Mallard Island cross-sectionalvariability: low-flow, moderate-flow, and high-flow events

SF Bay Sediment ProjectU.S. Geological SurveySacramento, CA

Item #2b

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

Item #2b

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)

Item #2b

Item #2b

Suspended-sediment record at Mallard IslandLow-flow, spring tide sampling: 8/31/2004, Q = 3,892 cfs

Item #2b

Suspended-sediment record at Mallard IslandModerate-flow, first-flush sampling: 1/4/05, Q = 64,000 cfs

Item #2b

Suspended-sediment record at Mallard IslandHigh-flow, first-flush sampling: 1/5/06, Q = 340,000 cfs

Item #2b

Comparison of cross-sectional, velocity-weighted SSC and Mallard Island upper SSC

Bias due to high SSC on north side of channel

Item #2b

Reasons to reserve judgment: NASA

Satellite image taken over SF Bay

Pulse at Mallard Island

Item #2b

• 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

Item #2b

Mean error = 25%McKee (2005) estimate = 30%

Reasons to reserve judgment: models

Daily averages: modeled cross-sectional SSC vs. modeled upper sensor SSC

Item #2b

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

Item #2b