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. 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.

Shoal-channel sediment exchange 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. Andreas Brand Steve Gladding Audric Collignon USGS-UC Berkeley NSF funded Mark Stacey Jessie Lacy Jim Hunt

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

Data collection Slope and Channel-Mid stations ADCP Top and bottom CTD/OBS 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 During each deployment: ADCP/CTD/OBS cross-channel transecting. SSC sampling for calibration. Grab samples of bed sediment.

Measuring suspended sediment OBSADV

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

What drives Sediment concentration?

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

What drives Resuspension? C W’C’

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 What drives Resuspension?

Sediment 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

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

Particle size Particle size influences settling rate and thus SSC.

Simple 2 nd order flocculation model 2 nd 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.

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.

Wave attenuation in Corte Madera Bay Wave measurements Jessie Lacy Bathy/Topo Bruce Jaffe Rob Kayen Delft 3D Modeling Bruce Jaffe Mick van der Wegen (UNESCO) 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. BCDC USGS EPA

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

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

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.

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

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.