INTERACTION PROCESS BETWEEN STREAM AND AQUIFER IN ALLUVIAL FAN OF THE YASU RIVER

摘要

Stream-water not only flows in a river channel but also has some relationship with riparian groundwater through the influent seepage into a river-bed or the effluent seepage from an aquifer. Although such exchange quantity between stream and aquifer may be relatively small in comparison with the stream discharge in the rainy season, the interaction between both is one of the important factors governing the stream flow condition with small discharge in the dry season and/or in the downstream from the intake dam. Because a remarkable decrease in the discharge affects both water quantity and quality and temperature, it may be necessary to keep a minimum discharge in order to preserve the river environment including ecological integrity. In this paper, for evaluation of the minimum discharge, the stream-aquifer interaction and its effects on the stream flow condition are investigated through field observations and a hydraulic model. According to the observations in an alluvial fan of the Yasu River in Japan, it is thought that variation of the stream discharge may be caused by the water exchange with aquifer. The findings from the hydraulic computations indicate that the model is a useful tool for quantitative evaluation of the minimum discharge required to maintain suitable stream flow conditions.