Prediction of ephemeral gully formation and erosion on hillslopes

Ephemeral gullies (channels) are formed by the concentration of surface and subsurface runoff on a hillslope and are usually obliterated by tillage. The objective of this research was to study the mechanisms of ephemeral gully formation and development and to predict channel erosion. A topographic analysis, which describes the effect of convergence of a landscape on runoff concentration, was used to predict the potential location of an ephemeral gully. A process-based model integrating the theory of shear stress with fundamental concepts of hydrologic and erosion processes was developed to predict channel erosion by concentrated flow.; Channel erosion rate was assumed to be a function of shear excess. Analyses of theoretical and experimental studies of boundary shear stress in channels with fixed cross sections led to the derivation of mathematical relationships of shear stress distribution in erodible channels. These analyses show that the distribution of boundary shear stress and the maximum shear stress vary with channel geometry, and that boundary shear stress increases exponentially from zero at the water-surface edges to the maximum at the bed centerline in channels with width-depth ratios greater than two. Both analytical solutions and numerical simulations suggest that an eroding channel in deep soils approaches a dynamic equilibrium in which hydraulic geometry and erosion rate are steady for a given discharge rate. A channel erodes downward only in equilibrium and its hydraulic geometry depends particularly upon the characteristics of boundary shear stress. When a nonerodible layer is encountered, however, the channel widens and approaches a final width. Changes of channel width and erosion rates due to the existence of a nonerodible layer are mathematically determined as an exponential function of flow rate, channel slope and roughness, and critical shear stress of the soils. Results of simulation studies for soils with uniform and nonuniform profiles further support the proposed mechanisms of ephemeral gully erosion.; Tests with experimental data also indicate that the model is adequate for predicting ephemeral gully erosion and geometry and can be easily incorporated to process-based models of interrill and rill erosion. Potentially, the model can be used to predict hydraulic geometry of stable stream channels.