HR: 1340h
AN: H53D-0516    [Abstracts]
TI: Physical Model of River Erosion Into Jointed Bedrock Through Quarrying
AU: * Dubinski, I M
EM: imdubi@cnr.colostate.edu
AF: Department of Geosciences, Colorado State University, Fort Collins, CO 80523 United States
AU: Wohl, E E
EM: ellenw@cnr.colostate.edu
AF: Department of Geosciences, Colorado State University, Fort Collins, CO 80523 United States
AB: A physical model of a jointed bedrock channel was used to study the geometry and rates of incision and knickpoint retreat of channels formed on resistant jointed bedrock where quarrying is the dominant erosional process. Resistant jointed bedrock channels commonly occur in crystalline lithologies such as granite, gneiss, quartzite, and basalt in a wide range of climatic and tectonic settings. Joints and other discontinuities in the bedrock are locally weak zones that may be preferentially weathered to form weakly or completely detached blocks that may be mobilized by flows. Such channels commonly have abrupt lateral or downstream discontinuities in bed elevation, including steps and knickpoints. The flume experiment was conducted in a 10 m long and 0.6 m wide flume with a constant bedslope of 0.03 located at Colorado State University's Engineering Research Center in Fort Collins, Colorado. Scaling was based on the Froude number where flows were constrained by an upstream entrance section to be critical or near critical flow with Froude numbers of 1 to 1.7. The bedrock consists of 3 layers of regularly horizontally and vertically jointed concrete where quarrying of blocks of consistent size can occur. The concrete is a 4 to 1 mixture of fine sand to Portland cement. A vertical knickpoint initially forms the downstream end of the bedrock channel. Three runs were conducted using different joint spacing: joint spacing of 3 cm both vertically and horizontally, joint spacing of 6 cm vertically and 3 cm horizontally, and mixed joint spacing of 3 cm and 6 cm vertically and 3 cm horizontally. The experiment was run until the channel form did not change over a period of 12 hr for a given discharge. Total run time was over 60 hr before this condition was met for a lower discharge of 0.11 m³/s at which point a higher discharge of 0.20 m³/s was introduced for a minimum period of 12 hr. A persistent retreating knickpoint with episodic quarrying of blocks at the knickpoint occurred during all runs. The slope of the retreating knickpoint changed nonlinearly over time. The higher discharge produced an observably higher erosion rate in all runs. Preliminary results suggest that joint spacing does at least qualitatively affect the erosion rate where smaller joint spacing yields lower erosion rates than the wider joint spacing despite the fact that smaller joint spacing produces blocks of less mass. This may be due to unequal scaling of the resistant forces caused by physical interaction between the lateral sides of the blocks when comparing the smaller joint spacing to the wider joint spacing.
DE: 1800 HYDROLOGY
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1847 Modeling
SC: Hydrology [H]
MN: Fall Meeting 2005