Analysis of transient seepage into equally spaced ditch drains from a ponded field

Abstract

An analytical solution is proposed for simulating transient seepage into a system of ditch drains placed in parallel in a homogeneous and anisotropic soil confined by an impermeable layer and receiving water from an infinitely extended ponded field. The solution assumes the ponding depth over the surface of the soil to be uniform and the soil fluid to be of constant density. The model can be applied for both equal and unequal water levels in adjacent drains. The accuracy of the developed solution is checked by comparing for a flow situation the hydraulic heads and streamlines as obtained from the proposed model with the corresponding results obtained from an available steady state solution of the problem as provided by Kirkham [14]. A numerical check on the proposed solution is also performed for a particular flow configuration of the problem in the transient zone. The study shows that flow in a ponded ditch drainage system is greatly influenced by anisotropy and directional conductivities of soils. The uniformity of surface flux distribution is found to be quite sensitive to the anisotropy ratio (the ratio of horizontal and vertical saturated hydraulic conductivities of soil) of a soil column -a higher anisotropy ratio favors and a low anisotropy ratio weakens this distribution. Furthermore, the time to attain steady state by a ditch drainage system can be considerable, particularly for situations where the drains are dug relatively deeper into a soil having a high anisotropy ratio. The analytical model presented here is significant as it can be used to design ditch drainage networks for reclaiming a salt-affected and/or waterlogged soil within a specified time.