HR: 1340h
AN: H43A-0360    [Abstracts]
TI: Experiments in eruption recovery: Channel bed and sediment transport adjustments as sand inputs decline
AU: * Gran, K B
EM: kbgran@u.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Mailbox 351310, Seattle, WA 98195 United States
AU: Montgomery, D R
EM: dave@ess.washington.edu
AF: Department of Earth and Space Sciences, University of Washington, Mailbox 351310, Seattle, WA 98195 United States
AU: Sutherland, D
EM: dsutherland@fs.fed.us
AF: Redwood Sciences Laboratory, Pacific Southwest Research Station, USDA Forest Service, 1700 Bayview Dr., Arcata, CA 95521 United States
AU: Lisle, T E
EM: tlisle@fs.fed.us
AF: Redwood Sciences Laboratory, Pacific Southwest Research Station, USDA Forest Service, 1700 Bayview Dr., Arcata, CA 95521 United States
AB: Transient sediment loading can alter channel bed conditions and sediment transport rates which complicates the prediction of sediment yields, yet evaluation of post-eruption sediment yields remains an important component of volcanic hazard assessment. The 1991 eruption of Mount Pinatubo, Philippines, introduced 1 km$^{3}$ of loose, sandy, pyroclastic flow debris to the Pasig-Sacobia-Abacan basins. River recovery is ongoing as sediment is removed from the basin or stabilized. Declines in sand inputs appear to be driving changes in surface composition and grain mobility. From 1996 to 2003, surface grain size increased, clast structures developed, and mobility declined on the Pasig-Potrero River. To investigate how further declines in sand input may affect bed organization and sediment transport, we conducted a series of four experiments in a 10m $\times$ 0.7m flume at Humboldt State University. Bed material and initial slope (2%) matched conditions on the Pasig-Potrero River at a 1:4 grain size ratio. Water and sediment discharge were set such that all grain sizes were visibly mobile. Median bedload size to depth ratios covered a similar range as the Pasig-Potrero, with higher average values. In the first run, the sediment feed matched the bed distribution with 70% sand. Subsequent runs had 60%, 50%, and 40% sand in the sediment feed. We monitored water surface and bed elevations, depths, sediment output rates and grain size distributions. Periodically, the bed was scanned at 1mm resolution and photographed. Bed configuration changed dramatically between runs. At 70% sand, the bed was highly mobile, with isolated gravel clasts rolling or sliding over a moving carpet of sand. At 60% sand, most stationary gravel was found in clusters. In runs with 50% and 40% sand, armored alternate bars developed, with pulses of sand and gravel jams moving downstream. As sand content decreased, the slope increased up to 4%, raising the shear stress to transport material of greater caliber. Bedload was finer than the feed during aggradation. Surface grain size adjusted faster than slope to the changes in sand content. Friction angles measured from topographic scans show a dramatic shift in distribution peaking at $0\deg$ in the 70% sand run to a flat distribution with 60% sand, to a distribution peaking at higher angles in runs with 50% and 40% sand. The average positive angle increased from $24\deg$ in the 70% sand run, to $56\deg$ with 40% sand. This corresponds to a 1.5- to 3.5-fold increase in dimensionless critical shear stress. Reach-averaged shear stress increased with declining sand content, but Shields stress ($\tau^{*}$) decreased an order of magnitude between 60% and 50% sand content. The range of $\tau^{*}$ and dimensionless bedload transport rates in these experiments spanned the range measured on the Pasig-Potrero River during the rainy seasons in 1997-98 and 2001. The data plot in two distinct sets, and the sharp transition illustrates bed sensitivity to sand content and could help explain strongly seasonal behavior on the Pasig-Potrero River. Low sand inputs during the dry season lead to channel consolidation, incision, and armoring, with the channel returning to wide, shallow, braided conditions in the rainy season. The sharp transition also illustrates the difficulty in predicting sediment yields during recovery from sediment loading, as evolving sediment rating curves may change dramatically through time.
DE: 1800 HYDROLOGY
DE: 1815 Erosion and sedimentation
DE: 1824 Geomorphology (1625)
DE: 1860 Runoff and streamflow
DE: 1894 Instruments and techniques
SC: Hydrology [H]
MN: 2004 AGU Fall Meeting