Post on 16-Jan-2016
description
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