HR: 16:15h
AN: H44B-02    [Abstracts]
TI: Channel Width Adjustments to Flood Flows in a Highly Erodible Landscape, North Fork Toutle River, Mount St. Helens, WA
AU: * Mueller, E R
EM: erich.mueller@colorado.edu
AF: Department of Geography, University of Colorado, Box 260, Boulder, CO 80309,
AU: Pitlick, J
EM: pitlick@colorado.edu
AF: Department of Geography, University of Colorado, Box 260, Boulder, CO 80309,
AU: Spicer, K
EM: krspicer@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg 10, Suite 100, Vancouver, WA 98683,
AU: Major, J J
EM: jjmajor@usgs.gov
AF: U.S. Geological Survey, 1300 SE Cardinal Court, Bldg 10, Suite 100, Vancouver, WA 98683,
AB: Heavy rainfall in November 2006 induced large-scale flooding and debris flows in the Pacific Northwest, most notably on drainages originating on the flanks of volcanoes. Field evidence and measurements from acoustic flow monitors suggest flooding on the North Fork Toutle River (NFTR), which originates in the crater of Mount St. Helens, was preceded by valley-spanning debris flows coupled with as much 5 m of fluvial incision in the headwaters. Peak flood discharges with a return period of approximately 20 years completely reset the width and depth of the NFTR. In this study, we document changes in channel geometry in response to the November 2006 floods by repeating measurements of width, depth, slope and grain size taken several months earlier. Because the NFTR flows through an unvegetated alluvial plain, we hypothesized that the channel would widen to dimensions determined by the flood magnitude, thereby establishing a new downstream hydraulic geometry. Repeat measurements were taken at 12 sites spaced 1-2 km apart, with reach-averaged gradients ranging from nearly 6% in the headwaters to about 2% downstream. Modeled flow estimates for the flood channel are in agreement with peak discharges estimated from regional flood-frequency analysis, increasing downstream from 40 to 150 cms. Flood flows more than doubled the width of the channel from an average of 12 m to more than 32 m during the flood, before returning to an average active channel width of 15 m by the summer of 2007. Downstream hydraulic geometry relations for channel width, w=aQ^b, have essentially the same exponents, b=0.49, 0.48 and 0.50, for conditions prior to, during and after the 2006 floods. This suggests that in erosive environments where channel adjustments can occur rapidly, the scaling relationship between width and discharge, w ~ Q^0.5, remains roughly constant for relatively common as well as infrequent flood events. Further, despite massive erosion and channel change, the NFTR was able to readjust its channel geometry to almost the same configuration as before the flood.
DE: 1815 Erosion
DE: 1821 Floods
DE: 1825 Geomorphology: fluvial (1625)
DE: 1856 River channels (0483, 0744)
SC: Hydrology [H]
MN: 2007 Fall Meeting