Bedrock incision by bedload: insights from direct numerical simulations

Guilhem Aubert,Pascal Allemand,Vincent J. Langlois

doi:10.5194/esurf-4-327-2016

Abstract

Abstract. Bedload sediment transport is one of the main processes that contribute to bedrock incision in a river and is therefore one of the key control parameters in the evolution of mountainous landscapes. In recent years, many studies have addressed this issue through experimental setups, direct measurements in the field, or various analytical models. In this article, we present a new direct numerical approach: using the classical methods of discrete-element simulations applied to granular materials, we explicitly compute the trajectories of a number of pebbles entrained by a turbulent water stream over a rough solid surface. This method allows us to extract quantitatively the amount of energy that successive impacts of pebbles deliver to the bedrock, as a function of both the amount of sediment available and the Shields number. We show that we reproduce qualitatively the behaviour observed experimentally by Sklar and Dietrich (2001) and observe both a "tool effect" and a "cover effect". Converting the energy delivered to the bedrock into an average long-term incision rate of the river leads to predictions consistent with observations in the field. Finally, we reformulate the dependency of this incision rate with Shields number and sediment flux, and predict that the cover term should decay linearly at low sediment supply and exponentially at high sediment supply.

Highlights

The incision of bedrock channels is one of the key processes that govern the formation and evolution of mountain ranges (Anderson, 1994; Howard, 1994; Whipple and Tucker, 1999)
One of the most commonly used approaches, the stream-power incision model, assumes that the incision rate within a river channel varies as a power law of both its local slope and its drainage area (Seidl et al, 1994; Whipple and Tucker, 1999)
The model that we adopt for the interaction between the water flow and the pebbles is rather simple, the dynamics of the bedload layer appears to be consistent with experimental observations: as shown by Fig. 6, the order of magnitude of the sediment transport rate is comparable to the one predicted by the Meyer-Peter–Müller law

Summary

Introduction

The incision of bedrock channels is one of the key processes that govern the formation and evolution of mountain ranges (Anderson, 1994; Howard, 1994; Whipple and Tucker, 1999). One of the most commonly used approaches, the stream-power incision model, assumes that the incision rate within a river channel varies as a power law of both its local slope and its drainage area (which is equivalent to introducing a dependence in the water discharge) (Seidl et al, 1994; Whipple and Tucker, 1999). The suitability of this model to adequately reproduce several features of bedrock channels has recently been reviewed extensively by Lague (2014). One of its main restrictions is that it does not take into account more detailed parameters such as the dynamics of the alluvial cover in the channel

Methods

Discussion

Conclusion