HR: 1340h
AN: H53D-0508    [Abstracts]
TI: The influence of bed roughness on partial alluviation in an experimental bedrock channel
AU: * Davis, J R
EM: jend@moorekb.com
AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Sklar, L S
H53D-0508 AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Demeter, G I
H53D-0508 AF: San Francisco State University, Department of Geosciences, San Francisco, CA 94132 United States
AU: Johnson, J P
H53D-0508 AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139 United States
AU: Whipple, K X
H53D-0508 AF: Massachusetts Institute of Technology, Department of Earth, Atmospheric and Planetary Sciences, Cambridge, MA 02139 United States
AB: The extent of alluvial cover on a bedrock channel bed strongly influences the efficiency of river incision, and can affect the quality of habitat for aquatic ecosystems. The extent of partial cover is commonly modeled as a simple function of sediment supply relative to the transport capacity of the stream, although other factors are likely to be important, particularly the roughness of the underlying bedrock surface. Here we report results of a set of laboratory experiments investigating the influence of bedrock channel bed topography on the dynamics of partial bed alluviation. The experiments were conducted in a tilting flume 8 m long and 0.3 m wide with an erodible bedrock bed made of a sand-cement mixture. The flume has a calibrated sediment feed and a double-basket sediment trap that provides a continuous record of sediment flux out of the downstream end. We used a uniform grain size of 5 mm, and varied the sediment supply rate from zero to that sufficient to create a fully alluviated bed. We created a variety of bedrock roughness conditions, from smooth, nearly planar surfaces to an egg-carton texture made with a plaster-coated foam mold. Intermediate roughness was achieved by chiseling into smooth beds and by allowing the bed topography to evolve by sustained bedload abrasion. We used a laser microtopography scanning device to make topographic maps of the bed surface, with a vertical resolution of 0.2 mm and a horizontal spacing of 5 mm. From these data we quantify bedrock bed roughness as the standard deviation of the distribution of bed elevations relative to a plane inclined at the mean bed slope. To guide our selection of bed roughness values we made topographic surveys of a number of bedrock channel beds, including partially alluviated channels where we dug trenches through alluvial deposits to expose the underlying bedrock surface. For each bed roughness condition we systematically varied the sediment supply rate and repeatedly mapped the extent of partial alluvial cover. We also calculated the volume of sediment stored within the channel as a function of time, from the cumulative difference between the sediment feed rate and measured sediment output. We find that low-roughness beds require a relatively high sediment supply before any alluvial patch formation occurs, and as supply increases, can accommodate only low levels of partial alluvial cover before a runaway alluviation process rapidly converts the bed to an aggrading alluvial condition. In contrast, highly rough bedrock surfaces partially alluviate at very low sediment feed rates, and do not experience runaway alluviation at high supply rates, and allow stable high fractional bed coverage. The dynamics of partial bed alluviation can be described by plotting sediment output from the flume as a function of sediment storage within the flume. Stable partial coverage corresponds to curves of increasing sediment output with increasing sediment storage, while the instability that leads to runaway alluviation corresponds to curves with a negative relationship between output and storage. For stable partial bed cover, sediment deposits within the flume have the effect of increasing the sediment transport capacity, presumably by altering the lateral distribution of boundary shear stress.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
DE: 1862 Sediment transport (4558)
DE: 8175 Tectonics and landscape evolution
SC: Hydrology [H]
MN: Fall Meeting 2005