Morphogenetic Role of Rainsplash Transport in Hillslope Evolution in Post-Orogenic Landscapes

Abstract:

The upper convex portions of hillslope profiles in soil-mantled landscapes have traditionally been interpreted as fundamentally different in origin from planar or concave hillslopes and have elicited different suggestions concerning their formation. In sub-humid landscapes, with sparse vegetation cover and little evidence for intense bioturbation, the upper convexity has usually been interpreted as the result of rainsplash. For the purpose of this study, we use a rainsplash transport equation developed from field and laboratory experiments and a model of ballistic trajectories to develop a method of predicting annual rainsplash transport rates under a range of environmental conditions, particularly related to climate reflected in mean annual rainfall, rainfall-intensity regime, and ground cover density. We derive diffusivity values of the kind frequently used in hillslope evolution models, and then examine the types of hillslope profiles that can be produced by this transport process. Under steady-state or decelerating uplift rates in post-orogenic environments in Africa, rainsplash transport is insufficient to create the observed convexities and lengths where erosion rates can be estimated. Additionally, the observed non-linear relationship between hillslope gradient and hillslope distance suggests additional erosive forces being in play, most likely that of sheetwash, despite this process usually being associated with concave hillslope profiles.