Welcome to Coastal Systems and Landscapes!
In this chapter, we are going to explore how the coast works like a giant, busy machine. Geographers call this a system. Instead of just looking at a beach as a pile of sand, we will look at how energy (like waves) and material (like sand) move in and out to create the amazing landscapes we see.
Don’t worry if some of the words sound like "science-speak" at first—we will break them down piece by piece. By the end of this, you’ll look at the beach in a whole new way!
3.1.3.1 Coasts as Natural Systems
Think of a coastal system like a bank account. You have money coming in (deposits), money sitting in the account (savings), and money being spent (withdrawals). A coast does the exact same thing with sediment and energy!
Key System Concepts
To understand the coast, you need to know these five "system" ingredients:
1. Inputs: These are things that "enter" the system. Examples include energy from waves, wind, and tides, or material like sediment from eroding cliffs.
2. Outputs: These are things that "leave" the system. This could be sediment moving out to the deep ocean or energy being dissipated (lost) on the shore.
3. Stores (or Components): These are parts of the landscape where material stays for a while, like a beach, a sand dune, or a spit.
4. Flows (or Transfers): These are the movements between stores. Longshore drift is a classic example of a flow moving sand from one part of the beach to another.
5. Dynamic Equilibrium: This is a fancy way of saying "balance." If the inputs and outputs are equal, the beach stays the same size. If one changes, the system works to find a new balance.
Quick Review: Feedback Loops
Sometimes the system gets pushed out of balance.
• Positive Feedback: This amplifies a change (e.g., a small breach in a sand dune allows more wind in, which blows away even more sand, making the hole bigger).
• Negative Feedback: This corrects a change to restore balance (e.g., a big storm erodes a beach, but the eroded sand forms an offshore bar which then trips the waves earlier, protecting the beach from further erosion).
3.1.3.2 Sources of Energy
Energy is the "fuel" that drives all coastal processes. Without energy, the coast would be a very still, boring place.
1. Waves: The Main Engine
Waves are created by wind blowing over the surface of the sea. The amount of energy in a wave depends on the fetch (how far the wind has travelled over the water), the wind speed, and how long the wind has been blowing.
Constructive vs. Destructive Waves
It is really important to know the difference between these two:
• Constructive Waves: These are "beach builders." They are low, flat, and have a strong swash (water moving up the beach) and a weak backwash (water moving back). They push sediment onto the beach.
• Destructive Waves: These are "beach destroyers." They are tall, steep, and have a weak swash but a very strong backwash. They pull sand away from the beach and into the sea.
Memory Aid: "Constructive builds it up, Destructive tears it down!"
2. Winds, Tides, and Currents
• Wind: Acts as an agent of erosion (blowing sand) and dictates where waves will hit the shore.
• Tides: These are the daily rise and fall of the sea level caused by the moon's gravity. The tidal range (the difference between high and low tide) determines which part of the cliff or beach gets hit by the waves.
• Currents: These are "rivers" of water within the ocean (like the Gulf Stream) that move sediment over very long distances.
High Energy vs. Low Energy Coasts
• High Energy Coasts: Think of rocky, stormy places like the Atlantic coast of Scotland. Here, the rate of erosion is higher than the rate of deposition. You’ll see cliffs and wave-cut platforms.
• Low Energy Coasts: Think of sheltered bays or the Mediterranean. Here, the waves are calmer, and deposition happens more than erosion. You’ll see big beaches and salt marshes.
Sediment Sources, Cells, and Budgets
Where does all the sand come from? It isn't just there by magic!
Sediment Sources: Cliffs being eroded, rivers bringing silt from inland, and even offshore sand banks being pushed landward.
Sediment Cells: Geographers divide the coast into "cells." These are lengths of coastline (usually between two headlands) that act as a closed system. This means sediment usually stays within that cell and doesn't leak into the next one.
Sediment Budget: This is the balance between sediment added to and removed from the coastal system. We can show it with a simple formula:
\( \text{Sediment Budget} = \text{Input} - \text{Output} \)
Quick Review:
• If the budget is positive, the beach grows (more sand coming in than leaving).
• If the budget is negative, the beach shrinks (more sand leaving than coming in).
Geomorphological Processes
"Geomorphological" is just a big word for "earth-shaping." There are four main ways the coast is shaped: Weathering, Mass Movement, Erosion, and Transportation.
1. Marine Erosion (The Attack)
Waves attack the land in five specific ways:
• Hydraulic Action: The sheer power of the water. Waves trap air in cliff cracks, compressing it until the rock literal explodes outward.
• Wave Quarrying: Similar to hydraulic action, but specifically the weight and impact of a breaking wave scooping out loose rocks.
• Corrasion/Abrasion: The "sandpaper effect." Waves hurl sand and pebbles against the cliff, wearing it away.
• Cavitation: When bubbles in the water implode under high pressure, sending tiny "shockwaves" into the rock.
• Solution: Weak acids in seawater dissolve certain rocks, like limestone.
• Attrition: This happens to the rocks, not the cliff. Rocks in the surf crash into each other, becoming smaller, smoother, and rounder.
2. Transportation (The Journey)
Once the sea has eroded the rock, it has to move it. The most important process here is Longshore Drift (LSD).
How Longshore Drift works:
1. Waves approach the beach at an angle (driven by the wind).
2. The swash carries sediment up the beach at that same angle.
3. The backwash carries the sediment straight back down the beach due to gravity.
4. This creates a "zig-zag" movement of sand along the coastline.
3. Sub-aerial Processes (The Silent Shapers)
While the waves attack the bottom of the cliff, the weather attacks the top. These are called sub-aerial processes.
• Weathering: Breaking down rocks where they sit. This includes mechanical weathering (like freeze-thaw) and chemical weathering.
• Mass Movement: When gravity pulls a whole section of a cliff down. This can be a landslide (sliding on a flat surface) or slumping (rotating on a curved surface—very common in clay cliffs).
• Runoff: Rainwater washing sediment down the face of a cliff into the sea.
Common Mistake to Avoid:
Don't confuse "Erosion" with "Weathering." Erosion involves movement (waves carrying stuff away), while Weathering is the breakdown of rock in its original place by the atmosphere or biology.
Summary Takeaways
• The coast is a system with inputs (energy/sediment), stores (landforms), and outputs.
• Constructive waves build beaches; destructive waves erode them.
• Longshore drift moves sediment in a zig-zag pattern along the coast.
• Coastal landscapes are shaped by both marine processes (at the sea level) and sub-aerial processes (from the sky/land).