Subsurface flow can be an important process in gully erosion through its impact on decreasing soil cohesion and erosion resistance as soil water content or pressure increases and more directly by the effects of seepage forces on particle detachment and piping. The development of perched water tables fosters lateral flow that can result in seepage at the surface and/or formation of soil pipes by internal erosion of preferential flow paths. Continued internal erosion of soil pipes can lead to gullies, dam and levee failures. However, the processes involved in particle and aggregate detachment from soil pipe walls and transport processes within soil pipes have not been well studied or documented. This paper reviews the limited research on sediment detachment and transport in macropores and soil pipes and applies the knowledge learned from the much more extensive studies conducted on streams and industrial pipes to hydrogeologic conditions of soil pipes. Knowledge gaps are identified and recommendations are made for future research on sediment detachment and transport in soil pipes. Copyright © 2017 John Wiley & Sons, Ltd.

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In agricultural regions, gully erosion is now recognized as a dominant source of soil loss, and the development and upstream migration of headcuts is critical to the initiation, incision, and dissection of these upland areas. This study examined the effect of soil texture on headcut development and migration using four common soils from the southeastern United States, tested on bed surface slopes ranging from 1 to 5% and overland fl ow rates ranging from 45 to 71 L min −1 . In response to this fl ow, actively migrating headcuts developed with migration rates ranging from 0.00001 to 0.0025 m s −1 , maximum scour depths ranging from 0.042 to 0.148 m, and sediment discharge rates ranging from 0.00017 to 0.0356 kg s −1 depending on the soil and boundary conditions imposed. Nearly all experimental runs resulted in steady-state soil erosion, wherein the headcut developed attained a constant rate of migration, shape, and sediment discharge as a function of time. These experimental observations were successfully compared with predictive equations for mass conservation, jet entry angle, maximum scour depth, and migration rate developed for actively migrating headcuts in upland concentrated fl ows. These results further demonstrate that systematic headcut development can occur in a range of soils, slopes, and fl ow rates, and that modifi ed jet impingement theory can be used to predict the erosional characteristics of these headcuts, thus providing the necessary analytical framework for improving soil erosion prediction technology.

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