Ephemeral gully channel width and erosion simulation technology

Abstract

Concentrated surface runoff, such as associated with ephemeral gully channels, increases erosion and transfers fine sediment and associated agrichemicals from upland areas to stream channels. Ephemeral gully erosion on cropland may contribute up to 40 % or more of the sediment delivered to the edge of agricultural fields, significantly threatening the health of downstream ecological services. Typically, conservation practices developed for sheet and rill erosion are also expected to treat ephemeral gully erosion, but technology and tools are needed to account for the separate benefits and effects of practices on various sediment sources. Without improved research studies, subjective observations will continue to be used to satisfy quality criteria in lieu of scientifically defensible, quantitative methods to estimate the impact of gully erosion. Ephemeral gully channels evolve by one, or combination of, complex physical process in the form of incision, headcut migration upslope, and channel sidewalls expansion (widening). This study focused on the latter, ephemeral gully channel widening relationships. The impact of various width functions on erosion can be very significant and is dependent on discharge, slope, soil properties, and management conditions. A description is provided on six ephemeral gully widening relationships, followed by recommended improvements, comparative application, and identification of future research needs. Improvements in the development of ephemeral gully width algorithms are critical to understanding the impact of conservation practices on controlling ephemeral gully erosion. Tools are needed to predict and quantify ephemeral gully erosion, including the capability to evaluate the effect of conservation practices to control erosion. An improved critical shear stress equation was developed and described that provides an approach to incorporating impacts of management practices on the resulting gully erodibility. Conservation management planning by organizations needs a systematic approach to determining the extent of ephemeral gully erosion problems on a field, watershed, or national basis, or to predict the recurring or new locations of ephemeral gullies prior to their development.