Direct rain splash and downwearing of internal surfaces as an important erosion process in alluvial gully development

Abstract

Rainfall and runoff are significant contributors to gully erosion, usually in the form of concentrated flow at the gully head. Yet differences in the erosion processes driving the development of alluvial gullies have yet to be adequately addressed. In considering the role of rainfall on gully development, downwearing of partly-eroded, internal surfaces by direct splash and wash erosion were investigated at 11 alluvial gullies in northeastern Australia. Rainwash processes are well described in the soil and agricultural erosion literature but are rarely considered important gully erosion processes. 293 erosion pins/plates were installed on internal buttress crests between 2019 and 2021. Over 70% of pins recorded erosion depths of 5–55 mm y−1 despite below-average rainfall in that period. Gully subsurface sediments had a rate of 24.5 mm y−1 (SD = 18.1), accounting for toppled pins, while adjacent surface materials had a rate of 1 mm y−1 (SD = 2). Downwearing volumes represent 50 – 80% of the sediment losses from measured gullies, with specific yields from downwearing alone of 162 ± 29–495 ± 62 ha−1 y−1. Soil chemistry results provide limited ability to understand the specific factors driving intra-site variability in erosion rates, but rates are related to strongly sodic and magnesic subsurface sediments that had strong dispersion and slaking properties. Total rainfall is a significant factor at the site scale, but no relationship was found with seasonal erosivity. From these results, we suggest that rainwash erosion on internal gully surfaces is an important and significant erosion process of large, alluvial gullies, with relevance to all open form gullies. Downwearing of internal surfaces and ongoing adjustment in gully depth results in a sustained growth phase over the medium-term compared with most hillslope gullies.