HR: 0830h
AN: H31C-0476    [PDF]
TI: Hydraulic Response of a Mountain Stream to a Large Debris Flow Sediment Pulse
AU: * Brummer, C
EM: cbrummer@u.washington.edu
AF: Dept of Earth and Space Sciences, Univ of Washington 63 Johnson Hall, Seattle, WA 98195-1310 United States
AB: Field and laboratory studies of landslide-induced sediment pulses have been used to develop numerical models of pulse dispersion and translation. Evolution of sediment pulses has been found to be dominated by dispersion when the grain-size distribution of the sediment wave is coarser than the ambient bed material and when the flood Froude number is high, whereas translation dominates the evolution of fine-grained sediment pulses in channels with low Froude numbers. A large debris flow, which delivered an estimated 750,000 m$^{3}$ of sediment and woody debris to Squire Creek in the North Cascades of Washington, provides a rare opportunity to examine the hydraulic response of a sediment pulse that includes clasts exceeding the flow competence of the receiving channel. The landslide initiated on a ridge approximately 670 m above Squire Creek during the morning of February 25, 2002 and filled a 400-m length of the valley floor to a depth of approximately 20 m. The pre-disturbance, step-pool channel had an estimated slope of 0.08 and drains an area of 25.7 km$^{2}$ at the study site. While the deposit has been incised to a depth of 5 to 10 m in the first 18 months following the event, considerable armoring of the bed surface appears to have stabilized a knickpoint at the toe of the deposit, a process hypothesized to reduce the dispersion rate of the sediment pulse. Because many of the clasts in the armor layer exceed the flood flow depth, rates of dispersion may be overestimated by the transport equations incorporated into numerical models. Relict boulders and steep reaches observed along Squire Creek and throughout similar mountain drainage basins in the Pacific Northwest suggest that channel-bed armoring and profile stabilization may be a more significant influence on the decay of large sediment pulses delivered by debris-flows than previously recognized. In particular, the persistence of such deposits has implications for large debris-flow dams as sediment capacitors in mountain channels that are confined by steep valley sideslopes prone to large infrequent landsliding. Monitoring of the Squire Creek profile over the next several years and comparison with ancient deposits identified elsewhere in the basin will investigate the mechanisms by which large landslide-induced sediment pulses disperse downstream and evolve into the boulder lag deposits that characterize many mountain streams.
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2003 Fall Meeting