Variations in rill morphology on unpaved road surfaces and their controlling factors

Abstract

AbstractRills play important roles in hillslope soil loss as both sediment sources and concentrated flow routes. The construction of unpaved roads in hilly regions changes the underlying surface conditions and leads to unique rill development. Via a terrestrial laser scanning‐aided field survey of eight typical unpaved road segments in the hilly red soil region of China, road surface rill morphology was quantified in detail based on 0.05 m‐resolution digital elevation models. The results illustrated that the rill network on a road segment was mainly composed of one major rill. The average rill network bifurcation ratio was 5.04, and the ratio was closely correlated with road surface area. Rill width and cross‐sectional area (CSA) generally increased along roads, and ephemeral gullies formed in the lower part of some road segments. Slope length, gradient and surface roughness of the underlying road were closely related to the rill morphology factors. The winding roads exhibited higher rill widths, depths and CSAs in straight or highly curved road segments than in other segments. Intergraded factors composed of slope length and gradient were proposed to establish prediction equations for rill and gully CSA. The critical slope gradient and length for rill initiation were negatively related to each other, and the average critical slope was 7.4%. Similarly, the positions of the peak CSA were determined by a function of slope gradient and contributing road length (L = 46.98/sin[S] − 174.3). These results can help guide the prediction of road surface rill soil loss and the design of conservation treatments.