The cross sectional shape of a bedrock channel reflects the channel's history of incision. Although current theory suggests that sediment supply is a dominant control on width-averaged bedrock incision rates, models predicting the cross sectional shape of bedrock channels have thus far neglected sediment supply effects. Here we present a simple model that predicts the evolution of the cross sectional shape of a bedrock channel under given conditions of sediment supply. Our model assumes that all erosion is the result of particle impacts of saltating bedload, and that bedload transport occurs at transport capacity along an active zone, the width of which is determined by the sediment supply, discharge, and channel cross sectional shape. We allow bedrock erosion to occur at locations within the active transport zone where the thickness of the sediment layer does not exceed a critical threshold. Preliminary model results are qualitatively similar to experimental observations documenting slot incision in response to a reduction in sediment supply and partial alluviation and peripheral erosion in response to a subsequent supply increase. Integrating our model predictions over the channel cross section to calculate width-averaged erosion rates indicates that the model captures the so-called `tools' and `cover' effects wherein width-averaged erosion rates reach a maximum value at an intermediate sediment supply rate. However, the specific nature of the relationship between average erosion rate and sediment supply is strongly influenced by the shape of the channel cross section. Our results may provide insight on estimating historical sediment supply rates from observed bedrock channel geometry.
AGU Fall Meeting Abstracts
- Pub Date:
- December 2007
- 1815 Erosion;
- 1825 Geomorphology: fluvial (1625);
- 1847 Modeling;
- 1862 Sediment transport (4558)