Lecture 5: Estuary circulation Flashcards
What is an estuary?
An estuary is:
(a) a semi-enclosed coastal body of water,
(b) which has a free connection with the open sea, and
(c) within which sea water is measurably diluted with fresh water derived from land drainage.
Why are estuaries important?
Estuaries are among the most productive
ecosystems on Earth
They are culturally, commercially and recreationally important
They also provide a range of valuable ’ecosystem services’
What are some major threats Estuaries face?
- Catchment clearing and modification
resulting in increased runoff - Sediment and nutrients loadings causing
eutrophication in estuarine ecosystems - Toxicants resulting from agriculture,
industry and shipping - Sea level changes that influence habitat
availability in estuarine habitats
Describe the Principle of continuity:
Because seawater is almost incompressible, a flow that is converging in one direction must be diverging in another direction.
Volume flux into a box must equal the
volume flux out of the region, adding up on all faces of the box
Describe Estuarine circulation:
Freshwater entering an estuary establishes an along-estuary salinity (density) gradient that results in long-term (order of months) surface outflow and net inflow underneath
What is the role of tides in Estuary circulation?
Tides drive estuarine circulation by mixing freshwater and seawater, influencing salinity gradients, and transporting sediments. Flood tides push seawater upstream, while ebb tides move mixed water seaward. Strong tides enhance mixing, reducing stratification, while weaker tides maintain distinct layers.
What is a Neap tide vs Spring tide:
Neap tides
(weaker tidal currents and reduced vertical mixing)
Spring tides
(strong tidal currents and intensified vertical mixing)
What are the five types of Estuary?
- Salt-wedge
- Strongly-stratified
- Partially-stratified
- Well-mixed
+ inverse estuaries!
Describe a salt wedge estuary:
Conditions:
Large river flow + weak tidal forcing .
Structure:
Sharp halocline (pycnocline) separates fresh surface water from salty bottom water.
Flow:
Strong seaward flow at the surface, weak landward flow near the bottom.
Mixing:
Minimal mixing; stratification is strongest during flood tide when seawater forms a “wedge” shape.
KNOW WHAT THEY LOOK LIKE
Describe a strongly stratified estuary
Conditions:
Moderate to large river flow + weak to moderate tidal forcing.
Structure:
Well-developed halocline (pycnocline) separates fresh surface water from salty deep water.
Flow:
“Two-layer” system—strong seaward flow at the surface, weaker landward flow below.
Mixing:
Minimal mixing between layers; stratification remains throughout the tidal cycle.
KNOW WHAT THEY LOOK LIKE
Describe a partially stratified estuary:
Conditions:
Weak to moderate river flow + moderate to strong tidal forcing .
Structure:
Weak pycnocline (salinity gradient), sometimes spanning the entire water column.
Flow:
Similar velocities in upper and lower layers due to strong tidal mixing.
Mixing:
More mixing than in strongly stratified estuaries, but some layering still present.
KNOW WHAT THEY LOOK LIKE
Describe a well mixed Estuary
Conditions:
Weak river flow + strong tidal forcing.
Structure:
Uniform salinity from surface to bottom, with a horizontal salinity gradient.
Flow:
Mean flow is weak and unidirectional, driven mostly by wind or tidal forcing.
Mixing:
Strong mixing prevents stratification, creating a vertically uniform water column.
Appearance:
No distinct layers—salinity changes gradually along the estuary.
KNOW WHAT THEY LOOK LIKE
