The management of runoff and sediment discharge in a mountain watershed requires a watershed-management system(WMS) that predicts runoff and sediment discharge in any stream section under given rainfall conditions. As part of this system, a rainfall-runoff model has been developed. As an example, the Sumiyoshi River on Mount Rokko has been selected. This paper discusses a sediment discharge model and a deformation of riverbed model as part of this system. We examined a WMS consisting of a rainfall-runoff model, a sediment discharge model and a deformation of riverbed model. The WMS was used to express almost exactly the movement of water and sediment in a mountain watershed, using the Sumiyoshi River as an example. 1) The mountain watershed was divided into many reaches with 0-order basins. Sediment discharge was evaluated as debris flow, not as shallow landslides like it was in most previous models. The discharge rate of landslides did not need to be considered with this method. 2) A rainfall-runoff model based on a kinematic wave method, the model which multilayered the intermediate flow layer in the upper layer and a lower layer was introduced, and the water-holding capacity of mountain land was modeled. 3) Sediment discharge was calculated by assuming erosion or debris flow based on the hydraulic quantity of 0-order basins. 4) The deformation of riverbed was calculated by considering the sediment discharge from the valley part computed by the sediment discharge model. And the influence of sediment passing over a Sabo dam, the change in water level caused by the Sabo dam, and the effect that a sediment discharge has in terms of reducing sediment removal were examined. The results showed that, each model reproduced the sediment transport observations of the Sumiyoshi River satisfactorily.

View full abstract

Index of freeze and thaw intensity was calculated by a thermal conductivity analysis using meteorological and topological input data for the Kizu River basin. In the analysis, all the detectable bare slopes were extracted by aerial photographs and the topological factors such as area, altitude, gradient and direction of each bare slope were obtained by GIS technique. Annual sediment inputs to five dam reservoirs constructed on tributaries of the Kizu rivers were calculated from change of topography in the reservoirs. Correlation between the index of freeze and thaw intensity and the annual sediment inputs were analyzed and the clear correlation indicates the availability of the freeze and thaw index estimating the sediment production and discharge. The sediment production and discharge from the total watershed area of the Kizu river was estimated to be 14,800 m³/year by the correlation, it seems to be overestimated comparing to the value by a past research. The method estimating the sediment production from a total river watershed can be improved by a higher accuracy method of bare slopes extraction and including a distinction of geology in the estimation.

View full abstract

There have been many previous experiments on sediment transport process in mountainous rivers. To measure the sediment discharge in a river, it is hard to catch and weigh transported sediment directly and consecutively. Therefore many indirectly measuring methods have been discussed and proposed. Especially, hydrophone fixed on river bed, and method using acoustic signal of hydrophone with sediment particles collision are widely used. However, specific and versatile method to analyze sediment discharge has not been developed. In order to develop them, it is necessary to make clear the relationship between amplitude of the hydrophone acoustic signal and the momentum of a sediment particle. The authors conducted a series of experiments on acoustic characteristics of sediment particles collisions with a hydrophone. Experiments results are as follows. (1) Amplitude of circumferential mode is linear to almost 0.7 powered of the momentum of moving particle, and amplitude of radial mode is linear to almost 2.1 powered of the momentum of moving particle. Therefore when only one amplification step is available, circumferential mode would be better. (2) Momentum of moving particle strongly receives influence of the movement speed of the particle. Moreover, the proportionality relation of particle diameter and amplitude is reversed above a certain domain. (3) The particle size distribution width measurable with one hydrophone was approximately 13 times. (4) Time duration of reverberant sound indicates that detection limit particle number was 31 sec-1. Detection rate was high when number of collision particle is smaller than 31 sec-1, and detection rate decreased rapidly when number of collision particle is larger. Therefore, it is necessary to consider the particle diameter when setting hydrophone pipe length. (5) Applying several hydrophones with different characteristic(e.g. fixation method, pipe thickness), wide particle size distribution width will be measurable. Furthermore, this method can cancel the relation of particle diameter and amplitude reverse.

View full abstract

A large landslide dam, around 100 m high, resulted from a deep-seated landslide that followed heavy rainfall during Typhoon No. 12 in September 2011 in the Kuridaira area, Totsukawa Village, Nara Prefecture. Although the Kii Mountain District Sabo Office had completed construction of a temporary overflow channel as a part of urgent countermeasures, the channel was damaged by erosion caused by the heavy rainfall (230 mm/day) during Typhoon No.17 in September 2012. Following this event, we studied the damage mechanisms by analyzing CCTV images and concluded that the flow in the temporary channel, with a head difference of over 90 m, scoured the foot of the channel slope. As result, the temporary channel was fatally damaged. Considering this finding, we have reassessed the countermeasures for large-scale landslide dams with high head differences and steep slopes. We conclude that, following the formation of landslide dams, it is necessary to construct not only an overflow channel, but also a sabo dam at the foot of the overflow channel to prevent scouring at the foot of the channel. It is also necessary to reduce the height of landslide dams by excavating the crown to reduce the water level.

View full abstract

Due to record-level rainfalls in the Tohoku region in late July and on August 9 th of 2013, landslides have occurred in Yamagata, Akita, and Iwate prefectures. Slope failures and debris flows have occurred intensively in these areas, becoming a tremendous calamity. From the torrential downpours, many areas throughout Tohoku recorded unprecedented rainfall levels. In particular, six people were tragically killed in the Kuyobutsu area of Semboku City (Akita Prefecture), when several homes were buried in a mudslide. Because there had been little rainfall in the Tohoku region up to this point, and because the region's rivers are narrow, there were many areas in which the damage was made even more extensive. In addition, many trees were swept up in the floodwaters, further compounding the damage to many areas.

View full abstract

The Sabo(Erosion Control)Act and the River Act, which date back to 1897 and 1896 respectively, constitute a basic legal framework for water-related disaster prevention policies in Japan. Added to the Forest Act, legislated in 1897, they have sometimes been labelled “three acts on flood control ”. Being one of the oldest piece of Japanese legislation in effect, the Sabo Act preserves, for the most part, its original contents and style, but the historical facts and circumstances which led to its enactment remain overall unclear. Referring to the process of the enactment of the River and Forest Acts and the political and economic background in that era, this study seeks to clarify the significance of the institution of the Sabo Act, which includes integration of flood control policies in lower and upper reaches of rivers, incorporation of regulatory measures to Sabo scheme and facilitation of financial aid to local authorities.

View full abstract