Flood control and water utilization have been considered as important factors in river works for some time now. However, in recent years, there is an emphasis on the Neo-Natural River Reconstruction Method which takes into account the biological environment of the river. The Ministry of Construction has been promoting this method since 1990, and has implemented it in the whole country. This Neo-Natural River Reconstruction Method recognize the biological environment and preserve a natural landscape. Therefore, it is possible to evaluate the method by observing the process of environment recovery after the structure is completed. Unfortunately, there have been no studies to observe this environment recovery process after the completion of the structure. Therefore, we had observed characteristic of gravel in each different riverbed condition point in natural riverbeds, and artificial riverbeds. As a result, we have concluded that this filed has various distribution of riverbeds gravel than natural riverbed, and total displacement length of gravel tracer was within 300 cm when high water level at May in 2006. In this paper, we describe evaluation of riverbeds gravel in stream preservation works of Neo-Natural River Reconstruction Method in Gifu prefecture.

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Rainfall, runoff and sediment yield were observed at four small watersheds to study difference of the sediment yield characteristics with natural recovery of vegetation in granite hilly mountain. These watersheds are forested watershed (A watershed), vegetation recovery watershed (B watershed), 85% forest recovered watershed (C watershed) and 50% forest recovered watershed (D watershed). The soil depth of A watershed is deeper than those of three other watersheds and that of C watershed is especially shallow. Perennial flow has observed at A watershed while ephemeral flow has observed at three other watersheds. High peak discharge and much sediment discharge have observed at D watershed while low peak discharge and little sediment discharge have observed at A watershed. In all watersheds, increasing of peak rainfall was followed by increase of peak discharge. The rate of increase in peak discharge to increase of peak rainfall of B watershed and D watershed is similar. Because the relationship between peak rain and peak discharge is positive, the sediment yield increased as the peak discharge increased in B, C and D watersheds. The runoff characteristics of B watershed were similar to that of D watershed, but sediment yield characteristics of B watershed was the middle of that of A watershed and D watershed. Total sediment discharge decreased with vegetation recovery. The change of the sediment yield characteristics is found to be changed with vegetation recovery, but more time is required for runoff response recovery than the surface vegetation change.

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Operational observation of sediment discharge in steep ravines is pivotal to make timely flash flood warnings as well as to build robust erosion control plans for effective disaster mitigation. A hydrophone sediment discharge measuring system (hereafter “hydrophone system”) has been installed in a 200 km²-scale river basin in order to develop a practical method for bed load observation. The hydrophone system counts the times that bed load sediments strike the acoustic sensor of the system. Five amplification levels (hereafter “channel”) can be tuned simultaneously to the same sound signal from the acoustic sensor to examine an appropriate set of the amplifications. A direct sediment discharge sampling apparatus was installed at the right upstream of the hydrophone acoustic sensor to calibrate its measuring. The observation of flood-induced sediment discharge shows that sediment discharge behaves in variance with water discharge as floods come about each season. In one example, sediment discharge fell in spite of steady rising water level, while it spiked sharply with water level plummeting at the latter part of the flood. Sediment supply in the river channel seems to be used up during the snow melt season, which reduced the sediment discharge in response to the rising water level. The relationship between sediment discharge and water level/discharge changes significantly as sediment supply is recharged at the outset of the flood season. A way of quantitative estimation of sediment discharge from the hydrophone system was suggested and compared with that of theoretical estimation based on hydraulic variables. Our result indicates that amplification level 16 for sediment discharge and amplification level 1024 for suspended load are proper for the upper Tedorigawa river basin.

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Debris flows occur when enough water supplied into materials on torrential streams. The occurrence criteria of debris flow has been evaluated with rainfall. However, the critical rainfalls triggering off debris flow may be differ by the supply process of water into materials. Material has been as riverbed deposit before the rainfall, or is supplied by slope failures and landslides during the rainfall. The authors selected Nishinogaito torrent for the test field, where debris flows has occurred frequently in these several years. The authors analyze the critical rainfalls based on the supply process of materials. In the Nishinogaito torrent, the material has been supplied before rainfall. Analyzing the accumulated rainfall within the time of concentration, it was estimated to be ten minutes and the critical rainfalls triggering debris flow in the Nishinogaito torrent was found to be 17 mm per ten minutes. From the result, we described a possibility to estimate the supply process of materials and or its occurrence process of debris flow from analyzing the critical rainfalls.

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Amid growing concerns over the natural environment and scenery of slopes in recent years, the trees' effect of reducing slope failure sediment movement has been drawing more attention, and more studies have been conducted aimed to establish its quantitative evaluation. To calculate the distance of slope failure sediment movement, we have presented equations, in this paper, which include the effect of trees as a resistive element based upon the existing slope failure sediment movement models. To indicate the trees' effect in the equations, we obtained its coefficient values, using the results of experiments with buffer forest zone models. Further, picking up some real cases of slope failure sediment movement that had actually occurred on a slope with trees, we calculated the distance of each slope failure sediment movement by using the equations. The calculation results indicated that in two cases where the lower part of the slope was rather gentle with an incline of 10 degrees, the sediment movement would stop halfway. This was found right since the calculated value on the sediment movement distance was relatively close to the actual measurement. Meanwhile, it was also learned that in other four cases in which the slope was evenly inclined within the range from 33 to 36 degrees, the sediment movement would not stop.

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In this paper, we report the results of field measurement on transport and deposition of large woody debris at rainfall in the Kanaso Creek, Miyake Island. Mud flows and floods are prone to occur in the Kanaso Creek, because the watershed were devastated by deposition of volcanic ash and affected by volcanic gas after the eruption of Mt. Oyama in 2000. At the lower reach of Kanaso Creek, large number of large woody debris were transported frequently at normal rainfall about 25 mm/h which occur several times every year on average. The number of transported large woody debris was greater during high intense storm events. The maximum length of large woody debris transported during the observed period was relatively smaller than channel width. The maximum length of large woody debris deposited at the upper of the Kanaso Creek. Percentage of deposited large woody debris with orientation ranging from 60 to 90° was two fold greater than values reported by Mizuhara (1979).

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As many as 19 landslide dams have been formed in the northern region of Nagano prefecture, central Japan, in last 500 years except one case. Of this number, seven were formed when the Zenkoji Earthquake occurred in 1847. This abundance is likely because of the geotectonic background of this area which is located at the western end of the “Fossa Magna, ” or Japan's central graben belt.
The Tobata landslide occurred in the early morning of June 24, 1757 due to heavy rain. Blocking the Azusa River, which is upstream of the Shinano River, a landslide dam with a height of 130 m and a storage capacity of 85 million m³ was formed. Around 10 a.m. on the third day (54 hours later), this landslide dam burst and its water flooded the area up to the confluence with the Narai River. According to calculation using the Manning's formula, it is estimated that the flood water ran down the river in a concentrated path with a velocity of 12 m/s and a peak flow of 27, 000 m³/s.
When the dam burst, local people were quickly ordered to evacuate and no casualties were caused during this flood.

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