HR: 1330h
AN: H52A-1177    [PDF]
TI: Event-Based Sedimentation in a Flood Bypass: Examples From the Sacramento Valley, California
AU: * Singer, M B
EM: bliss@bren.ucsb.edu
AF: Donald Bren School of Environmental Science and Management, 2400 Bren Hall University of California, Santa Barbara, Santa Barbara, CA 91306-5131 United States
AU: Aalto, R
AF: Department of Earth and Space Sciences, University of Washington 63 Johnson Hall, Box 351310, Seattle, WA 98195 United States
AU: Dunne, T
AF: Donald Bren School of Environmental Science and Management, 2400 Bren Hall University of California, Santa Barbara, Santa Barbara, CA 91306-5131 United States
AU: Nittrouer, J
AF: Department of Earth and Space Sciences, University of Washington 63 Johnson Hall, Box 351310, Seattle, WA 98195 United States
AU: Nittrouer, C A
AF: Department of Earth and Space Sciences, University of Washington 63 Johnson Hall, Box 351310, Seattle, WA 98195 United States
AB: Flood bypasses are miniature, anthropogenic analogs of flood basins, which abutted many lowland rivers before the advent of major flood control engineering. Whereas natural lowland floodplains are frequently inundated by diffuse overbank flow or localized crevasse splays, an engineered bypass is only activated when passive weirs near the main channel are overtopped during large floods. Consequently, a bypass represents a focused pathway for the transport of water and sediment that may reflect a legacy of event-based sediment accumulation, compared with the more chronic, diffuse sedimentation across natural flood basins. Our investigations suggest that bypasses are net receivers of suspended sediment from the main channel, with recent sediment accumulation patterns that reflect the dimensions, flow history, and frequency of over-weir sediment delivery. Our preliminary field observations and laboratory measurements indicate that sedimentation in Colusa and Yolo Bypasses may be dominated by a few large sedimentation events (of order $\sim$1 m of deposition per event). However, little is known about how sediment accumulation occurs in bypasses during floods. This study commences a detailed investigation of the links between hydrology and sedimentation in the flood bypasses of the Sacramento Valley. Our strategy is to 1) model the over-weir conveyance and intra-bypass accumulation of fine sediment during floods, and 2) sample, describe, and date these sediment deposits to verify the numerical model. We modeled the influence of the 1964-1965 hydrologic event on sedimentation in the Colusa-Sutter-Yolo Bypass system by combining hydrologic records with empirical models of suspended-sediment concentration and suspendibility calculations. We used records of flood discharge for the event period into and through Colusa, Sutter, and Yolo Bypasses to compute sediment concentration entering the system based on discharge-concentration relationships and simplified hydraulics within the bypass system. We computed daily deposition via suspendibility criterion of various grain sizes at selected loci. The model results are being empirically validated with accumulation thicknesses, textures, and dates derived from $^{210}$Pb CIRCAUS geochronology, sediment granulometry, and X-radiographs from 2-6-m-deep vertical cores collected at various locations throughout the bypass system.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
SC: Hydrology [H]
MN: 2003 Fall Meeting