Submitted to: Landform Analysis
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/5/2011
Publication Date: 12/31/2011
Citation: Romkens, M.J. 2011. Calculation of exit gradients at drainage ditches. Landform Analysis. 17:151-153.
Interpretive Summary: Seepage gradients at the soil-water interface in ditches and drains appreciably impact the detachment and subsequent transport of particles in flowing stream channels. Seepage studies have mostly been concerned with the amount of water moving from upland areas to the stream system by subsurface flow. The impact of subsurface flow on particle detachment from stream banks or drainage ditches has received very little attention. Moreover, the gradients that are operative differ in magnitude and direction on different locations of the soil-water interface. Therefore, to assess the impact of seepage gradients on soil detachment an exact approach of quantifying these gradients anywhere on the soil-water interface is needed. This article discusses how these exit gradients can be determined for the case where a ponded surface drains to a ditch or drain with a water level lower than that of the adjoining ponded land area and for which a close-form analytical solution was obtained.
Technical Abstract: Seepage gradients play an important role in the detachment of soil particles from the side walls of stream channels and drainage ditches. Most seepage studies have focused on water losses. Relatively few have addressed the determination of these gradients as causes of soil loss and incipient gully development. This paper presents the methodology of calculating these gradients on any point of the soil-water interface of a subsurface flow system, for which a close-form analytical solution was obtained (Römkens, 2009). Such a solution was derived using conformal transformations for a situation in which a ponded surface drains by subsurface flow into a ditch with a water table lower than that of the ponded surface. The derived relationships allow a close estimate of the soil detachment forces on the wetted drainage perimeter of the stream system.