HR: 10:20h
AN: H52A-01    [Abstracts]
TI: Field Monitoring of Bedrock Channel Erosion and Morphology
AU: * Johnson, J P
EM: joelj@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Whipple, K X
EM: kxw@mit.edu
AF: Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Ave, Cambridge, MA 02139 United States
AU: Sklar, L S
EM: leonard@sfsu.edu
AF: Department of Geosciences San Francisco State University, 1600 Holloway Avenue, San Francisco, CA 94132 United States
AB: We present field measurements of erosional morphology and fluvial incision into Navajo Sandstone bedrock along a short human-perturbed channel reach in the Henry Mountains, Utah. Bedrock rivers flow in self-formed channels and form diverse erosional morphologies. The parameters that collectively define channel morphology (e.g. width, slope, bed roughness, bedrock exposure, sediment size distribution) all dynamically adjust in poorly understood ways to imposed fluid and sediment fluxes. The picture of erosion that emerges from our field monitoring is consistent with laboratory flume experiments conducted under sediment-starved transport conditions. We find that erosion is a sensitive function of the evolving bed topography because of feedbacks between the turbulent flow field, sediment transport, and bottom roughness. To facilitate highway construction over Swett Creek in the early 1970s, the Utah Department of Transportation created a natural experiment by filling in part of the original canyon and routing flow through a culvert and blasted bedrock slot. The vertical-walled, blasted channel has an upper reach (the ``flume'', length ~80 m, slope ~0.022, width ~5 m) and a shorter downstream reach that steeply slopes into the original channel (the ``flume mouth'', length ~17 m, slope ~0.18). The field-scale flume thus constructed provides an excellent opportunity to observe the morphology of a bedrock channel that has eroded from a well constrained initial geometry over a known amount of time. Ephemeral channel flow only occurs during spring snowmelt and flash floods. We monitor water depth, and we have a limited number of direct bedload transport measurements. In addition, we have installed bolts in the bedrock and are currently monitoring active erosion and morphological changes in the channel reach. Bedrock incision into the Navajo Sandstone occurs mostly by abrasion, with dramatic sculpted forms apparently carved by the finer bedload that becomes locally suspended in intense vortices. As has been observed previously in other bedrock channels, erosional morphology varies with slope. The lower slope ``flume'' has longitudinal grooves, significant sediment cover and modest rates of erosion. Erosion in the higher slope ``flume mouth'' is not spatially uniform but is focused to form a narrow inner channel. We measured approximately 45 cm of vertical incision into Navajo Sandstone in the inner channel from the 2005 season of exceptional snowmelt flow. When transported sediment becomes focused in the center of the channel without depositing, an inner channel forms simply because abrasion only happens where sediment energetically impacts the bed. Positive feedback occurs between the sediment preferentially transported in topographic lows and the further abrasion of topographic lows. The inner channel in particular has potholes and sculpting; erosional bedforms such as these add roughness to the channel. Complementary laboratory experiments that also resulted in inner channel formation suggest that sidewall friction and tortuous flow conditions in the inner channel reduce the shear stress available to transport sediment, acting as the negative feedback that ultimately limits inner channel incision. Inner channels may often form where the sediment load imposed on the reach is well below the transport capacity of the flow, such as where the local slope is higher.
DE: 1815 Erosion
DE: 1825 Geomorphology: fluvial (1625)
DE: 1862 Sediment transport (4558)
DE: 1895 Instruments and techniques: monitoring
SC: Hydrology [H]
MN: Fall Meeting 2005