Abstract In most cases, debris flow tends to form a bore having a swelled front. Though bore is a nonsteady flow in itself, it can be treated as a steady flow in the moving coordinate system which moves the same speed with that of bore. To make a steady bore )n a laboratory, a belt-conveyor type channel with movable bed was structured. By setting up the belt speed to that of bore front, a static debris bore can be obtained in the channel. Flow of zero ; mean velocity is also obtainable by setting u p the belt speed to the mean flow velocity against a selected slope. In these flow, the detailed and long time observation of grain behaviour is available with enhanced measurement accu- racy. The results of the experiments on sediment-laden flow and debris flow using the belt conveyor channel are as follows (1) In a torrential stream, the gravels of a certain size within a certain range move faster than the mean flow velocity, while the grains larger and finer than this range move more slowly than the mean flow velocity. As the result, the sorting of grains is seen to occur in the direction of flow when the grains of nonuniform size are put into the flow. (2) Though the velocity distribution of debris flow seems uniform and looks like the Bingham flow in the fixed coordinate system, it is, as apparent from Fig. 5, not the Bingham flow but a shear flow. (3) The mean concentration of debris front is almost unvaried regardless of an angle of bed and a bore velocity, as far as the experiments is concerned. The distribution of ; concentration His uniform except in the vicinity of the bed where, contrary to the case of suspended sediment, the concentra- tion is lower than in the upper part. 4) At the front, as a large grain is more resistive to beeing caught in the flow than a small one, it tends to stay there. This is the reason for the occurence of large-grain concentration at the front. (5) The velocity of a debris bore can be calculated by the use of the equation of ideal hydraulic bore.

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This paper aims that we make clear the effects of some factors (such as the weight of the stone, the height of the dam, the angle of downstream slope so on) against the impact-wear of concrete surface caused by the sediment overflowing the sabo-dam. The results of experiments are as follows:
1), The relations between amount of the impact-wear on the concrete surface (M) and the weight of collisions (W), and between (M) and the falling height (H) are shown respectively by following equations M=aW^m, M=bHⁿ. In these case m takes 0.8-1.1 and n takes 0.8-1.9 depending on the materials of the concrete. Then, the amount of the impact-wear in proportion the kinetic energy of colliding sediment.
2), The relation between the weight of collisions (W) and the number of colliding times (N) which cause a certain amount of the impact-wear is shown the following equation, N·W^p=constant, p takes generally 0.8-1.1 (≈1.0). It shows that amount of the impact-wear are caused by the total weight of the sediment regardless of the size.
3), The ratio of the wear in the case of inclined to the holizontal place (R_M) will be estimated by the following equation, R_M=(1+Bsin²θ)cos²θ. θ is the inclination angle of the concrete surface, B is the proportional constant and takes 0.9-1.6 in this case.

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The danger index to forecast the outbreaking time of slope failures calculated from rainfall record has been searched and tested using two records of slope failures. One of the records is about slope failures in Kure City, Hiroshima Pref. reported by Fire Survice of Kure City. The other record is about slope failures in several prefectures, Hiroshima, Okayama, Yamaguchi and Ehime Pref. reported by the Ministry of Construction.
Attempts have been made to use the antecedent precipitation index and the storage amount of the Tank Model as a danger index of slope failures. These indexes are calculated from hourly rainfall record of the nearest observation station.
The best half life of the antecedent precipitation index for the forecasting of outbreaking condition is 24 hours. About the data of slope failures in Kure City (14 years, number of slope failures. 2690), 96 percent of the cases occured above the critical value, 55mm. The total period when the value of the index is greater than the critical value is about 90 hour/year under the weather condition of Kure. Slope failures occur in one hour out of 5 hours when the value of the index is above the critical value in Kure City.
These results show the antecedent precipitation index whose half life is 24 hours is available as a danger index to forecast slope failures. It is shown the storage amount of the Tank Model is also available as a danger index.
Using these indexes, the dangerous condition about 90 percent of the slope failures in Kure City can be pointed out more than one hour before the time of outbreaking. In this case, the dangerous period is about 90 hour/year.
The antecedent precipitation index with the same critical value in Kure City is available as the danger index of slope failures also about the records occured in each prefecture, Hiroshima, Okayama, Yamaguchi and Ehime.

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Alternate bars are apt to be formed in torrential rivers in alluvial fans and meandering and local scouring due to the bar formation often do damages to channel works.
Control measures against bars which have detrimental effects on channels are studied by means of hydraulic experiments, and cross dykes such as bed girdle and ground sill prove to be effective.
Cross dykes arrangement, footing depth and a height of revetment are discussed taking bars formation mechanisms into consideration.
1) The correlation between H_smax and ΔH_smax is as follows;
ΔH_smax=0.8 H_smax
where
H_smax is the height of alternate bars
and
ΔH_smax is a scouring depth.
2) A height of bars varies in proportion to the interval of cross dykes.
The optimum interval is approximately two (2) times of a channel width in the region on 5≤B/√Q≤10,
where
B is a channel width and Q is a designed discharge.
3) A length and height of bars can be decreased by the use of ground sills.
4) A height of bars and a scouring depth can be decreased by the use of bed girdles in the region of the transition between alternate bars and multiple bars under the condition of L/B=1.5, where L is a interval of bed girdles.

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