Estuaries and coasts

Question 1

What is an estuary?

Answer 1

A portion of the ocean:
- Diluted by freshwater input
- Semi-isolated by land

Question 2

What are the 3 key aspects of an estuary?

Answer 2

• Geometry
• Rate of freshwater input
• Strength of tidal flows

Question 3

What happens when fresh and salt water meet?

Answer 3

‘Lock-exchange experiment’

• Two layer flow
  → → less dense fresh water outflow above more dense salt water inflow (density-driven circulation)

Question 4

What is circulation modified by?

Answer 4

• Mixing between fresh and salt water
• Tidal movement of the salt water in and out of estuary

Question 5

What are the types of estuary?

Answer 5

• Salt wedge
• Partially-mixed
• Well-mixed
• Fjord
• Inverse (?)

Question 6

What occurs in salt wedge estuaries?

Answer 6

1. Fresh water flows out at surface (net seawards flow is almost all fresh water) from 0 ppt to 15 ppt
2. Salt water intrudes at depth, the salt wedge moves in and out of estuary with tide
3. Salt water is held back by fresh water → sharply tilted interface
4. Fresh water erodes saltwater → one-way mixing (entrainment

Question 7

What occurs in partially-mixed estuaries?

Answer 7

1. Surface flow exceeds freshwater flow
2. Salt inflow is due to advection and diffusion
3. density contours tilted

• Increased tidal mixing with reduced river flow mixes salt water into surface layer
• Rapid exchange of water

Question 8

What occurs in well-mixed estuaries?

Answer 8

1. Slow net seawards flow at all depths
2. Salinity becomes uniform with depth
3. Little or no inward transfer of seawater

Even greater tidal mixing with reduced river flow leads to well mixed estuary

Question 9

What occurs in Fjord estuaries?

Answer 9

1. River flow remains at surface and moves seaward with little mixing
2. Little inflow of water at depth (still acts as a barrier)
3. Deeper water may stagnate

• Deep basin with entrance sill
• Moderate river flow and little tide

Question 10

What are the features of an inverse estuary?

Answer 10

• Salinity increases up estuary
• Surface inflow of seawater

Question 11

What are inverse estuaries not strictly an estuary?
Why

Answer 11

As in hot, dry regions evaporation may dominate over precipitation and river inout

Question 12

What is true of calm/swell/summer beach profile changes?

Answer 12

• Low waves → weak undertow
• Accretion dominant → berm

Question 13

What is true of energetic/storm/winter beach profile changes?

Answer 13

• High waves → strong undertow
• Erosion dominant → offshore sandbar

Question 14

How are coastal dunes formed?

Answer 14

Formed from wind blown sand

Question 15

What do coastal dunes feature?

Answer 15

• Primary dune ridge
• Secondary dune ridge
• Foredunes

Question 16

What is true of coastal dunes?

Answer 16

• Vegetation is important as this stabilises the dunes, and the absence may lead to blow-outs
• They are susceptible to wave attacks

Question 17

What is aeolian sand transport? (Coastal dunes)

Answer 17

It is the first process of coastal dune formation and involves the movement and weathering of sand particles behind and parallel to the shoreline.

Question 18

What are barrier islands?

Answer 18

• Large sand deposits separated from mainland by estuary, bay or lagoon
  —- Abundant sand supply
  —– Low seafloor slope

Question 19

How are barrier islands formed?

Answer 19

• Submergence of coastal sand ridge
• Segmentation of sand pit
• Emergence of longshore sand bar

Question 20

What is true of clipped coasts?

Answer 20

• Changes often associated with spectacular failure
  —– Wave erosion at cliff base
  —– Land processes (groundwater)

Question 21

How are delta coasts formed?

Answer 21

Built for river sediments

Question 22

What are the 3 zones of delta coasts?

Answer 22

Delta plain, delta front and prodelta