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 M2 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

)

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)

Change in solution associatedwith 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.

Log10W/m2

Longitude Latitude

(B. Blanton)

Satilla River 1 m tide2-4 m mean depth50 m3/s avg riverflow0.5-1 m/s tidal currentsPristine, multiple channels in lower estuary5 km MHHW width, 1km MLW width

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

Field program in1999 – mooringsand surveying

Two deployment periods,spring and fall

Rapid survey tracks

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

Weirdexception

shelf

shore

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

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.

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 andnon-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 profilesshow x-channel structure extends to depth

Stratification weakerat spring tides butx-channel structurepersists

Axial velocity around Station 4

Spr

NpSpr

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 bendstrap the salinity gradient, slows upstreammovement of salt intrusion

Natural buffer to variationin salinity intrusion?

Average axial velocity profiles

Ebb > 0

Riv

er v

eloc

it y • Subtracting ur from profile still leaves no evidence for vertical exchange

• Provides strong suggestion for lateral exchange

intertidal