Upscaling sediment-flux-dependent fluvial bedrock incision to long timescales

Jens Turowski

doi:10.5194/egusphere-egu21-10291

Abstract

&lt;p&gt;&lt;span&gt;Fluvial bedrock incision is driven by the impact of moving bedload particles. Mechanistic, sediment-flux-dependent incision models have been proposed, but the stream power incision model (SPIM) is frequently used to model landscape evolution over large spatial and temporal scales. This disconnect between the mechanistic understanding of fluvial bedrock incision on the process scale, and the way it is modelled on long time scales presents one of the current challenges in quantitative geomorphology. Here, a mechanistic model of fluvial bedrock incision that is rooted in current process understanding is explicitly upscaled to long time scales by integrating over the distribution of discharge. The model predicts a channel long profile form equivalent to the one yielded by the SPIM, but explicitly resolves the effects of channel width, cross-sectional shape, bedrock erodibility and discharge variability. The channel long profile chiefly depends on the mechanics of bedload transport, rather than bedrock incision. In addition to the imposed boundary conditions specifying the upstream supply of water and sediment, and the incision rate, the model includes four free parameters, describing the at-a-station hydraulic geometry of channel width, the dependence of bedload transport capacity on channel width, the threshold discharge of bedload motion, and reach-scale cover dynamics. For certain parameter combinations, no solutions exist. However, by adjusting the free parameters, one or several solutions can usually be found. The controls on and the feedbacks between the free parameters have so far been little studied, but may exert important controls on bedrock channel morphology and dynamics. &lt;/span&gt;&lt;/p&gt;

Highlights

Mechanistic, sediment-flux-dependent incision models have been proposed, but the stream power incision model (SPIM) is frequently used to model landscape evolution over large spatial and temporal scales. This disconnect between the mechanistic understanding of fluvial bedrock incision on the process scale, and the way it is modeled on long time scales presents one of the current challenges in quantitative geomorphology
In addition to the imposed boundary conditions specifying the upstream supply of water and sediment, and the incision rate, the model includes four free parameters, describing the at-a-station hydraulic geometry of channel width, the dependence of bedload transport capacity on channel width, the threshold discharge of bedload motion, and reach-scale cover dynamics
While rivers may respond to this variability by visibly changing their shape over the course of a single flood event, over long time scales, it is thought that fluvial incision rates and average river morphology depend on some characteristic statistics of the distribution of discharge (e.g., Blom et al, 2017; DiBiase & Whipple, 2011; Lague et al, 2005; Molnar, 2001; Molnar et al, 2006; Scherler et al, 2017; Tucker, 2004)

Summary

Introduction

River processes are driven by flowing water. Water discharge varies over time, according to the spatial and temporal patterns of precipitation in the catchment, its size, and its hydrological properties (e.g., Deal et al, 2018). While rivers may respond to this variability by visibly changing their shape over the course of a single flood event, over long time scales, it is thought that fluvial incision rates and average river morphology depend on some characteristic statistics of the distribution of discharge (e.g., Blom et al, 2017; DiBiase & Whipple, 2011; Lague et al, 2005; Molnar, 2001; Molnar et al, 2006; Scherler et al, 2017; Tucker, 2004). Bedrock rivers are a key component of erosional landscapes such as active mountain belts. Fluvial bedrock incision is thought to be driven by the impact of moving bedload particles.

Results

Discussion

Conclusion