Rill length and plot-scale effects on the hydrogeomorphologic response of gravelly roadbeds

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Although unpaved roads are well-recognized as important sources of Hortonian overland flow and sediment in forested areas, their role in agriculturally-active rural settings still lacks adequate documentation. In this study, we assessed the effect of micro-catchment size, slope, and ground cover on runoff and sediment generation from graveled roadbeds servicing a rural area in southern Brazil. Fifteen replications based on 30-min-long simulated rainfall experiments were performed at constant rainfall intensities of 22–58 mm h−1 on roadbeds with varying characteristics including ˜3–7 m2 micro-catchment areas, 2–11° slopes, 2–9.7-m-long shallow rill features, and 30–100% gravel cover. The contributions of micro-catchment size and rill length were the most important physical characteristics affecting runoff response and sediment production; both the size of the micro-catchment and the length of the rills were inversely related to sediment loss and this contradicts most of the rill erosion literature. The effect of micro-catchment size on runoff and sediment response suggests a potentially problematic spatial-scale subjectivity of experimental plot results. The inverse relationship between rill length and sediment generation is interpreted here as related to the predominance of coarse fragments within rills, the inability of the shallow flows generated during the simulations to erode this sediment, and their role as zones of net sediment storage. Copyright © 2015 John Wiley & Sons, Ltd.

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