Suspended Substance Carried by Snow Melt Runoff in the Mogami River Basin.
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概要
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The Mogami River basin enclosed by high mountains of Asahi, Iide and Ohu, has snow melt runoffs which begin at the middle of March and last about two months. The total outflow of the runoffs occupies about 40% of the annual outflow, becoming a major resource of the water supply. The runoff also transports huge amount of various materials suspended or solved in the water. Illuminating the suspended substance in the runoff, this study analyses the temporal and spatial characteristics of the concentration and the total flux of the transport. The observation was performed on May 2, 1996 by using hand made samplers specially designed for the high speed flow, covering the confluence of five major branches. NOAA AVHRR data, and time sequential data of the discharge and turbidity have been collected to examine the snow covered area, and temporal characteristic, respectively. Spatial distributions of concentration and sediment transport indicate that the catchment of Nagasaki, which includes Iide and Asahi mountains having wide snow covered area, yields dominant amount of the wash load. Time sequences of the concentration and discharge at Shimono result in the suspended substance transport integrated in April and May 1996 of about 1.5E8kg. It is shown that the concentration is proportional to 1.44 power of the discharge, but the coefficient of proportionality decreases for the later runoff. This phenomena may be interpreted that the decrease of wash load material in the basin by washout influences the concentration. This mechanism is modeled to reproduce the concentration by using only the time history of the discharge.
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