In this study, the author applied the theory of sediment discharge to a mountain stream and examined its applicability for determining the sediment yield resulting from showers as well as the long-term applicability of the results for mountainous watersheds. Furthermore, using hydrological data obtained in the past 60 years and data of sediment yields, the author analyzed the influence of the process of forest recovery on sediment yield. This study adopted one such formula, the Brown formula is considered applicable for bed material load. It became clear that sediment yield in Shirasaka watershed (the weathered granite zone) is proportional to runoff to the 1.40th power. However, coefficient values obtained by observation and by calculation are different. This fact suggests that other factors, for example washload, may have a significant impact on sediment yield. Next, using sediment data recorded in the past 60 years for Shirasaka watershed and the result obtained from the sediment discharge theory, the author analyzed the relationship between sediment yield and the forest. Sediment yield has a tendency to decrease gradually. Here, variations in the Erosion Index (E_i) was studied thoroughly, where “E_i” is the quotient when sediment yield is divided by runoff to the 1.40 th power. Changes in the “E_i” align well with the changes in the bare-land ratio (L_a). Thus as a result of having analyzed relation of “E_i” and “L_a”, it was expressed in an exponential function.

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This paper presents an impact analysis of steel cell dam supported by truss frame. In advance to this study, a collision test with a model steel cell dam was carried out in order to investigate impact force on a steel cell and transmitted impact force on a truss frame. The axial force of the truss frame which supports the steel cell become significantly smaller than the impact force at the collision point. In order to evaluate this result from analytical point of view, the simulation analysis was performed by using finite element method to prove the validity of the transmitted impact force obtained from the collision test. Furthermore, how the stress wave propagates through the steel cell is discussed.

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The durability, methods for in-situ measurement of the degree of deterioration, and methods for evaluating the bending strength of wooden members, which are important factors for the planning, design, and maintenance of wooden erosion control structures, were studied. Wooden members were collected as test specimens from a wooden groundsill and a revetment which were constructed in Nigori Creek which runs on the southern flank of Mt. Ontake, Nagano Prefecture, in 1985. Four types of test were conducted on the wooden members to assess the degree of deterioration : the ultrasonic pulse test, stress wave impact test, pin penetration test and drilling resistance test, and bending test. The correlations between the bending strength and values measured by the four tests and oven-dry density indicate that the bending strength of wooden members is most closely correlated with the values derived from the drilling resistance test. The test results also indicate that the bending strength of the deteriorated part is almost zero and the bending strength of the non-deteriorated part is the same as that of sound wood. Methods for evaluating the bending strength and bending capacity of deteriorated wood were obtained by using the ratio of the thickness of the deteriorated part measured by the drilling resistance test. The investigation revealed that the difference of circumstances of wooden members, even those for the same structures, greatly influences the degree of deterioration. The wooden members which were wetted with water or soaked in water at all times have been slightly deteriorated in 17 years, while the wooden members which are not usually wetted with water had deteriorated significantly. Except for the wooden member at the highest portion of the revetment, the members of the 17-year-old wooden groundsill and revetment have maintained sufficient bending strength to assure the safety of the structure.

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Some factors for the occurence of the large-scale landslide in Tsaoling, induced by the 1999 Chi-Chi earthquake in Taiwan, were discussed. The results are as follows : 1) The three components of acceleration were all very large, and the predominant direction of the seismic ground motion agreed with the direction of the slope. 2) The geology is Neogene, i.e. relatively new strata, and the bedding planes are roughly parallel with the ground surface. 3) This slope was under the ground conditions that a landslide was apt to occur by stress release or gravitational creep. 4) The ground has been weakened by salt weathering. Especially, 3) and 4) may be controlling the repetition of landslides in Tsaoling. Therefore, the observation of these phenomena may offer effective data for the prediction of next landslide.

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The typhoon No. 10 with heavy rainfall had attacked to Hokkaido from August 8 to 10, 2003. This paper promptly reports the outline of the sediment related disaster. The authors had carried out aerial reconnaissance and field surveillance to detect the disastrous circumstances due to the heavy rainfall. The most heavily damaged areas were Ribira River, Wenteshikan River and Bin River; upstream of the Atsubetu River, where had been extremely deposited sediment and woody debris. The results show that there were shallow landslides on the hill slopes, bank erosion, sediment yield (runoff) from gullies and temporal retention of woody debris on existing check dam basins in upper streams. In lower streams, large amount of sediment and woody debris were also deposited on the riverbed. The maximum size of woody debris was approximately sixty centimeters in diameter and twenty meters in length. The Prefectural Government of Hokkaido will carry out the emergency measures against remained sediment and woody debris. We concurrently study the totalized measures against sediment related hazards through the study committee with the forest agency.

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