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Physical Processes on Intertidal flats:  Waves, Currents, and Sediment Transport

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. .

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Physical Processes on Intertidal flats:  Waves, Currents, and Sediment Transport

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

  2. 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. San Francisco San Francisco Low Tide High Tide

  3. 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

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

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

  6. Theoretical Preliminaries Our approach: Focus on sediment transport budget MHHW Flux ~2 m Marsh MLLW Erosion – Deposition = Flux offshore Flux offshore = mass per unit time = sum of u * C over water column (dispersion neglected) 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?

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

  8. Results

  9. Stratification • Stratification significant • >0.5 kg/ m3 between 10 cm and 50 cm depth • Fluctuations at seiche frequency during ebb

  10. 7 hours 70 minutes 60 seconds 60 seconds Velocity Data Observation: Many time scales of variability

  11. (~0.002 Hz) ~0.1 Hz ~ 0.5 Hz Power Spectrum • Multiple frequencies of motion present over a tidal period <0.0001 Hz

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

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

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

  15. X marks experiment site = Seiching Forcing 2-3.5 m ~ 2-3 km Lower frequency seiching Bathtub Analogy X X • Forcing = wind, perhaps tide • When forcing stops, sloshing begins Wave Speed = sqrt ( g* H)  Wave period ~ 250-700 seconds

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

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

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

  19. Comparison of waves • 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 Tide 8 Concentration Concentration Velocity Tide 5 Velocity

  20. 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’

  21. 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’

  22. 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: Sediment Fluxes Positive Transport Direction

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

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

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

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

  27. 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

  28. 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

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

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

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

  32. Summary of Hydrodynamic forcing Near-bed flow characterized by following motions: • Tidal Motions (T ~ 12 hours) • Large changes in depth (0 -1.5 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

  33. 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)

  34. Conclusions • 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.

  35. 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

  36. Conclusions • 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

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

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

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