Flow-retarding effects of vegetation and geotextiles on soil detachment during concentrated flow

Roughness elements at the soil Surface (e.g. crop residues, rock fragments, vegetation, geotextiles) strongly reduce the erosivity of overland flow (both interrill and concentrated overland flow) and hence soil detachment rates. Common methods for shear stress partitioning that allow the calculation of effective flow shear stress in the presence of roughness elements originate from river hydraulics but seem invalid for overland flow. An alternative method to estimate the effective flow shear stress in the presence of a soil surface cover has been presented and tested for soil detachment by concentrated runoff on soil surfaces covered by crop residues by Knapen el al., (2008). In this method, the estimation of effective flow shear stress is based on the recalculation of the hydraulic radius for covered soil surfaces using flow hydraulics oil uncovered surfaces. However, the applicability of this method for roughness elements different from crop residues and under field conditions needs to be tested to confirm its validity. Therefore, runoff data from three experimental studies (conducted oil geotextile and grassed-covered Soil Surfaces) are analysed in this Study. The results show that effective flow shear stress, calculated using this method, is not only a good soil detachment predictor for soil Surfaces covered with crop residues but also for the tested soil surfaces with a veaetation or geotextile cover. However, the geotextile experiments point to one of the shortcomings of the method. At high flow shear stress levels, vortex erosion due to flow turbulence is reported for the thickest geotextiles. These flow turbulences are not accounted for since the method is based oil average flow characteristics. Copyright (C) 2009 John Wiley & Sons, Ltd.