cross-sectional variability: low- flow, moderate-flow, and high-flow events Bay Sediment Project S. Geological Survey cramento, CA Item #2b
Mar 27, 2015
Mallard Island cross-sectionalvariability: low-flow, moderate-flow, and high-flow events
SF Bay Sediment ProjectU.S. Geological SurveySacramento, CA
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Mallard Island Cross-sectional Variability
• Validation of sediment load calculation methodologyby McKee et al. (2005)
• Low-flow measurement on August 31, 2004•Freshwater outflow at a minimum (~ 113 cms = 4,000 cfs)•Peak spring tide •Large wind-waves (>1 m)
• Moderate-flow measurement on January 4, 2005•Freshwater outflow moderate (~ 1830 cms = 64,000 cfs)•2 days after neap tide •No wind-waves
• High-flow measurement on January 5, 2006•Freshwater outflow highest since 1997 (~ 9700 cms = 340,000 cfs)•3 days after peak spring tide •No wind-waves
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Protocol
• ADCP: Perform discharge measurement, identify five discharge centroids
• CTDO profiling: After ADCP measurement, return to five centroids, lower and raise CTDO package through water column at each centroid
• Water sampling: Periodically collect water samples for optical sensor calibration, at multiple depths
• Compare discharge-weighted SSC with SSC at gagehouse (used for McKee calculation)
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Suspended-sediment record at Mallard IslandLow-flow, spring tide sampling: 8/31/2004, Q = 3,892 cfs
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Suspended-sediment record at Mallard IslandModerate-flow, first-flush sampling: 1/4/05, Q = 64,000 cfs
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Suspended-sediment record at Mallard IslandHigh-flow, first-flush sampling: 1/5/06, Q = 340,000 cfs
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Comparison of cross-sectional, velocity-weighted SSC and Mallard Island upper SSC
Bias due to high SSC on north side of channel
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Reasons to reserve judgment: NASA
Satellite image taken over SF Bay
Pulse at Mallard Island
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• NASA-ASTER image March 3, 2000, Q=150,000 cfs
• Centroid of sediment pulse seaward of Suisun Bay
• 17 days after peak Dayflow, 12 days after peak SSC at Mallard Island
• Sediment storage in Honker Bay affecting SSC in Mallard Island X-S
Reasons to reserve judgment: NASA Item #2b
Reasons to reserve judgment: models
Modeled cross-sectional SSC vs. modeled upper sensor SSC
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Mean error = 25%McKee (2005) estimate = 30%
Reasons to reserve judgment: models
Daily averages: modeled cross-sectional SSC vs. modeled upper sensor SSC
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Next steps• Not enough information to definitively modify McKee’s flux
estimates
• Are extremes bracketed? I think so…
• Need quasi-continuous velocity and SSC data over the duration of a multi-day flood event, across the channel
• Two possible approaches
- Deploy multiple instrument packages in channel- Perform a few cross-sectional surveys everyday for the duration of a 3-5 day flood pulse
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