1. Estuaries
Chapter 8 – Talley et al.
Outline:
What is an estuary?
Geologic classification of Estuaries
Classification based on salinity structure
Estuarine Dynamics (Tidal and Tidally-Averaged)
Coriolis effects

2. Semi-enclosed and coastal body of water
What is an Estuary?
Semi-enclosed and coastal body of water
2) Free connection with open sea
3) Sea water is measurably diluted with fresh water derived from land drainage
River (salinity ~ 0)
Estuary 5 10 15 20 25 35 30
Ocean (salinity ~ 35)
Cameron and Pritchard (1963, Estuaries)

3. Estuarine Classification
Morphology/Geology
Coastal Plain
Delta System
Bar-built
Tectonic
Fjords
Salinity Structure
Salt Wedge
Partially-Mixed
Well-Mixed
Tides
Microtidal: 2 m > range
Mesotidal: 2 > range > 4 m
Macrotidal: 4 > range > 6 m
Hypertidal: range > 6m
Passive Margins
Active Margins
Glaciated Regions

4. Gioronde Estuary France
Estuaries are short lived (thousands of years) on geologic time scales (millions of years)
Rapidly varying Sea Level creates estuaries by eroding valleys and canyons during sea-level fall and drowning them during sea-level rise.
Constant Sea Level eliminates estuaries by sediment infilling.
Continued Existence of Estuaries requires sea level>= sediment infilling.
Chesapeake Bay
Gioronde Estuary France
Sea-level > Infilling
Sea-level ~ Infilling

5. Mississippi River Delta
Sea-level < Infilling

6. Palmiet Estuary, South Africa
Bar-Built Estuary
Palmiet Estuary, South Africa
Longshore current

7. Tectonic Estuary
San Francisco Bay

8. Gullmar Fjord, Norway
sill
sill

9. Classification by Salinity Structure:
a) Salt Wedge
b) Partially mixed
c) Well Mixed (vertically)
d) Well Mixed (vertically and laterally)

10. Tidal Dynamics of Estuaries
Daily variations of tidal height at the mouth drives water in/out
river
ocean
z=η
FLOOD
z=η
river
ocean
EBB
Stronger gradients during ebb gives more mixing
Force balance for flow
is surface elevation
and friction

11. Sub-tidal (tidally-averaged) Dynamics of Estuaries
z=-h
z=0
river
ocean
z=η z y x
Baroclinic PG
Stress Divergence
Barotropic PG
Assumes that Az and
dρ/dx are constant in z.
Barotropic PG + Baroclinic PG balanced by turbulent friction of mean circulation + =

12. Derivation of Estuarine Circulation
x-momentum:
Integrate twice in z:
Apply boundary conditions: 1) no stress at surface; 2) no flow at bottom (no slip condition)
This gives:
Plug in C1 and C2:
Solve for dη/dx, using continuity:
…or if river velocity is small:
Gives:
Plug in expression for dη/dx:

13. The tidally averaged circulation is called “Estuarine Circulation” or “Gravitational Circulation”
Solutions assuming H = 10 m and dS/dx = 4×10-4 psu/m

14. Tidally-averaged Circulation from the Hudson River:
Spring tides cause stronger mixing;
Neap tides have weaker mixing
Velocity contours in cm/sec
From Lerczak et al. (2006, J. Phys. Oceanogr.)

15. River flow and wind stress effect

16. The wind has a stronger effect on the shallow flow
The wind has a stronger effect on the shallow flow. To maintain a constant transport (average flow speed * width * depth), the deep water is pushed upwind.

17. The tidally-averaged estuarine circulation makes estuaries very effective traps for materials
River
Estuary
Ocean
Estuarine Turbidity Maximum—Convergence between seaward river flux and landward estuarine circulation leads to accumulation of sediment leading to high turbidity near the limit of salt intrusion (~1 psu)

18. This promotes the creation of density stratification
Estuarine Circulation brings salt water up the estuary along the bottom and fresh water out at the surface.
This promotes the creation of density stratification
z=-h
z=0
river
ocean
z=η z y x
The balance between advection and turbulent mixing determines the strength of vertical density stratification

19. Tidally-averaged Salinity from the Hudson River
Velocity contours psu (deviation from cross-sectional mean value)
From Lerczak et al. (2006, J. Phys. Oceanogr.)

20. Both Length of the Estuary and Stratification are Impacted by River Discharge
Stratification (ΔS)
river
ocean
Estuary Length
Stratification (ΔS)
river
ocean
Estuary Length

21. Amazon River Continental Shelf
When river discharge gets too big, the entire estuary gets pushed out onto the shelf.
Amazon River Continental Shelf
From Kineke and Sternberg, 1995

22. Fjord circulation Narrow and deep Intermediate or shallow sill
Weak runoff
Thin surface layer
Weak mixing
Hypoxic deep water
Often at high latitude (Norway, Greenland, Chile, New Zealand

23. Coriolis effects on estuaries
Flow deflects to the right
Outflow deflects to the right

24. Water goes downwind in the bay
Direct and remote effect of winds on enclosed bay connected to the ocean
Water goes downwind in the bay
Ekman transport adds or removes water from the bay.
High water
Low water
Low water
High Water

25. Next Class – coastal circulation
Chapter 8 in Talley et al