Land-Ocean Interactions:

Estuarine Circulation

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

Schematic of a typical Estuary

very fresh

quite sa

lty

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

Smaller estuary: salinity shows tidal variability

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

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

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:

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

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

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

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.

Some velocity profile data from the Hudson Riverocean river

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

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

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

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

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

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

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

Secondary flowsDensity driven lateral tidal cells - axial convergence

Salt-wedge

PartiallyMixed

Well-Mixed

http://www.youtube.com/watch?v=feBHj0HAZLI