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Waves and resuspension on the shoals of San Francisco Bay

Waves and resuspension on the shoals of San Francisco Bay. Jessie Lacy USGS-CMG. Motivation. San Francisco Bay is a shallow estuary. Historically, most hydrodynamic data collected in channels. Resuspension processes differ in the channel and shoals.

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Waves and resuspension on the shoals of San Francisco Bay

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  1. Waves and resuspension on the shoals of San Francisco Bay Jessie Lacy USGS-CMG

  2. Motivation San Francisco Bay is a shallow estuary. Historically, most hydrodynamic data collected in channels. Resuspension processes differ in the channel and shoals. Restoration and maintenance of marshes creates a sediment sink, especially with sea level rise. Most of the sediment supplied from, or transported across, shoals. Mudflats provide important habitat for invertebrates and birds.

  3. Shoal-channel sediment exchange USGS-UC Berkeley NSF funded Mark Stacey Jessie Lacy Jim Hunt Andreas Brand Steve Gladding Audric Collignon • Goals • Evaluate mechanisms of sediment transport from shoals to channels in South Bay. • Investigate seasonal variation in their importance. • Investigate resuspension dynamics in subtidal shoals.

  4. Study Site Spring: Feb 23-Mar 16, 2009 Fall: Sep 9-Oct 6, 2009

  5. Data collection Slope and Channel-Mid stations ADCP Top and bottom CTD/OBS During each deployment: ADCP/CTD/OBS cross-channel transecting. SSC sampling for calibration. Grab samples of bed sediment. Shoal stations ADV pair CTD/OBS Benthic ADV/OBS/pressure 36 and 72 cmab 8 min burst every 12 min LISST 2 CTDs Imaging sonar

  6. Measuring suspended sediment OBS ADV

  7. Measuring suspended sediment: calibration ADVs and OBSs calibrated against field data.

  8. What drives Sediment concentration?

  9. turbulentflux settling With ADVs we can directly measurement turbulent flux of sediment. During resuspension events, first term>>second term. Calculation of <w’c’> requires separation of waves from turbulence. We used Shaw and Trowbridge method.

  10. What drives Resuspension? W’C’ C

  11. What drives Resuspension? 1) Waves at low tide 2) Maximum sediment flux at maximum current shear 3) Sediment resuspension after flood tide 4) Elevated resuspension due to reduced critical shear stress

  12. What drives Resuspension? Before wind wave event Loosening of sediment by waves during slack Resuspension under combined current wave shear Resuspension of weakly consolidated sediment after waves event Sediment

  13. Fall Deployment: Sediment concentrations ShN: Red, Be: black

  14. Particle size Particle size influences settling rate and thus SSC.

  15. Simple 2nd order flocculation model • 2nd order kinetics due to 2 particle collisions • SSC decrease due to formation of particles too large to be kept in suspension by turbulence • Rate of flocculation (k) measured by decrease in SSC over time • Hypothesis: k is a function of fluid shear rate.

  16. South Bay Conclusions • Elevated sediment concentrations (and thus transport) on the shoals is produced by: • Wind-wave resuspension, especially at low tide, AND • Turbulence created by tidal currents, which mixes sediment up into the water column, and contributes to bed shear stress. • Particle size varies over the tidal cycle due to flocculation, influencing settling dynamics. • Wind-wave resuspension response depends on the pool of erodible sediment. • Less frequent strong wind events very important to total flux.

  17. Wave attenuation in Corte Madera Bay BCDC USGS EPA What is the role of marshes and mudflats in attenuating waves in SF Bay? Risk of diminishing this function by loss of wetlands due to sea-level rise. Delft 3D Modeling Bruce Jaffe Mick van der Wegen (UNESCO) Wave measurements Jessie Lacy Bathy/Topo Bruce Jaffe Rob Kayen

  18. Instrument deployment Jan 22-Mar 23, 2010 S stations ADV, OBS 0.5-1.5 m MLLW M stations Bursting pressure sensor DP ADCP, top & bottom CTD bottom OBS

  19. Additional sampling: Water samples for SSC calibration. Grab samples of bed sediments.

  20. Data from S3 ADV 25 cm above the bed • 3 events with wave heights > 0.3 m. • Wave direction usually 120º • Period 2-3 s. • Peaks in ub and SSC at low tides. • Clear spring-neap signal in OBS voltage.

  21. Instantaneous burst data Waves at high tide may inundate marsh. Ferry wakes occur at all tidal stages.

  22. Knowledge gaps • Bed sediment properties (density, biofilms) and their relationship to critical shear stress • Temporal variability • Variation with depth • Particle dynamics: grain size in suspension and settling. • Transport mechanisms: between subtidal and intertidal shoals, intertidal mudflats and marshes. • Timescale of retention of sediment in San Francisco Bay.

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