Title: Subsurface exit gradients at a drainage ditch with lower water table than the groundwater table Author
Submitted to: Federal Interagency Sedimentation Conference Proceedings
Publication Type: Proceedings
Publication Acceptance Date: March 19, 2010
Publication Date: June 26, 2010
Citation: Romkens, M.J. 2010. Subsurface exit gradients at a drainage ditch with lower water table than the groundwater table. Federal Interagency Sedimentation Conference Proceedings. (CD-ROM) Interpretive Summary: Soil erosion on upland areas is a highly complex phenomenon involving many component processes. One of the least explored processes is the role of subsurface flow. Quantifying the subsurface flow regime is essential for estimating seepage to drainage ditches and channels, for assessing the transport of contamination (especially nitrates) to surface waters, and for evaluating the effect of subsurface flow on bank erosion and slope stability. This article discusses an analytical approach, based on conformal theory, to estimate for a given steady state flow regime the exit gradients to drains and ditches with a water table lower than the groundwater table in the adjoining land areas. The solution represents a significant step forward in models describing subsurface flow draining areas and estimating contaminant transport to open water by subsurface flow.
Technical Abstract: Rill and gully development are usually thought of as a consequence of overland flow in which at selected locations in the landscape the shear stress has exceeded a threshold value. In those cases, the flow regime causes local scour that might lead to headcuts and incisions in the concentrated flow pathways. In most of these studies, the role of the subsurface water regime as a contributing factor is being ignored. Field and laboratory studies have shown that in addition to surface flow, the subsurface soil water regime may appreciably affect conditions for incipient rilling and ephemeral gully development. Also, in places where the groundwater table intersects the surface topography, seepage occurs that leads to localized soil erosion which manifests itself in terms of incisions and headcuts. This article summarizes an analysis of the subsurface flow regime in a homogeneous isotropic aquifer with a water table that is higher than the water level in the incised drainage ditch. The aquifer with a finite depth has an open surface boundary and is infinite wide. To facilitate the analysis, a steady state flow condition is considered in which the analysis is based on a solution of Laplace’s equation for the flow region. Expressions for the exit gradients at the water/soil inter-phase were determined. The model considers a buffer strip where no water enters the soil profile. Estimates are made for seepage losses for different buffer strip widths. The analysis yields a close-form solution for this simplified physical realization that will facilitate estimations of subsurface flow in runoff and erosion models.