Welcome to the Coast!
Hello! Today we are exploring the dynamic world of coastal processes. This is one of the most exciting parts of Geography because the coast is always changing—sometimes slowly over centuries, and sometimes in a single afternoon during a storm. Whether you live near the sea or hundreds of miles away, understanding how waves and tides shape our world is vital for managing our environment and protecting communities. Don't worry if some of the terms seem a bit technical at first; we will break them down step-by-step with simple analogies!
1. The Engine of the Coast: Waves
Waves are the primary force that shapes the coastline. They are essentially "packets" of energy moving through the water, mostly created by wind blowing over the surface.
How Waves are Formed
The size and energy of a wave depend on three main factors:
1. Wind Strength: How hard the wind is blowing.
2. Wind Duration: How long the wind has been blowing.
3. Fetch: This is the distance of open water over which the wind blows. The longer the fetch, the bigger the wave!
Wave Types: Friends or Foes?
In Geography 9696, you need to distinguish between two main types of waves. Think of them as the "builders" and the "destroyers."
Constructive Waves (The Builders)
These waves "construct" or build up the beach. They are usually low in height and have a long wavelength (they are spread out).
- Strong Swash: The water moving up the beach is powerful and carries sediment (sand/shingles) onto the shore.
- Weak Backwash: The water moving back down the beach is weak, so it leaves the sediment behind.
Destructive Waves (The Destroyers)
These waves "destroy" or erode the beach. They are tall, steep, and happen frequently.
- Weak Swash: Very little sediment is moved up the beach.
- Strong Backwash: The water rushing back to the sea is very powerful, "stealing" sand and rocks away from the beach.
Quick Review Box:
Constructive = Strong Swash (Adds sand)
Destructive = Strong Backwash (Removes sand)
Did you know? Waves don't actually move the water molecules very far forward; they move the energy. It's like a "Mexican Wave" in a stadium—the people stay in their seats, but the wave travels around the circle!
2. Coastal Erosion: How the Sea Attacks
Erosion is the wearing away of the land. The sea uses four main "weapons" to break down cliffs:
1. Hydraulic Action: This is the sheer power of the water. As waves smash against a cliff, they trap air in cracks. The pressure of the compressed air acts like a small explosion, shattering the rock.
2. Abrasion (or Corrasion): Think of this as sandpapering. The waves pick up pebbles and hurl them against the cliff, scraping the surface away.
3. Attrition: This happens between the rocks themselves. Pebbles carried by the waves bump into each other, becoming smaller, smoother, and rounder over time.
4. Solution (or Corrosion): Some rocks (like limestone or chalk) are dissolved by the chemicals in seawater.
Memory Aid: Use the acronym "H.A.A.S" to remember them:
Hydraulic Action
Abrasion
Attrition
Solution
Key Takeaway: Erosion is most active during storms when wave energy is highest. Harder rocks (like granite) erode slowly, while softer rocks (like clay) erode very quickly.
3. Coastal Transportation: The Conveyor Belt
Once the sea has eroded the rock, it has to move that material (the "load") somewhere else. This is called transportation.
Longshore Drift (LSD)
This is the most important transportation process you need to know. It moves sediment along the coastline in a "zigzag" pattern.
1. Waves approach the beach at an angle (usually dictated by the prevailing wind).
2. The swash carries material up the beach at that same angle.
3. Gravity pulls the backwash straight down the beach (at a 90-degree angle).
4. This repeats, moving a pebble along the coast bit by bit, like a zig-zagging traveler!
Common Mistake to Avoid: Many students forget to mention that the backwash always moves straight down the beach due to gravity. Don't forget that part!
4. Coastal Deposition: Dropping the Load
Deposition happens when the sea loses energy. If the wind dies down or the water enters a sheltered bay, it can no longer carry its load and drops it. This creates landforms like beaches, spits, and bars.
What causes deposition?
- Shallow water.
- Sheltered areas (like bays).
- Little to no wind.
- Where a river enters the sea (reducing the flow speed).
5. Sub-aerial Processes: The "Silent" Workers
It's not just the sea that changes the coast! Sub-aerial processes happen on the cliff face, above the water line. These include weathering and mass movement.
Weathering
- Salt Crystallisation: Salt spray gets into cracks. When the water evaporates, salt crystals grow and wedge the rock apart.
- Freeze-Thaw: Water enters cracks, freezes, expands by 9%, and cracks the rock (though this is rarer at the coast than in mountains).
Mass Movement
This is when soil or rock moves down a slope due to gravity.
- Rockfall: Pieces of rock fall off a cliff (usually due to weathering).
- Slumping (Rotational Slip): Common in clay cliffs. After heavy rain, the soil becomes heavy and "greasy," causing a whole chunk of the cliff to slide down in a curved motion.
Analogy: Imagine a wet sponge on a tilted tray. If it gets too heavy with water, it will eventually slide down. That’s essentially what happens during a coastal slump!
6. Coastal Landforms (The Results)
Geography is all about the "Shapes of the Land."
Erosional Landforms
- Cliffs and Wave-Cut Platforms: The sea erodes a "notch" at the base of a cliff. Eventually, the overhanging rock collapses. This repeats, and the cliff retreats, leaving a flat platform of rock at the base.
- Caves, Arches, Stacks, and Stumps:
1. Waves exploit a crack to make a Cave.
2. The cave is eroded all the way through to make an Arch.
3. The roof of the arch collapses, leaving a pillar of rock called a Stack.
4. The stack is eroded until it falls, leaving a Stump.
Depositional Landforms
- Spits: Long, narrow ridges of sand/shingle that stick out into the sea. They form where the coastline changes direction, but Longshore Drift keeps moving sediment in the original direction.
- Bars: If a spit grows all the way across a bay, it creates a Bar, trapping a Lagoon behind it.
Quick Review: Erosion creates "holes" and "pillars" (caves/stacks). Deposition creates "stretches" of sand (spits/bars).
7. Sea Level Change
Sea levels aren't constant. They change for two main reasons:
1. Eustatic Change: A global change in the volume of water in the oceans (e.g., ice melting due to climate change).
2. Isostatic Change: A local change where the land itself rises or falls (e.g., after the weight of an ice sheet is removed, the land "bounces" back up).
Memory Trick:
Eustatic = Everywhere (Global)
Isostatic = Island (Local land movement)
8. Coastal Management: Protecting the Shore
Humans try to stop erosion using two methods:
Hard Engineering (Building big structures):
- Sea Walls: Concrete walls that reflect wave energy. Very expensive!
- Groynes: Wooden fences built into the sea to stop Longshore Drift and keep sand on the beach.
Soft Engineering (Working with nature):
- Beach Nourishment: Adding new sand to a beach. It looks natural and protects the coast.
- Managed Retreat: Letting the sea flood low-value land to protect more important areas elsewhere.
Key Takeaway: Hard engineering is effective but can be ugly and expensive. Soft engineering is more sustainable and cheaper but doesn't always provide the same "solid" protection.
Congratulations! You've just covered the core concepts of Coastal Processes for your Geography 9696 syllabus. Keep reviewing these terms, and try drawing the diagrams for Longshore Drift and the Cave-Arch-Stack-Stump sequence—it really helps the info stick!