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A study of environmental issues regarding prolonged turbid water in a large-scale dam reservoir

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Observed catchments and Geology


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Concept of the prediction model
of suspended-solids outflow


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Predicted amounts
of suspended solids


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Accumulation of suspended-solid
outflow in the whole study area.


ABSTRACT:
Recently, prolonged water turbidity induced by heavy rainfall has been identified at many reservoirs, and the effects on the environment in downstream regions due to discharge of the turbid water have been a serious problem. The retention of turbid water is thought to be caused by the existence of suspended solids generated in the upper river basin, which are then carried into the reservoir by rivers.

In this research, possible factors of water turbidity were extracted and quantitatively analyzed using a geographical information system. Additionally, the water level and turbidity were measured in the upper river basins of a dam located in southeast Kyushu Island. Using the collected data, a model that evaluates the amount of suspended-solid outflow was developed by modifying the tank model. Using this model, the researchers were able to quantitatively determine the efflux characteristics of suspended solids in the catchment area.

DETAILED:
In the recent years, prolonged outflows of turbid water from dams after intense precipitation, such as occurs during a typhoon, have become an environmental problem. To solve this problem, several studies on river turbidity have been conducted by dam management agencies. However, none of the research on turbidity has investigated the relationship between turbidity currents and the topographical, geological and land-cover factors in the research area to clarify the source of the turbidity currents. This study extracted as many potentially influential factors as possible, and developed a comprehensive model to characterize and quantify the turbidity currents and to analyze their causative factors. This was done by applying remote-sensing technology, a geographic information system (GIS) spatial analysis method, a statistical approach, and a hydrological model of the study area, which is located in southeastern Kyushu. Since continuous turbidity currents have been identified in the study area for several years, continuous monitoring of turbidity and analysis of its contributing factors are important. These measures are needed to determine the potential deposition of sediment in the reservoir and downstream areas. Hydrological characteristics can then be determined through monitoring and analysis. By monitoring turbidity, the dynamics of the turbidity outflow quantity and its correlation with other data can be obtained.

Furthermore, by analyzing historical data on turbidity and using identified mechanical factors, important mechanical factors that contribute to the existence of the turbidity current can be analyzed. This in turn is vital to effective watershed management, and to the prediction of suspended-solid production in the entire watershed. On the basis of these results, such as the relationship between factors of turbid-water occurrence and efflux and the distribution characteristics of suspended solids, a hazard map of turbid-water outflows can be drawn and the potential source of turbidity currents can be pinpointed. In particular, the potential risk of suspended-solid outflow in the upper river basin of the reservoir can be quantitatively evaluated on the basis of the spatial distribution of quantified mechanical factors.

Environmental Geotechnology Laboratory
Professor Yasuhiro Mitani
Assistant Professor Hiro Ikemi
Assistant Professor Ibrahim Djamaluddin
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