Physical Processes on Intertidal flats: Waves, Currents, and Sediment Transport Stefan Talke Feb. 18, 2009

What is an Intertidal flat? A muddy or sandy area without vegetation, generally in an estuary, which is exposed to air during low tide and covered by water during high tide. Low TideHigh Tide San Francisco

Why do we care? Estuarine boundary condition –Important for modeling estuarine physics correctly –Shoreline storm buffer (absorbs energy) Source and sink of sediments, contaminants –Transfer to/from deeper waters (e.g., shipping channels) –Pollution often buried in mudflats Mercury and other heavy metals, PCB’s, hydrocarbons, pesticides, etc. buried in the San Francisco Bay Ecologically important –Intertidal flats very productive Benthic diatoms account for 50% of primary production in Ems estuary, The Netherlands (de Jonge) Nursery for juvenile (small) fish, invertebrates (protected from predation) Clams, oysters, worms, shrimp live in mud Benthic diatoms Photos from V. de Jonge

Experiment Location: San Francisco Bay N 200 m 0 x Location of Experiment Heavy metal pollution Research question: What governs fate of contaminated sediment?

Sediments Contaminants Nutrients/Biota Theoretical Preliminaries Marsh Clams Worms Amphipods MHHW MLLW ~2 m SettlingTurbulenceBedload Transport Question: How can one understand the fate and transport of contaminants?

Theoretical Preliminaries Marsh MHHW MLLW ~2 m Our approach: Focus on sediment transport budget Erosion – Deposition = Flux offshore Flux Flux offshore = mass per unit time = sum of u * C over water column u = horizontal velocity C = suspended sediment concentration Basically accounting—if you know u(z) and C(z), can predict evolution of tidal flat  easy, right? (dispersion neglected)

Experimental Setup Multiple velocity, turbidity, and salinity sensors Deployed autonomously for days to weeks Acoustic Doppler Velocimeters Optical Backscatter Conductivity Temperature Depth probes

Results

Stratification  Stratification significant >0.5 kg/ m 3 between 10 cm and 50 cm depth Fluctuations at seiche frequency during ebb

7 hours 70 minutes 60 seconds Velocity Data Observation: Many time scales of variability

Power Spectrum Multiple frequencies of motion present over a tidal period (~0.002 Hz) ~0.1 Hz ~ 0.5 Hz < Hz

Wind data correlation Only onshore directed wind considered Conclusion: The highest frequency of variation occurs due to local wind waves N Wind from south Experiment Correlation wind/wave energy Significant correlation for f > 0.3 Hz

Ocean buoy wave spectrum Mudflat power spectrum Comparison of buoy wave spectrum and mudflat power spectrum

Bathymetrical Map of San Francisco Bay X Typical direction of ocean swell Refraction Bathymetry makes it unlikely for ocean swell to reach mudflat—yet they do! X X marks experiment site Reflection?

X marks experiment site = Seiching Lower frequency seiching X X Wave Speed = sqrt ( g* H)  Wave period ~ seconds Bathtub Analogy Forcing ~ 2-3 km m Forcing = wind, perhaps tide  When forcing stops, sloshing begins

What drives suspended sediment concentrations? Not tidal velocities (exert too little stress) Often, local wind waves But not always

Seiching produces anomolous response What drives suspended sediment concentrations? Bedload Transport: Settling on lee side of ripples Unidirectional flow Vortex eats at lee side of ripple, ejects sediment into water column Reversal of flow due to wave action ? Unlikely…

Velocity Reversals During positive seiching motion, frequency of velocity reversal is much greater Why? Tide 5

Comparison of waves Tide 5 Tide 8 Concentration Velocity Tide 5: Ocean swell dominated --As depth decreases, goes non-linear -- concentration fluctuations at ocean swell frequency Tide 8: Wind wave dominated -- Are not nonlinear over measured depths

What drives suspended sediment concentrations? Large gradients in salinity ‘trap’ sediment in fronts Not directly related to wind waves or seiching Called the ‘turbid tidal edge’

Large gradients in salinity ‘trap’ sediment in fronts Not directly related to wind waves or seiching Called the ‘turbid tidal edge’ What drives suspended sediment concentrations?

Sediment Fluxes Positive Transport Direction Definitions Separate raw velocity and concentration data into following frequency bands --Wave: 0.5 sec < T<100 sec --Seiche: 100 sec< 1500 sec --Tidal: T> 1500 sec Question: What are the implications of multiple frequencies of motion to sediment erosion, deposition, and transport? Flux:

Tidally averaged sediment fluxes Flow rate near bed (~ 10 cm) Suspended sediment concentration near bed (~ 10 cm) Storm period tides 1-4 Effect of freshwater discharge, storm surge

Tidally varying sediment fluxes Thought experiment: symmetric conditions Velocity time Suspended sediment concentration (SSC) time What is the net flux (uC) over the tidal (wave) period?(none) + shorewards

Tidally varying sediment fluxes Slightly different case: Asymmetric SSC Velocity time Suspended sediment concentration (SSC) time What is the net flux (uC) over the tidal (wave) period? (towards the shore) + shorewards

Tidally varying sediment fluxes Third thought experiment: Asymmetric velocity Velocity time Suspended sediment concentration (SSC) time What is the net flux (uC) over the tidal (wave) period?(offshore) + shorewards

Tidally varying sediment fluxes Conclusion: Asymmetries in velocity and/or SSC drive sediment fluxes over a tidal (or wave) period Here, we construct symmetric functions to determine whether SSC or velocity variation drives transport

Tidally varying sediment fluxes During storm (tides 1-4), asymmetries in tidal velocity and SSC drive fluxes After storm (tides 5-8), asymmetries in only SSC drive fluxes

Total sediment fluxes Big difference in magnitude, direction, and cause of sediment fluxes between storm and non-storm periods

Longer View Wave forcing inherently over only a portion of a tide  Tidally varying fluxes ubiquitous

Longer View Long term bias towards large events occurring during the flood tide Day of year Time (hrs*12)

Summary of Hydrodynamic forcing Near-bed flow characterized by following motions:  Tidal Motions (T ~ 12 hours) –Large changes in depth ( m) –Spring-Neap variation –Ebb Dominated  Seiching Motions (T~ 500 seconds) –Seiche in the Inner Richmond Harbor, most likely due to wind wave setup or asymmetric tidal phasing  Ocean Swell (T ~ 8-15 seconds) –Varies on time scale of days –Follows Rayleigh Distribution  Wind Waves (T ~ 1-3 seconds) –Extremely energetic events with timescale of hours or several days –Correlates with local winds and storm events

Summary of SSC Sediments are observed in water due to  Wind waves and ocean swell forcing  Seiching Motions (T~ 500 seconds)  Turbid tidal edge (frontal convergence process)

Tidally averaged fluxes due to order of magnitude increase in u and C occur during stormy periods –Freshwater forcing important Asymmetry in C over tidal period dominates sediment fluxes in non-stormy periods Elevated C observed due to waves, frontal processes, or seiching motions. These processes drive asymmetry during non-storm periods. Conclusions

Estuaries have many of the physical characteristics of lakes –Semi-enclosed bodies of water lead to seiching (‘bath tub sloshing’) –Locally driven wind waves important However, wave climate also analogous to open shore beach –Tides –Ocean swell, but scaled by smaller magnitudes and less violent forces Intersection of these paradigms and time scales creates unique, challenging environment

Intertidal Environment dominated by asymmetry and nonlinearity –Asymmetry Tides Wave forcing (e.g., Wind waves) Ripples Sediment concentration –Non-linearity Waves (ocean swell) Interaction between waves and currents Conclusions

Wind Waves Sediments Contaminants Nutrients/Biota Updated view of Intertidal Mudflat Marsh Clams Worms Amphipods MHHW MLLW ~2 ft Settling Turbulence Bedload Transport + sediment pumping Ocean Swell Seiche Stratification Tidal Forcing Erosion: & velocity reversals Other processes?

Thanks to an NIEHS project for support, and many colleagues for field assistance