1. Land-Ocean Interactions: Estuarine Circulation

2. Land-Ocean Interactions: Estuarine Circulation
Estuary: 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 mouth
Estuary
Estuary head
River
A typical estuary has most of the freshwater entering at its head, and has a transitional section between the body and the coastal ocean.
It is a common practice to classify estuaries into different categories, using varying schemes (based on their geomorphology or circulation) for the purposes of enabling us to study, qualitatively, common properties across multiple geographic locations. Subtleties of the terrestrial inputs and unique circulations affected by complicated coastline and bathymetry conspires to make most estuaries more unique than alike. But we can understand common physical dynamical processes that estuaries have in common…

3. Schematic of a typical Estuary

4. very fresh
Chesapeake – a really big estuary
quite salty

5. 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
About 90% of the nutrients that enter the Chesapeake from river runoff are assimilated by plankton in the Chesapeake Bay and exported as inorganic nutrients into the coastal ocean.
The estuary acts as a large filter, processing and modifying the inputs.
The capacity of an estuary to act this way depends heavily on the residence time, which is affected by the physics of the estuarine circulation.

6. Smaller estuary: salinity shows tidal variability
The James River – a much smaller estuary feeding into the Chesapeake Bay - shows a noticeable signature of tidal variability in the surface salinity distribution. Many estuaries have a pronounced cycle in the vertical stratification associated with the tides.
Smaller estuary: salinity shows tidal variability

7. 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 and momentum
typically small: wind, Coriolis, long time scale coastal sea level
significant forces: friction (mixing), pressure, nonlinearity, acceleration (time variability)
tides are important
most common dynamic balance is 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

8. 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
Long and narrow
Approximated by 2-dimensional vertical/along axis flow (relatively little across axis flow)
Mathematically we have balance equations for salt, mass and momentum
possible forces are friction, pressure, nonlinearity, unsteady (acceleration)
typically small: wind, Coriolis, coastal sea level (expect where it drives the tides)
relative magnitude of these terms one classification scheme
Dynamic balance is between pressure force and friction/mixing
Mixing affects the salt balance – which affects the pressure distribution and pressure gradient
Common features:
strong tidal forcing
large density difference between river and ocean
complex topography
Can classify estuaries baaed on their topography/geomorphology

9. Fjords Glacial valleys flooded by rising sea level
Topography classification:
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
Alaska, British Columbia, Norway, Scotland
Chile, New Zealand
River valleys deepened by glaciers
Very deep due to glacial scouring
800 m deep
Shallow sill at mouth (terminal moraine of glacier)

11. 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
Sketch partially mixed or salt wedge type section

13. 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

15. 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
It’s Physics!

16. 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.
Long and narrow
Approximated by 2-dimensional vertical/along axis flow (relatively little across axis flow)
Mathematically we have balance equations for salt, mass and momentum
possible forces are friction, pressure, nonlinearity, unsteady (acceleration)
typically small: wind, Coriolis, coastal sea level (expect where it drives the tides)
relative magnitude of these terms one classification scheme
Dynamic balance is between pressure force and friction/mixing
Mixing affects the salt balance – which affects the pressure distribution and pressure gradient
Common features:
strong tidal forcing
large density difference between river and ocean
complex topography
Can classify estuaries baaed on their topography/geomorphology

17. Tides are the principal source of mixing energy
Velocity shear and turbulence are generated
in the bottom boundary layer from friction and bottom drag
in the velocity shear across the halocline
but density difference works against mixing

18. to ocean
river

19. to ocean river flood tide ebb tide to ocean river
Geyer, W.R. And P. MacCready, Annu. Rev. Fluid Mech :175–97, doi: /annurev-fluid
to ocean
river
Profiles of velocity, density anomaly, and eddy viscosity as they evolve over a tidal period for the strain-induced periodic stratification regime: There is complete destratification during the flood tide, leading to strong mixing and almost no vertical shear. On the ebb tide, substantial shear may develop due to suppression of turbulence by stratification, which originates from the straining of the horizontal density gradient. The stratification at the end of the flood tide results from lateral straining, whereas the increasing stratification during the ebb tide results from along-estuary straining.
Geyer, W.R. And P. MacCready, Annu. Rev. Fluid Mech :175–97, doi: /annurev-fluid

20. ocean
river
ocean
river
ocean
river

21. (averaged over several tidal cycles) V1 = V2 S2/S1 Volume balance:
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 , S4
V2 , S2
Difference between upper and lower transport is always R
This is the same logic we used in determining the exchange flow through the Strait of Gibralter in the Nov 17, 2010, lecture

22. Mass transport in a highly stratified estuary
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.
Entrainment adds salt to the surface layer, so salinity increases seaward
Vigorous circulation aids flushing
Can enable upstream biological transport (retention of larvae in estuaries)
© 1996 M. Tomczak

23. Salinity in a salt wedge estuary
Top: As a function of depth and distance along estuary
Bottom: Vertical salinity profiles for stations 1-4
Surface salinity is close to zero at all stations. Bottom salinity is close to oceanic.
If the tidal excursion, or tidal volume, is small compared to the freshwater input (R times T_tide)
freshwater floats on saltier water w/o much mixing
wedge of salt water extends into estuary with little mixing
most of the water exchange is at the front
© 1996 M. Tomczak

24. Salinity in a slightly stratified (partially-mixed) estuary
Top: As a function of depth and distance along estuary
Mixing is indicated by the circles.
Bottom: Vertical salinity profiles for stations 1-4 Surface and bottom salinity increase from station 1 to 4, but surface salinity is always slightly fresher.
If the tidal volume is increased we get more mixing
gives a slightly stratified or “partially mixed” estuary
significant vertical mixing everywhere – dilution of the lower layer salinity
© 1996 M. Tomczak

25. Salinity in a vertically well-mixed estuary
Top: As a function of depth and distance along estuary
Bottom: Vertical salinity profiles for stations 1-4 Surface and bottom salinity increase from station 1 to 4, but surface and bottom salinity are always nearly identical
If tidal volume and/or tidal mixing increases still further, locally the salinity stratification is nearly eliminated.
Well mixed estuary
Salinity increases toward the sea but does not vary with depth
Penetration of salt upstream is by horizontal mixing only – no longer any overturning estuarine circulation
An estuary can transition between the well mixed and partially mixed regimes on the spring-neap tidal cycle
This can actually mean the estuary is less flushed on the more energetic spring tide
the river outflow transport is distributed over the entire water depth, so the outflow current is weaker, whereas when the estuarine circulation is active there is faster outflow at the surface balanced by inflow at the bottom.
© 1996 M. Tomczak

28. Secondary flows Density driven lateral tidal cells - axial convergence
Asymmetry is because of friction acting on different water depths

29. 3-dimensional circulation
Slightly stratified estuary with weak Coriolis effect (northern hemisphere).
Slightly stratified with strong Coriolis effect
Vertically mixed estuary with Coriolis effect
Coriolis force concentrates flow on the right bank
both ebb and flood
Get weaker mean flow on left (looking seaward) than right
If the effect is strong, the mean flow on the left can be into the estuary producing a strong secondary circulation
Blue (dark) arrows indicate
upper layer flow, and red (light) arrows bottom flow
© 1996 M. Tomczak