land-ocean interactions: estuarine circulation. estuary: a semi-enclosed coastal body of water which...
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Land-Ocean Interactions:
Estuarine Circulation
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Land-Ocean Interactions:
Estuarine CirculationEstuary: a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage. (Pritchard,1963)
Coastal Ocean
Estuary mouthEstuary
Estuary head
River
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Schematic of a typical Estuary
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very fresh
quite sa
lty
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Density
gradient
along axis of
estuary
… and in the
vertical
(strongly
stratified)
Stratification evolves over time in response to freshwater inflow – shows time scale of estuary residence time is long
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Smaller estuary: salinity shows tidal variability
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Characteristics of estuaries• Most estuaries:
– strong tidal forcing– large density difference between river and ocean– complex topography– Long and narrow – can often be approximated by 2-dimensional vertical/along-axis
flow (relatively little across axis flow)
• Mathematically we have equations for salt, mass (volume) and momentum– significant forces: friction (mixing), pressure, nonlinearity, acceleration
(time variability)– typically small: wind, Coriolis, longer that tidal period coastal sea level
(tides are important)– most common dynamic balance is between pressure and friction/mixing
• Mixing affects the salt balance …• … which affects the pressure distribution and pressure gradient
• Can classify estuaries based on their physics (relative magnitude of different terms), or topography/geomorphology
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Physics essentials:
• Fresh river water encounters salty ocean water• Fresh = light; salty = heavy• Freshwater flows seaward at the surface• Get landward flow of more dense, salty, water
– estuarine or gravitational or baroclinic circulation– time scales of ~1 day … so Coriolis force is usually of
secondary importance– circulation is evident averaged over a few tidal cycles– mixing and entrainment processes are central to
details of the salt and volume transport balance
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Fjords• Glacial valleys flooded by
rising sea level
• Found poleward of 43o latitude
• Narrow, deep inlets
• Shallow sill connect fjord with ocean
• Freshwater flows out in a thin surface layer
• Deep water is near oceanic salinity and relatively motionless
Topography classification:
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Coastal Plain Estuaries
• River valleys flooded by sea level rise following glacial period (sometimes sediment-filled fjords)
• Little sedimentation
• Ancient river valleys determine the topography
• Shallower than fjords and more uniform in depth
• Extent of salt influence depends on forcing more than bathymetry
• Tides are often the most important source of mixing
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Bar-built and Lagoon Estuaries
• Drowned river valleys with high sedimentation rates
• Very shallow
• Often branch toward mouth into a system of shallow waterways (lagoons)
• Narrow connections to the ocean
• Sediment accumulates at mouth contributing to bar formation
• Shallow lagoons can be well-mixed by tides and winds
• Complex topography: channels, island and shoals
• Multiple sources of freshwater
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Classification based on salinity structure (= physics yay!)
• The majority of estuaries in populated coastal regions are in the coastal plain category (locally: Chesapeake, Delaware, Hudson)
• Within this group there are large differences in circulation patterns, density, residence time, and mixing
• A better classification is one based on salinity and flow characteristics
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Physics essentials:
• Fresh river water encounters salty ocean water• Fresh = light; salty = heavy• Freshwater flows seaward at the surface• Get landward flow of more dense, salty, water
– estuarine or gravitational or baroclinic circulation– time scales of ~1 day … so Coriolis force is
usually of secondary importance– circulation is evident averaged over a few tidal
cycles– mixing and entrainment processes are central
to details of the salt and volume transport balance
Mixing across the strong vertical salinity gradient is significant
Turbulence driven by velocity shear affects mixing rates
Density stratification works against mixing but does not prevent it.
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Some velocity profile data from the Hudson Riverocean river
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Top: As a function of depthand distance along estuary
Bottom: Vertical salinityprofiles for stations 1-4
Surface salinity increases from station 1 to station 4, but bottom salinity is close to oceanic at all stations
Salinity in a highly stratified estuary
© 1996 M. Tomczak
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River volume flow is R.
Outflow from the estuary in the upper layer is 10R.
This is balanced by oceanic inflow of 9R.
The net outflow at the ocean end is, of course, still only 1R.
Mass transport in a highly stratified estuary
© 1996 M. Tomczak
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Salt balance:
Salt in = V2S2 + R So
Salt out = V1S1
V1S1 = V2S2
(averaged over several tidal cycles)
V1 = V2 S2/S1
Volume balance:
R + V2 = V1
R = V1 – V2
= V2(S2/S1) – V2
= V2(S2/S1 – 1)
V2 = R / (S2/S1– 1) or
= S1 R / (S2 – S1)
V1 = S2 R / (S2 – S1)
R V1 , S1
V2 , S2
Vertical flux of salt through entrainment
R V1 , S1 V3 , S3
V4 , S4V2 , S2
Difference between upper and lower transport is always R
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Salinity in a salt wedge estuary
Top: As a function of depthand distance along estuary
Bottom: Vertical salinityprofiles for stations 1-4
Surface salinity is close to zero at all stations. Bottom salinity is close to oceanic.
© 1996 M. Tomczak
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Salinity in a slightly stratified (partially-mixed) estuary
Top: As a function of depthand distance along estuaryMixing is indicated by the circles.Bottom: Vertical salinityprofiles for stations 1-4
Surface and bottom salinity increase from station 1 to 4, but surface salinity is always slightly fresher.
© 1996 M. Tomczak
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Salinity in a vertically well-mixed estuary
Top: As a function of depthand distance along estuaryBottom: Vertical salinityprofiles for stations 1-4
Surface and bottom salinity increase from station 1 to 4, but surface and bottom salinity are always nearly identical
© 1996 M. Tomczak
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3-dimensional circulation
a. Slightly stratified estuary with weak Coriolis effect (northern hemisphere).
b. Slightly stratified with strong Coriolis effect
c. Vertically mixed estuary with Coriolis effect
Blue (dark) arrows indicate upper layer flow, and red (light) arrows bottom flow
© 1996 M. Tomczak
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Secondary flowsDensity driven lateral tidal cells - axial convergence
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Salt-wedge
PartiallyMixed
Well-Mixed
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