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

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)

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

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

Mississippi River Delta Sea-level < Infilling

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

Tectonic Estuary San Francisco Bay

Gullmar Fjord, Norway sill sill

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

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

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 + =

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:

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

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.)

River flow and wind stress effect

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.

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)

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

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

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

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

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

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

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

Next Class – coastal circulation Chapter 8 in Talley et al