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Tides and the salt balance in a sinuous coastal plain estuary

Tides and the salt balance in a sinuous coastal plain estuary. H. Seim, UNC-CH J. Blanton, SkIO. Tides Residual circulation Salt balance. Modeled M 2 elevation without estuaries – tide experiences two-fold amplitude increase and notable phase change in SAB. NC. SC. GA. FL.

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Tides and the salt balance in a sinuous coastal plain estuary

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  1. Tides and the salt balance in a sinuous coastal plain estuary H. Seim, UNC-CH J. Blanton, SkIO Tides Residual circulation Salt balance

  2. Modeled M2 elevation without estuaries – tide experiences two-fold amplitude increase and notable phase change in SAB NC SC GA FL • Finite Element • Nonlinear • 2D (ADCIRC) • Western North Atl. • Crossshelf Amplification • Equatorward phase propagation • Latest phase along GA/FL border • Shelf response sensitive m (B. Blanton)

  3. In the SAB large sections of the coastline are backed by extensive estuaries depth (m) (K. Smith, D. Lynch)

  4. M2 Solution Elevation Difference Amplitude Ratio Est sol’n Amp -------------------------- > 1 NoEst sol’n Amp Phase Diff (in red) Est Phase - NoEst Phase >0 (B. Blanton)

  5. Change in solution associated with energy flux into estuaries. Estuaries must be a sink of energy (high dissipation)

  6. Including estuaries increases dissipation >25%... Strange result – inclusion of highly dissipative estuaries leads to 10% increase in tidal range. Log10W/m2 Longitude Latitude (B. Blanton)

  7. Satilla River 1 m tide 2-4 m mean depth 50 m3/s avg riverflow 0.5-1 m/s tidal currents Pristine, multiple channels in lower estuary 5 km MHHW width, 1km MLW width

  8. Bottom topography – not well known (last full survey in 1920s), not maintained

  9. Field program in 1999 – moorings and surveying Two deployment periods, spring and fall

  10. Rapid survey tracks

  11. Tidal analysis • Derived tidal constituents (using t-tide) from 2 month-long records at mooring locations • Compared to shelf observations in Blanton et al. 2004

  12. M2 tide – maximum in estuary… shore * shelf *

  13. M2 currents – increasing landward, big phase change shore shelf

  14. Tide increasingly ‘progressive’ moving inshore shore Weird exception shelf

  15. Hypersynchonous estuary • Strongly convergent geometry (Lb<<λ) • No reflected tidal wave • Wave speed close to √(gH) • Phase difference typically like standing wave but sensitive to geometry, friction

  16. Energy flux and dissipation • Big energy flux at mooring sites (7000-21000 W/m) • Infer large dissipation rates (0.5-1.5 W/m2) • Equivalent to 10-4 W/kg, 10,000-100,000 open ocean values.

  17. Roving survey analysis • Performed least-squares fits to zero, semi-diurnal and quatra-diurnal frequencies • Q: is there cross-channel structure to the flow?

  18. Depth-scaling accounts for ~25% of variance – rest due to bends and non-linearities?

  19. Flow around bends in rivers – big influence, but simple topography, trickier in the estuary

  20. Tidal energy – can be dissipated or transferred to other frequencies….. generate STRONG depth-averaged mean circulation, amazing pattern associated with bends

  21. Subtidal flow – nearly all moorings show seaward flow – in deep channel inland seaward

  22. Spr Np Example axial velocity map Seaward Landward

  23. Salinity regime SAT 1 SAT 2

  24. Alongchannel salinity field response – rapid adjust to discharge pulses, slow recovery

  25. Salinity response to discharge

  26. Alongchannel salinity • Obvious maximum gradient, often in region of the bends • Asymmetric temporal response to discharge changes – fast seaward, slow landward

  27. Mean surface salinity – show strong cross-channel structure

  28. Mean salinity profiles show x-channel structure extends to depth

  29. Stratification weaker at spring tides but x-channel structure persists

  30. Spr Np Axial velocity around Station 4 Spr Np

  31. 1D longitudinal dispersion fit requires salt flux of 0.1 PSU*m/s…

  32. Spring • Flow in deep channels carries salt seaward • Seaward salt flux is exchanged by landward flow upstream from cross-over flow • Exchange system is part of system of tidal eddies Speculation on lateral exchange in salt balance Circulation at bends trap the salinity gradient, slows upstream movement of salt intrusion Natural buffer to variation in salinity intrusion? • Landward flux is result of tidal asymmetry in favor of flood

  33. River velocity intertidal Ebb > 0 • Subtracting ur from profile still leaves no evidence for vertical exchange • Provides strong suggestion for lateral exchange Average axial velocity profiles

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