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ETM: The Estuarine Turbidity Maximum

ETM: The Estuarine Turbidity Maximum. Puget Sound Oceanography 2011. ETM. The Estuarine Turbidity Maximum (ETM) is a region of locally-elevated suspended matter concentration that occurs near the landward end of salt intrusion in estuaries

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ETM: The Estuarine Turbidity Maximum

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  1. ETM: The Estuarine Turbidity Maximum Puget Sound Oceanography 2011

  2. ETM • The Estuarine Turbidity Maximum (ETM) is a region of locally-elevated suspended matter concentration that occurs near the landward end of salt intrusion in estuaries • It relies on the existence of a region of erodible sediments (the “Mud Reach”) which are resuspended on tidal time scales • The patch of erodible sediments exists because of persistent along-channel convergence of sediment transport

  3. St. Lawrence R. Elbe River Hudson R. Chesapeake Bay Gironde River Columbia R. San Francisco Bay Estuarine Turbidity Maximum (ETM) Depth River Ocean

  4. 5 10 May 2,1998 15 5 10 May 17,1999 15 10 20 30 40 50 60 70 Stratification, turbidity, salt front location influenced by freshwater flow 200 May 1998 (above average freshwater flow) 0 Depth (m) May 1999 (below average freshwater flow) NTU 50 0 Kilometers from River Mouth Sanford et al. 2001, North and Houde 2001

  5. Physics Part 1: The Rouse Profile • Sediment concentration is highest near the bottom • Results from a balance between particle sinking speed and turbulent diffusion Slow-sinking particles, high turbulence Fast-sinking particles, low turbulence

  6. Physics Part 2: Along-channel Convergence • The exchange flow modifies the tidal currents • EBB is strongest in the river end • FLOOD is strongest (at the bottom) in the estuary

  7. Observations about particle trapping in ETMs • Particle trapping in ETMs occurs by asymmetrical tidal transport of a pool of resuspendible particles with a limited range of settling velocities • Fine sediments in estuarine environments almost always exist in aggregated (flocculated) form. Aggregation and disaggregation can be active processes, depending on concentration, stickiness, and small scale shear. • Settling velocities of fine sediments trapped in ETMs are determined by the aggregate properties (size and specific density), not the individual particle properties • Typical particle settling speeds: • Fine silt 0.1 mm/s • Medium silt 1 mm/s • Fine sand 3 m/s • Flocs: can be much faster

  8. ETM particles are packed with organics 20 to 63 um 10 to 20 um > 63 um < 10 um Close-up DAPI stained Columbia River Crump & Baross, 2000

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