Water flow from a horizontal tunnel in an unsaturated zone

Abstract

Flow from a horizontal tunnel in an unsaturated zone is an important factor in calculation of water loss from the conveying tunnel and in predicting the amount of water table build up. This study has investigated the three-dimensional unsaturated flow from a tunnel by using the linearized Richards' equation. It superposes the point source solution along the tunnel axis to yield the line source solution. For treating the tunnel surface as a saturated-water-content boundary, this study first provides an empirical function to numerically calculate the flux distribution along the tunnel axis. It then provides an analytical solution to calculate flow from the tunnel. We find that the recharge rate from a horizontal tunnel decreases with time and gradually approaches a steady-state rate. The recharge increases with increasing tunnel length and radius, and also with increasing the pore size distribution parameter of the soil. The vertical downward recharge beneath the tunnel becomes constant and equals the total recharge from the tunnel at late time.