山体トンネル湧水と雨水浸透

Abstract

To elucidate the recharge mechanism due to infiltration of rainwater into anaquifer in a mountainous watershed, discharge of seepage into a tunnel, which issited at Yura, Wakayama, is measured at four locations (A-1 to A-3 and B) sinceMarch 1988. The mountain is composed of fissured rocks, that is, A and B locationsare mainly composed of sandy rock and chart, respectively. Electric conductivity ofthe seepage water is, in particular, measured in A-3 location since June 1989.Based on analysis about observed data, the following conclusions were reached. 1)Infiltrating water moves downward through two domains. One is a flow through alarge fractured space (fissure flow). The other is a flow through a small fracturedrock (matrix flow). The fissure flow is speedy and contributes to increase ofseepage discharge just after rainfall event occurs. The matrix flow shows slowmovement and contributes to the base flow of the seepage. 2) Filter velocity, q, ofthe fissure flow in A-3 location can be given by that q=a9n, where 9 is the watercontent and a and n are constant. Value of a in cold season is larger than that inhot season, though n is always kept 2. Therefore, the recharge process due to fissureflow can be analyzed through kinematic wave theory. 3) Water-redistributionprocess in matrix in A-3 location after rain-infiltration may be also analyzed bykinematic wave model. The hydraulic conductivity may be given by am9n, where n= 2 and am is around 1/20 times as large as a for fissure flow in hot season. 4)Fissure flow becomes dominant to the seepage in B location when it rains over 20mm, but the observed seepage does not persist over a week, so that seepage causedby matrix flow is not large in magnitude. The fissure flow is quite speedy so thatthe traveling time through a surface soil layer is dominantly larger than in thedownward rocks. On the other hand, in A location, seepage due to matrix flowappears at any season.