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

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

4. M 2 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. Log 10 W/m 2 Longitude Latitude (B. Blanton)

7. Satilla River 1 m tide 2-4 m mean depth 50 m 3 /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

11. Rapid survey tracks

12. Roving surveys – tried to sample cross-channel structure; Shallow depths limited where this could happen

13. 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

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

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

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

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

18. Energy flux and dissipation Big energy flux at mooring sites (7000- 21000 W/m) Infer large dissipation rates (0.5-1.5 W/m 2 ) Equivalent to 10 -4 W/kg, 10,000-100,000 open ocean values.

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

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

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

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

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

28. Example axial velocity map Spr Np Landward Seaward

31. Salinity regime SAT 1 SAT 2

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

34. Salinity response to discharge

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

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

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

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

42. Axial velocity around Station 4 Spr Np Spr Np

44. 1D longitudinal dispersion fit requires salt flux of 0.1 PSU*m/s…

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

46. CONCLUSIONS Landward flow over shallows and seaward flow over deeps consistent with Li & O’Donnell (2005) model for a short estuary in spring tides Deviates from model at neap, suggesting importance of vertical stratification Tidal asymmetry particularly strong at neap which enhances landward flow in the shallower channels

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