CSDMS 2014 annual meeting poster Kristin Sweeney

From CSDMS
Presentation provided during CSDMS annual meeting 2014

Testing bedrock incision models in a mixed bedrock-alluvial system: High Cascades, Oregon

Kristin Sweeney, University of Oregon Eugene Oregon, United States. sweeney3@uoregon.edu

Abstract:

There is abundant field evidence that sediment supply controls the incision of bedrock channels by both protecting the bed from incision and providing tools to incise the bed. Mixed bedrock-alluvial systems are uniquely suited to test these models, as the transition from bedrock to alluvial morphology can constrain parameters like sediment supply that are otherwise difficult to measure. Here, we use Lidar data and field observations from a fluvial channel cut into a Holocene lava flow in the High Cascades, Oregon to explore the ability of the full physics of models of abrasion by saltating bedload to predict observed incision. The blocky andesite of Collier lava flow erupted from Collier Cone ~1500 years ago, paving over the existing landscape and erasing fine-scale landscape dissection. Since the eruption, a 6 km stream channel has been incised into the lava flow, though the channel is currently dry for most of the year. The channel is comprised of two alluvial reaches and two bedrock gorges. The division between these reaches follows the background slope of the lava flow, with alluvial areas corresponding to low slopes and gorges corresponding to high background slopes. The alluvial reaches are characterized by deposits up to 2 m thick and gravel-bedded self-formed channels; gorges are incised up to 8 m into the flow. Using a simple finite difference scheme with airborne-Lidar-derived pre-incision topography as an initial condition, we predict incision in the two gorges with the saltation-abrasion model. We examine model outcomes as a function of water discharge, grain size, and sediment supply, fully populating the parameter space with several model runs. Water discharge and grain size are set as free parameters and we use a 1D energy equation to calculate channel shear stress. To constrain sediment supply, we assume that there is no incision in the alluvial reaches, such that sediment supply is greater than or equal to alluvial transport capacity. Our initial results using this approach suggest that for most parameter values, gorge incision is dominated by the tools effect, whereby greater channel shear stress and greater sediment supply result in higher channel incision.

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