Estimating Seepage and Exit Gradients to Incised Ditches in a Homogeneous Isotropic Soil.

Knowledge of the soil water regime in land adjoining incised streams is of great interest to those assessing the hazards of gully development, soil erosion and water losses by seepage, and subsurface chemical pollutant transport. The importance of the subsurface water regime has generally been recognized. However, the development of tools capable of adequately describing the groundwater regime have been lacking. This lack is attributed to a number of factors, including the complexity and heterogeneity of the soil material and the geometric variations in depth and expanse of the water-carrying groundwater aquifer at the field scale near incised streams. While these complex conditions seems to be insurmountable, reasonable descriptions of the flow region can be made to a first approximation, making simplifying assumptions related to the size, shape, and homogeneity of the subsurface flow region. In this paper, seepage losses and exit gradients are calculated for the case of an incised ditch with a water level lower than the groundwater table in the adjoining homogeneous, isotropic land area. The approach is based on potential flow theory. Emphasis is placed on the evaluation of exit gradients and seepage forces at different parts of the soil-water interface boundary. This information will be helpful as a reference to measured values for a similar sized and shaped area of less homogeneity and may be helpful in explaining incidences of seepage erosion and gully development.