Many slope failures and debris flows occurred in 1993, in the area of fallen trees by the 19th typhoon of 1991. We thought to be disturbed and weakened the slope surface layer by shaking due to the typhoon, because there was not an amount of rainfall caused slope failures. In this paper, considering that the vitality of trees indicates the weakness of slope surface layer, the relation between vitality of trees and slope failures was studied. As the results, it was appeared that the decrease of normal vegetation index (NVI) obtained by airbone MSS data agreed well with slope failures occurred in a medium amount of rainfall areas.
Revetments constructed on river bends must be structurally stronger and more deeply embedded than those on straight river sections, because such revetments are susceptible to damage caused by local scouring. At locations, where likelihood of occurring sever local scouring is high, increasing the embedding depth of revetments will be unfavorable to the execution, and also will increase construction costs. In such situations, measures must be taken to reduce the local scouring. Generally, revetment foundations on straight river sections are protected by constructing bed girdles between the consolidation work, but it is not clear that the bed girdles can play similar roles on bends. It is known that on river bends with gentle slope, vane works effectively prevent the erosion of riverbanks and the local scouring of river bends. However, whether this method can be applied to steep slope rivers is still not well established. This experimental study examined the following issues focussing on the curvature radius to river channel width ratio (R/B) and the river channel width to water depth ratio (B/H), on the assumption that there are the factors with significant effects on the local scouring. At first, the effects of R/B and B/H on the scouring depth in a bend were clarified and method for determining it was proposed. Next, a comparison of the local scouring with and without bed girdles was performed to examine the function of bed girdles in local scouring reduction, thereby, demonstrating the limits of applicability of bed girdles. Finally, the functions of vane works on a bend in steep slope rivers were studied, proposing their shape and installation method (e. g. height, length, installation angle, and interval) .
In order to control wind-blown sand on beaches foredunes have commonly been constructed using sand fences. Recently artificial embankments are often employed instead foredunes because construction of the embankment can be rapidly and economically done by using large earth-moving construction machines. However, the functioning of embankments for wind-break effect and wind-blown sand control is not fully understood yet. Thus, using a blown-off type wind tunnel, which was 1.1 m high, 1.0 m wide and 20 m long, the wind field around model embankments was examined. Two series of embankments were included in the experiments. One series involved trapezoidal type embankments where the foreslope was changed between 1:0, 1:0.5, 1:1, 1:2, 1:3, and 1:5 keeping the ratio of B/H=1.0 (where B is the crown width and H is the height of the embankment) and a backslope of 1:1. The other series encompassed rectangular type embankments with varying ratios of B/H=0.05, 0.5, 1.0, and 2.0. Two main types of measurements were carried out, namely a tracer experiment to visualize the flow pattern and measurements of the wind speed field. The experiments produced the following main results : To keep a large area where the wind speed decreases on the downwind side of the embankment, the foreslope of the embankment should be steeper than 1:1 and the ratio of B/H should be smaller than 1.0. An area where the wind accelerated appeared behind the embankment within elevations from H to 5H. The area of acceleration considerably decreased when the foreslope became gentler than 1:1 and ratio of B/H became larger than 1.0. The wind speed amplification at the upwind edge of embankment crown was the largest when the foreslope of embankment had 1:1 and the width of crown did not give influence to the wind speed amplification.
During the process of initiation and motion of a debris flow triggered by a landslide, failure of clay blocks (megablocks) of in the landslide developed. It is important to clarify the mechanism of this failure in order to develop a model for this type of debris flow. We studied the fracture of megablocks by surveying deposits of the debris flow that occurred at Hachimantai, Akita Prefecture in May 1997. This study revealed the following. 1) The Landslide C block contained well-weathered tuffs as a source of megablock. 2) The debris flow deposits included many megablocks of several meters in diameter, slightly smaller than the depth of the deposits. The megablocks found in the downstream area had a larger degree of failure. 3) The typical form of failure was due to shear. Thus, as a landslide or debris flow moved downward, shear failure of the megablocks developed. The matrix became fine-grained as a result of the failure of megablocks. 4) The depth of failure (δ) of megablocks was roughly proportional to the distance from the landslide area to the position of a trench (x).
How sediment was controlled by a slit sabo dam was reported based on data obtained from the September '98 storm which hit the Yunoiri-sawa basin, a branch of the Matsukawa River. Photogrephs and survey data taken before and after the event were analyzed with the following results. The slit sabo dam checked the sediment during the storm and released it probably during the recession period of the storm. The sediment newly deposited was easily and quickly eroded away during and after the storm. The wings of an ordinary closed typed sabo dam built downstream of the Yunoiri-sawa also worked as a slit sabo dam.
Slope type fishways are more suitable for various species than classic types : “pool fish ladders”. However, in many cases, they need discharge control to maintain fish passage conditions. In this situation, experiments related to fish behavior with various “discharges” and “pool lengths” have been scrutinized to extract the basic information. Following the former study, the desirable minimum flow depth for swimming and critical flow velocity are studied. Finally, the critical relationship for the swimming behavior between flow depth and velocity was obtained, and it was also found that the enough pool existence along slope type stream is needed. With these results, hydraulic conditions in sample slope type fishways are investigated in situ by mean of velocity, water depth, turbulent intensity “It”, and pool length. A sample fishway with concrete blocks doesn't have suitable hydraulic conditions for the passage of fish, but the turbulence intensity was found below the natural habitat conditions.