There are two important parameters governing suspended load concentration distribution. One is the ratio of a shear velocity to a falling velocity of sand particles. As the distribution proposed by Rouse (1937) shows, it is only a parameter included in the previous diffusion theory. Another is the distribution of source points of suspended particles. Higher source points result in a larger concentration. A sediment supply on the whole water surface produces the uniform concentration distribution. Previous diffusion theory has not contained the parameter concerning source points because it has been developed only for the sand particles already being in suspension. We have found that the Rouse's distribution does not agree with the concentration distribution measured in a steep channel with large roughness. One of the reasons is that the diffusivity is much higher than that in the case of mild slope channels, but another seems to be the lack of a concept of source points. In this paper we have pointed out the necessity and importance of source term in the diffusion equation. Then we have developed a new framework of analysis of suspended load concentration distribution, and presented a diffusion equation with a source term as a more general form for suspended load. This framework can remove the ambiguous points from the previous diffusion theory. A method for calculating the vertical distribution of source points of suspended load was also presented considering the sheltering effect of bed gravels. The calculation results on source points and sediment concentration have indicated that the presented model could express the effect of bed gravels on their distributions as well as their experimental ones.

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Rapid changes in bedload and suspended transport occur in mountainous torrents because of drastic changes of flow conditions with respect to precipitations. Although understandings of bedload and suspended sediment transport in steep mountainous torrents are important for estimating rate and extent of sediment transport in channel networks, relatively few field data have been collected compared to downstream counterparts. Therefore, the main objective of this study is to demonstrate bedload and suspended sediment transport based on intensive field samplings. Then, we discussed the mechanism of bedload and suspended sediment transport in a mountainous torrent. During the selected five storm events from 2000 to 2001, we collected bedload (by a mesh basket) and suspended sediment (grab sampling) with 5-15 minutes interval in the Yanagisawa River, Southern Shizuoka city. Precipitation and stream discharge were also monitored during the study period. All size classes of bedload sediment tend to show counterclockwise hystersis loop, whereas clockwise trend was predominant for suspended sediment. The shapes of suspended sediment loops differed to ones of bedload even in the same event. In counterclockwise hysteresis event of bedload, maximum rate of coarser sediment appeared sooner after the peak discharge and maximum rate of the finer sediment appeared relatively later to the peak discharge. The changes in the bedload rate of each grain size sediment suggested that the characteristics of bedload transport would be influenced by changes in grain size distribution of bed surface material. In contrast, clockwise hystersis loop of suspended sediment suggested that characteristics of suspended sediment would be affected by sediment supply conditions around the channel. Our findings will help to comprehend sediment dynamics in mountainous torrents as well as in channel networks for developing sediment control plan.

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Grid type sabo dam is commonly constructed in order to control the debris flow. The opening of grid dam is choked with big rocks included in the front part of the debris flow, and the dam can trap the following muddy flow as a closed type dam after the blockage of opening. In this paper, we proposed a new simulation method that can reproduce the one-dimensional riverbed variation in a mountainous river with a grid dam. In the model, we regard the grid dam as a closed type dam whose height varies in time through the blockage. The one-dimensional governing equations for a stony debris flow are employed and the sediment material is composed of two grain-size classes. Only the bigger sediment causes the stoppage of the grid dam opening, and increase the height of the dam. The temporal change of the dam height is estimated by using the probability of the blockage that is influenced by the sediment concentration, flow depth, sediment diameter and distance of columns of grid dam. We try to clarify the grid dam's function for the debris flow control by the simulation model. From the calculation, it is shown that the grid dam can reduce the peak sediment discharge of a debris flow more effectively than the closed type sabo dam.

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The standard rainfall of traffic regulation applied in practice in Yamaguchi Prefecture was set 30 years ago, therefore, its application to the extraordinarily heavy concentration rainfall occurred in recent years needs to be proved. The current standard is not very reliable in predicting the occurrence of disasters and the setting theory is not clarified. Moreover, the time to prepare the traffic regulation is not considered in the current standard, and there is not a clear standard to remove the traffic regulation. For these reasons, it is difficult to implement traffic regulation before the occurrence of a disaster. In this study, a new standard rainfall and the application methods are investigated by using 1989 to 2003 rainfall data. The radial basis function network (RBFN) is used to determine standard rainfall more accurately and more objectively, and the effective rainfall, which is often used as a standard rainfall for erosion and sediment control projects, was adopted as a rainfall index. The new standard rainfall turned out to be a more reliable than the current standard rainfall for the standard of an impending disaster, reducing the frequency of unnecessary preparations because of unreliable predictions. In this paper, the setting methods of the standard rainfall to start and to remove traffic regulation on the basis of rainfall predictions are proposed. This method of administration makes it possible to start traffic regulation well in advance of a potential disaster and to remove traffic regulation objectively.

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Many sediment disasters have been caused around the west parts of Japan by Typhoon No. 23 (Tokage). This paper mentioned about two districts where caused severe sediment disasters. One of the sediment disasters caused five casualties in Tamano City, Okayama Pref. and the other disaster caused three casualties in Sanuki City, Kagawa Pref. on 20^th October 2004. The cause of rise such casualties was regarded that these torrents were not anticipated as debris flow torrents. The volume of sediment discharge was rather small, comparing the heavy rain fall, and that is to say, the scouring river bed was intensified, when sediment supply to river channel was little, and it became a cause of collapses of bank protection works. And another thing, it was recognized that the slope failures in the Monnyu river catchments occurred mainly in about 15 years old artificial forest.

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On October 23, 2004, the Niigata-chuetsu earthquake hit the Niigata-chuetsu region of Niigata Prefecture, Japan causing many landslides along the Imokawa basin. Some of these landslides dammed off the flow of the Imokawa river. Wire-sensors and water level gauges were installed near these landslide dams to monitor the discharge rate of the river and to warn local residents living downstream concerning the dangers of flooding if the dams broke. In fact, a small dam burst on November 4, 2004. Its flood hydrograph was observed, and data was recorded. The flood discharge rate increased sharply for ten minutes and subsided over the next 50 minutes. Although not all possible situations are understood from the data, it did provide valuable data on predicting the peak discharge rate in the case where a landslide dam bursts.

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Hydrologic data such as rainfall, flow discharge rates and amounts of sediment transport are measured at Sabo offices. However, except for rainfall the data is not put to much practical use. In order to control mountain basins and mountain torrents, this type of data needs to be used. Herein is proposed an ideal system for collecting and applying hydrologic information. An example of data analyses is illustrated based on a storm that hit the Joganji river May 17, 2004. Hydrologic data; rainfall, flow discharge rate, bedload measured by using a hydrophone and suspended load sampled directly were obtained from the river. New sabo dams with some sort of a sediment control shutter are considered to be the ideal sabo structures and require all of this type of hydrologic information.

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