope, and the branch flume was 3.0 m long and 0.2 m wide and had slopes of 1/40 - 1/25. The angle of the two flumes at the confluence was 60 degrees. The model hydrographs of the trunk and branch were the same shape. There were two experimental patterns : in one, the peaks of the branch and trunk rivers were at the same time, and in the other, the peak of the branch river preceded the peak of the trunk river. As a result, the timing of the flooding of the trunk and branch rivers and sediment discharge were shown to deform the riverbed markedly. Therefore, in runoff analyses, it is necessary to remember that in nature, there is often a difference in the timing of the sediment discharge of trunk and branch rivers in mountainous districts, where the sediment discharge is very large.

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The collapse of caldera walls of Mt. Bawakaraeng on 26^th of March 2004 in South Sulawesi is one of the largest mass movements in the history of Indonesia. The collapsed material, calculated at approximately 232 million m³, covers 8 km of the upstream part of Jeneberang River. The main purpose of this research is to evaluate the magnitude of the impact on the change of basin and how the impact decreases over time. A given amount of rainfall produces less discharge after the collapse and peak flows which normally occurred in the months of January are now found in the months of March. Before the collapse, every daily amount of rainfall corresponds to 0.6 value of daily discharge, though in 2006, it decreased to 0.45. Daily discharges are mostly low flows (less than 5 mm/day) except in the year of the collapse (2004). After the collapse, medium flows occurred more frequently and high flows (flow of over 90 mm/day) occurred in less frequency and lower values. Analysis of turbidity and discharge relationship at Bili-bili Dam located 31 km downstream of the collapse may be applied to understand the sediment exhaustion and the basin's potential rates of recovery. Bili-bili Dam's maximum turbidity rate increased significantly from 407 Nephelometric Turbidity Units (NTU) to 125,159 NTU after the collapse and turbidity's respond to discharge alters after the collapse. Peak turbidity rates normally fall faster than the discharge falling limb ; nonetheless after the collapse, both values fall by approximately 50% daily.

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There are many researches on deterioration rate of wood, but almost these researches just focused on finding out the deterioration rate according to time directly basing on the in-field sampling. These researches mentioned that weather factors are the reasons that cause the deterioration of wood ; however, did not show the relationship between the deterioration rates with these factors. Therefore, we have measured the deterioration from 6.0 to 9.0 years at the seven wooden crib dams in Kyoto Prefecture. Wood species are Cryptomeria japonica (Sugi) and Chamaecyparis obtusa (Hinoki). In addition, we calculated the design high water level of overflow depth, warm index, and maximum snow depth at the dam sites to clarify the relationships between them. The design high water level was calculated from Manning's formula. Warm index and maximum snow depth were obtained from the previous studies. The obtained results showed that there is a significant relationship between deterioration rates with design high water level. In the case of dam built at the torrent bed, the warm index and maximum snow depth have not much influence on the deterioration rate.

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