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Title: The Effect of the Subsurface Soil Water Regime on Sediment Production and Movement

item Romkens, Mathias

Submitted to: International Symposium on River Sedimentation
Publication Type: Proceedings
Publication Acceptance Date: 7/30/2010
Publication Date: 9/6/2010
Citation: Romkens, M.J. 2010. The Effect of the Subsurface Soil Water Regime on Sediment Production and Movement. International Symposium on River Sedimentation. CD ROM.

Interpretive Summary: Soil erosion on upland areas is commonly thought of being caused by drop impact by rainfall and overland flow. While the preponderance of soil loss on sloping land are indeed caused by these surface processes, the role of the subsurface water regime is often ignored but is in increasingly recognized as a serious contributor to erosion problems, particularly as it concerns incipient rilling and gully development and growth. This paper summarizes past experimental research of the author in which the impact of the subsurface regimes on soil erosion was measured under controlled laboratory conditions using a flume and rainfall simulator and on-going analytical research in which seepage gradient expressions are derived for a well defined subsurface flow regimes. The results of the laboratory experiments clearly demonstrated the huge impact of subsurface soil water pressures on erosion, especially rill development, and sediment transport rates. The analytical study is one of the very few studies in which a quantitative evaluation is made of seepage forces on particle entrainment and in slope stability problems.

Technical Abstract: Soil erosion and sediment transport in agricultural watersheds are highly complicated processes involving a large array of soil properties, surface conditions, and flow regimes. The preponderance of early research on upland areas in agricultural watersheds was motivated by concern for the loss of agriculturally productive soil. During the last 30 years the concerns were extended to include environmental issues especially the contamination and pollution of streams, lakes and ponds by sediment and agri-chemicals. In recent years, there is an increased realization that the subsurface soil water regime may appreciably affect soil erosion processes from hillslope land by causing incipient rilling and gullying, and by the movement of chemical and colloidal constituents into the stream system. This paper examines the role of the subsurface water regime on sediment generation in the upland sediment source areas of watersheds. Specifically, experimental results will be discussed that describe the effect of soil water pressures on the sediment concentration in sediment laden overland flow. Also, exit gradients are determined of subsurface flow into drainage ditches and channels for situations with water levels lower than the groundwater table in the adjoining areas. In this analytical study, a conformal solution approach is sought for a well defined aquifer with known boundary conditions and hydraulic properties. The exit gradients, important in seepage erosion, are determined from the general solution for this case.