Desertification becomes significant problems in the grazing land in semi-arid Mongolia. The purpose of this study is to evaluate the present soil erosion processes using Cs-137. We investigated two small watersheds (BGN : underlain by granite, vegetation cover in rainy season is 70%, KBU : underlain by sedimentary rocks, vegetation cover in rainy season is 30%). In the undisturbed soils, most of the Cs-137 is concentrated uppermost layer and the depth distribution exhibits an exponential decline with depth. Using Cs-137, we estimated the depth, rates and processes of soil redistribution by comparing the reference value at the reference site. The soil erosion rate for the past 40 years using Profile-Distribution Model was estimated as net erosion rate in BGN watershed of 0.3 t/ha/y, and the sediment delivery ratio of 12%. In contrast, in KBU watershed, the net erosion rate is estimated as 3.8 t/ha/y, which is about 13 times as large as BGN watershed, and sediment delivery ratio is estimated as 78%. The significant differences for many phenomena in connection with soil erosion were found between watersheds with different vegetation cover.
Abstract Although many sabo dams have been constructed in mountainous riparian zones, few studies have examined the subsequent transition of riparian vegetation as a result of sediment deposition. In the future, the maintenance of sabo dams and their sediment trap areas must consider the effects of invading vegetation on the river environment, while ensuring the dams' disaster-prevention function. This study investigated the long-term vegetative invasion of sediment trap areas following the construction of sabo dams from time-series aerial photographs, and examined related factors. The study sites were the sediment trap areas of 27 sabo dams in the Mogami River basin, in Yamagata Prefecture, Japan. We found that vegetation invaded the sediment trap areas of 23 of the sabo dams. The vegetation invasion could be classified into six types, according to the timing of invasion or the dynamics following invasion, type-A had no vegetation invasion because of impounded water ; type-B had vegetation invasion before the dam pocket was filled ; type-C 1 had vegetation invasion as soon as the dam pocket was filled; type-C 2 had lost the invading vegetation ; type-C 3 took several years for the vegetation invasion after the dam pocket was filled ; and type-D had no vegetation invasion because of constant sediment transport. These invasion types clearly corresponded to the number of years since the dam pocket was filled (Y). The degree of sediment transport, which affects the ability of vegetation to invade the sediment trap area, is a function of the sediment trap capacity (V) and average annual sedimentation volume (Qd)(or V, watershed area (A) and average specific annual sedimentation volume (Qd*)). These factors result in the vegetation invasion having the same value of Y.
Natural mountainous rivers with step-pool bed form provide good habitats for fish. However, once too much sediment is naturally or artificially supplied there, the habitats must be damaged by the severe sediment deposition. The following clear water flow tries to restore the habitat, but the impact could continue until the bed condition is back to the previous situation. Important information on impact evaluation of the sediment movement is the deposition volume in pools, the erosion volume and the time necessary to be thoroughly restored. The aim of this paper is to make the restoration process from a fully sediment deposition clear. Firstly, a field experiment on artificial sediment flushing from a check dam was carried out in the Hiru-dani torrent. The restoration process from fully sediment deposition in the pools was monitored over one year. As a result, the pools had the different restoration rate and some pools did not return to the previous situation. This is because restoration process depends on the experienced water discharges. Then, a restoration process was analyzed with a focus on the bottom velocity in pools and a method for evaluating the restoration rate was presented. The method could explain the restoration process in the Hiru-dani torrent.
To evaluate the impact of sediment flushing on fish, we need to predict the physiological impact of turbid water. For some indices used to evaluate the impact, the concentrations of suspended solids and dissolved oxygen are very im-portant. In this study, we clarified the mechanism of the physiological impact of turbid water on fish and constructed a model that calculates the rates of mortality, or asphyxiation, based on the concentrations of suspended solids and dissolved oxygen. First, we examined the temporal variation in the concentrations of suspended solids and dissolved oxygen at three sites subject to sediment flushing. Then, we conducted two tank experiments. The first investigated the relationship between the concentration of suspended solids and the mortality rate of char with sufficient dissolved oxygen, and the second examined the physiological impact of an increase in the concentration of suspended solids. Third, we modeled the results of the tank experiments. We postulated that as the concentration of suspended solids increases, the volume of oxygen absorbed decreases as the gills become covered with sediment. Finally, we examined the physiological impact of sediment flushes at the three sites using the model. The tank experiments showed that an increase in the concentration of suspended solids and a decrease in the concentration of dissolved oxygen both lead to asphyxia. Given the temporal variation in the concentrations of suspended solids and dissolved oxygen, the model has the potential to roughly estimate the rates of mortality or asphyxiation of fish.
Many slope failures and landslides occurred in the focal region by the mid Niigata prefecture earthquake, 2004. Along the River Imo several landslide dams were formed and the channel were occluded. The mid Niigata prefecture area is a very eminent area of heavy snowfall and especially the maximum depth of snow of the basin of the River Imo which suffered an enormous quake damage exceeds 3 m. Therefore, the enough attention against sediment movement which will occur during snow thawing season is necessary. From the field investigation, though remarkable new slope failures and expanded old slope failures were not found, unstable sediment which was carried by snowmelt water from the upper part of slope is accumulating at the foot of slope failures by the earthquake. There is a great possibility that the unstable sediment will be moved rapidly again at the time of rainy season or typhoon and develop into debris flows. Therefore, it is necessary to intensify a monitoring system against sudden sediment movement and aggradation at the lower reaches in the flood season.