Annualmeeting:2017 CSDMS meeting-128: Difference between revisions

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|CSDMS meeting abstract title=Using a landscape evolution model to evaluate the role of pulses of uplift on bedrock valley width and channel mobility
|CSDMS meeting abstract title=Downstream sweep erosion as a mechanism for bedrock valley widening: comparison between model simulations and field examples
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|CSDMS meeting abstract=Understanding the factors that control lateral erosion rates in bedrock channels is a frontier in geomorphology. When lateral bedrock erosion rates exceed the rate of vertical incision on the channel bed, wide bedrock valleys are produced; if vertical incision rates then exceed lateral erosion rates, a strath terrace can be produced. Lateral erosion rates and the evolution of wide bedrock valleys are linked to bedrock lithology, sediment supply in the stream, discharge variability, and uplift.  
|CSDMS meeting abstract=Understanding the factors that control lateral erosion rates in bedrock channels is a frontier in geomorphology. Lateral erosion rates and the evolution of wide bedrock valleys are linked to bedrock lithology, sediment supply in the stream, and shear stress exerted on channel walls. I use a newly-developed lateral erosion component in the Landlab modeling framework to explore how model results compare with recently published field examples of downstream sweep erosion as a mechanism for gorge eradication and bedrock valley widening. The lateral erosion component dictates that lateral erosion rate is proportional to shear stress exerted on the channel walls in a bend in the river; therefore sharp bends with a smaller radius of curvature will produce faster lateral erosion. Cook et al. (2014) identified a similar mechanism they call downstream sweep erosion (DSE). They suggest that bedrock gorges can be rapidly eroded by DSE when a wide flood plain with a laterally mobile stream exists upstream of the gorge, requiring a sharp bend in the channel to enter the gorge. I set up the model domain to recreate conditions of a low relief area with a mobile channel in the upper part of the model domain and a narrow, high relief gorge in the downstream end of the model domain. I ran modeling experiments under a range of water flux and sediment mobility conditions. The model results show gorge widening that propagates downstream as described by Cook et al. (2014) and preferential erosion of blocks that protrude into the channel. The enhanced lateral erosion at channel bends and the resulting downstream sweep erosion emerge naturally from the models, matching observations in many field areas. Together this suggests that channel curvature is of fundamental importance to lateral erosion rates in bedrock channels.
We use a newly-developed lateral erosion component in the Landlab modeling framework to explore how the width of bedrock valleys develops under competition between lateral channel mobility and uplift rate. Two model formulations are presented, one representing the slow process of widening in a bedrock canyon, the other representing undercutting, slumping, and rapid downstream sediment transport that occurs in softer bedrock. We ran modeling experiments with a range of bedrock erodibility, sediment mobility, and uplift conditions. In order to determine the role changing uplift patterns has on channel mobility and bedrock valley width, two modeling scenarios were run: one with a fourfold increase in base level lowering and one with a fourfold decrease in base level lowering.
In the total block erosion models, which represent hard bedrock, the migration of knickpoints through the channels controls the onset of changes in valley width. As the channel comes into a new equilibrium slope, lateral erosion is either stalled (in the case of increased uplift) or accelerated (in the case of decreased uplift). In the undercutting-slump models, increasing uplift results in short-lived valley narrowing as the knickpoint moves up the channel. But after the model domain has reached equilibrium, higher uplift results in significantly wider bedrock valleys. This counterintuitive response reflects the nature of the model algorithm, which represents undercutting, slumping, and rapid downstream transport of sediment that occurs in soft bedrock. This class of models shows that in soft bedrock, higher uplift can increase channel mobility and result in wider bedrock valleys, rather than promoting less mobile channels and valley narrowing. Through these model experiments, we evaluate the conditions under which changes in base level fall result in wider bedrock valleys and more mobile channels and conditions under which the opposite occurs.
|CSDMS meeting posterPDF=Langston_CSDMS_2017_poster.pdf
|CSDMS meeting posterPDF=Langston_CSDMS_2017_poster.pdf
|CSDMS meeting posterPNG=Langston_CSDMS_2017_poster.png
|CSDMS meeting posterPNG=Langston_CSDMS_2017_poster.png
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Latest revision as of 16:16, 20 July 2017






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Downstream sweep erosion as a mechanism for bedrock valley widening: comparison between model simulations and field examples

Abigail Langston, Kansas State University Manhattan Kansas, United States. alangston@ksu.edu
Gregory Tucker, University of Colorado Boulder Colorado, United States. gtucker@colorado.edu


Langston CSDMS 2017 poster.png

Understanding the factors that control lateral erosion rates in bedrock channels is a frontier in geomorphology. Lateral erosion rates and the evolution of wide bedrock valleys are linked to bedrock lithology, sediment supply in the stream, and shear stress exerted on channel walls. I use a newly-developed lateral erosion component in the Landlab modeling framework to explore how model results compare with recently published field examples of downstream sweep erosion as a mechanism for gorge eradication and bedrock valley widening. The lateral erosion component dictates that lateral erosion rate is proportional to shear stress exerted on the channel walls in a bend in the river; therefore sharp bends with a smaller radius of curvature will produce faster lateral erosion. Cook et al. (2014) identified a similar mechanism they call downstream sweep erosion (DSE). They suggest that bedrock gorges can be rapidly eroded by DSE when a wide flood plain with a laterally mobile stream exists upstream of the gorge, requiring a sharp bend in the channel to enter the gorge. I set up the model domain to recreate conditions of a low relief area with a mobile channel in the upper part of the model domain and a narrow, high relief gorge in the downstream end of the model domain. I ran modeling experiments under a range of water flux and sediment mobility conditions. The model results show gorge widening that propagates downstream as described by Cook et al. (2014) and preferential erosion of blocks that protrude into the channel. The enhanced lateral erosion at channel bends and the resulting downstream sweep erosion emerge naturally from the models, matching observations in many field areas. Together this suggests that channel curvature is of fundamental importance to lateral erosion rates in bedrock channels.


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