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

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)

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

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)

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

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)

Satilla River 1 m tide 2-4 m mean depth 50 m 3 /s avg riverflow m/s tidal currents Pristine, multiple channels in lower estuary 5 km MHHW width, 1km MLW width

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

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

Rapid survey tracks

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

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

M2 tide – maximum in estuary… * * shore shelf

M2 currents – increasing landward, big phase change shelf shore

Tide increasingly ‘progressive’ moving inshore Weird exception shelf shore

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

Energy flux and dissipation Big energy flux at mooring sites ( W/m) Infer large dissipation rates ( W/m 2 ) Equivalent to W/kg, 10, ,000 open ocean values.

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

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

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

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

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

Example axial velocity map Spr Np Landward Seaward

Salinity regime SAT 1 SAT 2

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

Salinity response to discharge

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

Mean surface salinity – show strong cross-channel structure

Mean salinity profiles show x-channel structure extends to depth

Stratification weaker at spring tides but x-channel structure persists

Axial velocity around Station 4 Spr Np Spr Np

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

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?

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

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