Abrasion of river bedrock: from the laboratory to the field

The incision of bedrock by rivers is a key process that controls river morphology, drives valley downcutting, determines the evolution of hillslopes base level, and shapes landscapes. Understanding the mechanisms of fluvial erosion of bedrock is therefore crucial in geomorphology. In this study, we investigate the factors controlling bedrock erosion rates, both experimentally to document the pebble abrasion process and in the field to highlight the respective roles of abrasion and block detachment. Experimentally, we use an annular flume to simulate different hydrodynamic conditions and sediment transport regimes, in order to conduct a parametric study and to explore the influence of sediment size (from fine to coarse gravel size), sediment quantity and flow velocity. Our experiments partly reproduce the semi-theoretical abrasion model of Sklar and Dietrich (WRR, 2004) in particular the dual role of the sediment load: a “tool” effect for gravel quantity lower than the amount required to cover the flume bottom, and an increasing “cover” or protective effect beyond. To complement these controlled laboratory observations, we are carrying out in situ erosion measurements in three different small gorges carved into silt- to sand-stones units across the Himalayan front in Nepal. We use terrestrial Lidar to follow the rocky banks evolution both at a scale of several tens of meters to quantify block detachment rates, and at the scale of locally protruding sandstone bars sediment in order to document the surface abrasion and its spatial variations. This micro-topographic study is complemented by hydrologic measurements and by an in-situ erosion sensor that records the timing of the erosion. These field data provide insight into the mechanisms of bedrock erosion in a monsoon-driven climate. Together, laboratory and field approaches provide a comprehensive framework for understanding bedrock abrasion, with implications for predicting landscape evolution in mountainous river systems.

Sklar, L. S., & Dietrich, W. E. (2004). A mechanistic model for river incision into bedrock by saltating bed load. Water Resources Research, 40(6).