| Budget Amount *help |
¥3,220,000 (Direct Cost: ¥3,100,000、Indirect Cost: ¥120,000)
Fiscal Year 2007: ¥520,000 (Direct Cost: ¥400,000、Indirect Cost: ¥120,000)
Fiscal Year 2006: ¥600,000 (Direct Cost: ¥600,000)
Fiscal Year 2005: ¥2,100,000 (Direct Cost: ¥2,100,000)
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| Research Abstract |
The Mizunashi river, located in the southern part of Niigata prefecture, is one of the rivers where slushflows often occur. The middle and lower reaches of the river flows in the alluvial fan and joins the Uono river In order to make clear hydrogeological and hydrometeorological backgrounds lurking behind the repeated occurrence of slushflow, hydrological observations, including water levels of the stream and groundwater levels, glaciological observations and subsurface structure investigations were carried out in this river The following are main results obtained.
1. Disappearance of surface water and snow accumulation on river bed
At the lower edge of the fan, the surface water often disappears entirely from the river channel. The position where surface water disappears depends on the discharge at the apex of fan. Since the apex discharge decreases to 2 m^3/s or less during the snow season, the snow accumulation on the river bed is possible between the confluence point and 5 km point t
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o the upstream side. This fills one of the necessary conditions of slushflow occurrence.
2. Subsurface structure and groundwater level
Electric prospecting surveys and existing boring data permitted the estimation of subsurface structure along the river. The water table becomes the deepest at the 45 km point in concert with the drastic drop of basement It appears certain that this structure contributes to the disappearance of surface water mentioned above.
3. Fluctuations of river water level during the snow season and slushflow releases
Measurements of river water level showed that water flowed continuously throughout the winter at the apex of fan. Meanwhile, surface water completely disappeared from the beginning of snow season to midwinter When large quantity of water was supplied to the river basin by the passage of low-pressure systems in winter, sudden emergence of surface water and rapid rise of water level were observed several times at the middle and lower reaches of the river where its bed was covered with snow. At those cases, after the entire snowpack had been saturated with water in several minutes, water was flowing over the snow surface over 5-10 m in width and the slushflow released in some cases.
4. Conclusions and future issues
Slushflows in the Mizunashi river are characterized by hydrological environments peculiar to alluvial fans, a extremely-low slope angle and a lack of impermeable bed surface, such as permafrost, ice and rock. Aiming at the development of the forecast method of slushflows in the Mizunashi river, it is necessary to develop the behavior model of surface water in which interaction relationships between groundwater and river water must be considered It is likely that the frequency of rainfall in midwinter is increasing in Japan due to effects of global warming; the clarification of slushflow may become important more and more. Less
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