Lecture 6: Estuary circulation 2

Question 1

The pattern of stratification (and circulation) that develops in an estuary depends upon the relative strength of…

Answer 1

• freshwater input (adds stratification via
  buoyancy input)
• tidal currents (breaks down stratification
  through vertical mixing)

Question 2

Describe a typical circulation in a fjord?

Answer 2

Typical circulation in a fjord with

(i) an outflowing low salinity layer at the surface,

(ii) a landward flowing salty ocean water at depth and

(iii) an isolated, often poorly ventilated third layer below sill depth.

Question 3

Are Fjords good carbon sinks.

Answer 3

Yes

• 11% of annual marine organic carbon (OC) burial [Smith et al. 2015] - only 0.16% of ocean area
• Atmospheric models suggest that Fiordland sinks a similar amount of CO2
  as New Zealand emits per year

Question 4

Altering River Flow Has Consequences: name some

Answer 4

Changes in freshwater input, such as river diversions, can significantly impact estuarine circulation, water quality, and ecosystem health.

Question 5

What is the concept of flushing time?

Answer 5

The concept of a Flushing Time
Flushing time, calculated using methods including the tidal prism and freshwater fraction approaches, helps estimate how quickly estuarine water is renewed.

Question 6

Describe the relationship between Stratification vs. Mixing Balance:

Answer 6

The relative strength of river discharge (buoyancy input) and tidal velocities (vertical mixing) determines whether an estuary is well-mixed, partially stratified, strongly stratified, or forms a salt wedge. Can be quantified via the Estuarine Richardson number.

Question 7

Estuarine Circulation is Driven by Density Gradients:

Answer 7

Freshwater input creates a density gradient that drives a two-layer circulation, with surface outflow and deep inflow