HR: 16:30h
AN: H44B-03 INVITED    [Abstracts]
TI: The dynamics of bedrock channel adjustment: Modeling the influence of sediment supply, weathering, and lithology on channel cross-sectional and longitudinal shape
AU: * Wobus, C
EM: cameron.wobus@colorado.edu
AF: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Tucker, G
EM: gtucker@cires.colorado.edu
AF: Cooperative Institute for Research in Environmental Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Tucker, G
EM: gtucker@cires.colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Anderson, R
EM: robert.s.anderson@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Anderson, R
EM: robert.s.anderson@colorado.edu
AF: Institute for Arctic and Alpine Research, University of Colorado, Boulder, CO 80309, United States
AU: Kean, J
EM: jwkean@usgs.gov
AF: U.S. Geological Survey, P.O. Box 25046, MS-966, Denver, CO 80225, United States
AU: Small, E
EM: eric.small@colorado.edu
AF: Department of Geological Sciences, University of Colorado, Boulder, CO 80309, United States
AU: Hancock, G
EM: gshanc@wm.edu
AF: Department of Geology, College of William and Mary, Williamsburg, VA 23187, United States
AB: The cross-sectional form of a natural river channel controls the capacity of the system to carry water off a landscape, to convey sediment derived from hillslopes, and to erode its bed and banks. Numerical models that describe the response of a landscape to changes in climate or tectonics therefore require formulations that can accommodate changes in channel cross-sectional geometry through time. We have developed a 2D numerical model that computes the formation of a channel in a cohesive, detachment-limited substrate subject to steady, unidirectional flow. Boundary shear stress is calculated using a simple approximation of the flow field in which log-velocity profiles are assumed to apply along vectors that are perpendicular to the local boundary surface. The resulting model predictions for the velocity structure, peak boundary shear stress, and equilibrium channel shape compare well with the predictions of a more sophisticated but more computationally demanding ray-isovel model. For example, the mean velocities computed by the two models are consistent to within ~3%, and the predicted peak shear stress is consistent to within ~7%. The efficiency of our model makes it suitable for calculations of long-term morphologic change both in single cross-sections and in series of cross-sections arrayed downstream. For a uniform substrate, the model predicts a strong tendency toward a fixed width-to-depth ratio, regardless of gradient or discharge. The model predicts power-law relationships between width and discharge with an exponent near 2/5, and between width and gradient with an exponent near -1/5. Recent enhancements to the model include the addition of sediment, which increases the width-to-depth ratio at steady state by favoring erosion of the channel walls relative to the channel bed (the "cover effect"). Inclusion of a probability density function of discharges with a simple parameterization of weathering along channel banks leads to the formation of model strath terraces. Downstream changes in substrate erodibility or tectonic uplift rate lead to step-function changes in channel width, consistent with empirical observations. Finally, explicit inclusion of bedload transport allows channel width, gradient, and the pattern of sediment flux to evolve dynamically, allowing us to explore the response of bedrock channels to both spatial patterns of rock uplift, and temporal variations in sediment input.
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
DE: 1860 Streamflow
SC: Hydrology [H]
MN: 2007 Fall Meeting