HR: 1330h
AN: H33A-09    [Abstracts]
TI: Event-based washload transport and sedimentation in and around flood bypasses: Case study from the Sacramento Valley, California
AU: * Singer, M B
EM: bliss@bren.,ucsb.edu
AF: Institute for Computational Earth System Science, UCSB, University of California, Santa Barbara Institute for Computational Earth Systems Science 3060, Santa Barbara, CA 93106 United States
AU: Aalto, R A
EM: aalto@u.washington.edu
AF: Earth and Space Sciences, University of Washington, 310 Condon Hall Box 351310, Seattle, WA 98195 United States
AB: In large river systems, suspended sediment transport and deposition patterns are often affected by channel constraints engineered for flood conveyance or navigation. Such managed channels typically have a limited number of overflow loci through which suspended sediment enters the river's floodplain. Engineered flood bypasses are narrow relic floodplains that are supplied by overflow diversion weirs along managed river channels, and support agriculture and complex aquatic and riparian habitats that are sensitive to the delivery of floods, fine sediment, and adsorbed contaminants. They function as wide, shallow conveyance channels parallel to the main river, and therefore present an opportunity to assess the applicability of existing theory for delivery to and settling of suspended sediment within floodplains. This study is an investigation of hydrograph characteristics, sediment delivery, and sedimentation within the upstream reaches of flood bypasses closest to the weir. We present analysis of hydrologic and sediment records and modeling in the Sacramento River basin. The effects of a single large flood in 1964-1965 were analyzed by documenting hydrograph characteristics, computing event-based sediment discharges and reach erosion/deposition through the bypass system, modeling bypass deposition, and comparing modeled results near the weirs with dated sediment cores. The rapidly rising, slowly declining 1964 flood was generated by storm runoff in the Sierra Nevada. The modeling results indicate: washload discharge through the lower valley 0.5 to 1.7 times long-term annual averages; mainstem reach erosion/deposition 0.5 to 1.25 times annual averages; and centimeter scale deposition in flood bypasses. The results are corroborated by a set of sediment cores extracted from Sacramento Valley bypasses, which were dated with ²¹⁰Pb geochronology and analyzed for grain size. The modeling and data suggest net sediment accumulation between the channel and flood weirs and in the `hydraulic shadow' of the flood weir, the length of which varies depending on flow and sediment characteristics. Net accumulation in the hydraulic shadow is hypothesized to be associated with infrequent, episodic erosion of stored upland mining legacy sediments. As a result, more frequent, relatively clear-water flooding erodes prior bypass sediment deposits at the downstream end of the hydraulic shadow and propagates upstream toward the weir. Such sediment remobilization and scour events were extensively documented in our cores and have implications for the fate and transport of contaminants such as mercury, left over from decades of foothill mining, and for sediment and contaminant delivery to the Sacramento-San Francisco Bay-Delta. The modeling and field data highlighted shortcomings in conventional theory for event-based sediment concentration profiles and particle settling. These limitations could be addressed with appropriate data collection and model revision to account for the processes of sediment transport over weirs and into flood conveyance channels.
DE: 1803 Anthropogenic effects
DE: 1815 Erosion and sedimentation
DE: 1821 Floods
DE: 1824 Geomorphology (1625)
SC: Hydrology [H]
MN: 2005 Joint Assembly