Sediment transport and depositional landforms

Introduction: The Coastal Conveyor Belt

Have you ever wondered why some beaches are made of fine sand while others are covered in large pebbles? Or how a thin finger of land like a sand spit can stretch out miles into the sea without being washed away? Welcome to the world of sediment transport and deposition. In this chapter, we explore how the ocean acts like a giant conveyor belt, moving material along the coast and "parking" it to create some of the most beautiful and iconic coastal landforms. This is a key part of Topic 2B: Coastal Landscapes and Change.

1. Moving the Goods: Longshore Drift

The most important process in this chapter is Longshore Drift (also known as littoral drift). This is the primary way sediment (sand, silt, and pebbles) moves along a coastline.

How it Works: The Zig-Zag Pattern

Longshore drift doesn't move in a straight line. It follows a very specific "zig-zag" movement driven by the wind and waves:

1. The Angle of Approach: Most waves don't hit the shore perfectly straight. Instead, they approach at an angle (usually dictated by the prevailing wind).
2. The Swash: As the wave breaks, the swash (the water moving up the beach) carries sediment up the beach at that same angle.
3. The Backwash: Gravity always wins! As the water runs back down to the sea, the backwash carries sediment straight down the steepest slope (90 degrees to the shoreline).
4. The Result: By moving up at an angle and down vertically, the sediment gradually migrates along the coast in a zig-zag pattern.

Quick Tip: If you are standing on a beach and notice the wind is blowing from your left, the waves are likely approaching from the left, meaning the sediment is moving from left to right!

The Wave Approach Angle

The angle of wave approach is the "engine" of longshore drift. If waves hit the coast head-on, sediment just moves up and down the beach. But the more acute the angle, the faster and further the sediment is transported along the coast.

2. Coastal Systems: The Sediment Cell Concept

Geographers don't just look at one beach; they look at the whole system. A sediment cell (or littoral cell) is a largely "closed" length of coastline where the movement of sediment is self-contained.

Think of a sediment cell like a bank account:

• Sources (Inputs): Where the sediment comes from. This includes eroding cliffs, offshore bars, or rivers carrying silt to the sea.
• Transfers: Processes like longshore drift and currents that move the sediment from one place to another.
• Sinks (Outputs/Stores): Where the sediment is deposited and "stored," such as in beaches, spits, or sand dunes.

In theory, sediment does not move between cells because they are separated by major headlands or deep-water channels. This concept is vital for Coastal Management (which you will study later) because if you stop sediment movement in one part of the cell (e.g., by building a groyne), it will affect the "budget" of the rest of the cell.

Key Takeaway: Coastlines exist in a state of dynamic equilibrium. If the inputs and outputs are balanced, the coastline stays stable. If the balance shifts, the coast either grows or erodes.

3. Depositional Landforms: Nature’s Coastal Architecture

When the sea loses energy, it can no longer carry its "load," and deposition occurs. This creates distinctive landforms. Don't worry if these seem tricky at first; they are all essentially just different ways the sea "parks" sand and shingle!

A. Beaches

Beaches are the most common depositional landform. They can be sand (low gradient) or shingle (steeper gradient). They are often classified by how they align with the waves:

• Swash-aligned beaches: Face the prevailing waves head-on. Sediment moves up and down the beach but not much along it.
• Drift-aligned beaches: Form where waves hit at an angle. These are shaped by longshore drift and often have a "spit" at one end.

B. Spits

A spit is a long, narrow ridge of sand or shingle that is attached to the land at one end and extends out into the sea or an estuary.

How it forms:

1. Longshore drift moves sediment along a coastline.
2. The coastline suddenly changes direction (e.g., at a river mouth or bay).
3. The sediment continues to be deposited in the same direction, extending out into the open water.
4. The end of the spit often curves (becomes a hook) because of changes in wind direction or wave refraction.
5. Salt marshes often form in the sheltered, low-energy water behind the spit.

C. Bars (Bayhead Bars)

A bar forms when a spit grows all the way across a bay, joining two headlands. This traps the water behind it, creating a lagoon. In some cases, a bar can form offshore due to waves breaking and dropping sediment (an offshore bar).

D. Tombolos

A tombolo is a ridge of sand or shingle that connects an island to the mainland. It forms because the island protects the area behind it from waves (a "wave shadow"), allowing sediment to accumulate via deposition.

Example beyond the UK: The Angel Road of Shodoshima in Japan.

E. Cuspate Forelands

These are triangular-shaped headlands extending out from the main coastline. They form when longshore drift occurs from two different directions. Where the two "currents" of sediment meet, they deposit material in a point, creating a series of ridges.

Example: Dungeness in Kent (UK) or Cape Hatteras in North Carolina (USA).

4. Common Mistakes to Avoid

• Confusing Swash and Backwash: Remember, Swash goes up the beach (at an angle); Backwash goes down (straight).
• Thinking Spits are permanent: Spits are highly dynamic. A single major storm can breach a spit or change its shape entirely.
• Ignoring the "Beyond the UK" rule: In your exam, you must be able to mention examples from outside the UK to access the top marks!

Summary Table: Factors Influencing Deposition

Note: To understand how these landforms are affected by rising sea levels or human management, please refer to the chapters on "Sea level change" and "Coastal management approaches."