Coastal Deposition Landforms

While erosion creates dramatic cliffs and headlands, deposition builds new landforms from the sediment produced. This lesson examines how beaches, spits, bars, tombolos, sand dunes, and salt marshes form and change over time.

---

Why Does Deposition Occur?

Deposition happens when waves lose energy and can no longer carry their load of sediment. This occurs in several situations:

• Constructive waves with a strong swash push material up the beach, but the weak backwash cannot remove it.
• In sheltered areas (bays, behind headlands, inside estuaries), wave energy is reduced.
• When there is an obstacle (e.g., a headland, groyne, or existing spit) that interrupts longshore drift.
• Where a river meets the sea, the sudden reduction in current speed causes the river to deposit its sediment load.
• During periods of calm weather, when wave energy is low.

---

Beaches

Beaches are the most common depositional landform. They are accumulations of sand, shingle, or a mixture of both, deposited by waves between the high and low tide marks.

Types of Beach

Feature	Sand Beach	Shingle (Pebble) Beach
Material	Fine sand particles (0.06-2 mm)	Pebbles, cobbles, and gravel (> 2 mm)
Profile	Gently sloping, wide	Steep, narrow
Why this profile?	Fine particles are easily moved by swash and backwash, creating a gentle gradient	Coarse material is pushed up by swash but the water drains through the gaps (percolation), weakening the backwash; material accumulates steeply
Berms	Low, indistinct ridges	Prominent storm berms (ridges of coarse material thrown up during storms)
Example	Bournemouth Beach (Dorset)	Chesil Beach (Dorset)
Wave type	Constructive waves	Both constructive and storm waves deposit material

Beach Features

Feature	Description
Berm	A ridge of sand or shingle at the top of the beach, marking the limit of normal high tide swash
Storm berm	A higher, larger ridge formed by powerful storm waves throwing material above the normal high tide mark
Cusps	Small, semi-circular depressions in the beach surface, formed by the interaction of swash and backwash
Runnels and ridges	Low points (runnels) and high points (ridges) running parallel to the shoreline, visible at low tide
Ripple marks	Small-scale undulations in the sand, formed by wave or current action

Exam Tip: When explaining why sand beaches have a gentle profile and shingle beaches have a steep profile, the key factor is percolation. On a shingle beach, water drains through the gaps between pebbles during the backwash, reducing its power. This means less material is pulled back down the beach, so it piles up steeply.

---

Spits

A spit is a long, narrow ridge of sand or shingle that extends from the coastline out into the sea or across a river mouth. It is attached to the land at one end and free at the other.

How a Spit Forms

graph TD
    A["Longshore drift transports<br/>sediment along the coast"] --> B["Coastline changes<br/>direction — e.g., at a<br/>river mouth or estuary"]
    B --> C["Sediment continues to be<br/>deposited in the original<br/>direction of drift"]
    C --> D["A ridge of sand/shingle<br/>builds out from the coast<br/>— a SPIT"]
    D --> E["The end may be curved<br/>by wave refraction or<br/>secondary winds"]
    E --> F["Sheltered water behind<br/>the spit allows mud<br/>to accumulate"]
    F --> G["Salt marsh develops<br/>behind the spit"]

Step-by-step formation:

1. Longshore drift transports sediment along the coastline.
2. The coastline changes direction — for example, at a river mouth, estuary, or where the land curves inward.
3. Longshore drift continues to carry sediment in the same direction, depositing it beyond the change in coastline.
4. A narrow ridge of material extends out from the land into open water — this is the spit.
5. The tip of the spit may be curved or hooked by wave refraction, wind from a secondary direction, or by currents.
6. Some spits develop recurved ends — multiple hooks formed during storms or changes in wind direction.
7. The sheltered area behind the spit (between the spit and the mainland) has low wave energy, allowing mud flats and salt marshes to develop.
8. The spit cannot grow all the way across a river mouth because the river current keeps the channel open.

UK Case Study: Spurn Point

Spurn Point (also called Spurn Head) is a spit extending 5.5 km into the Humber Estuary from the Holderness coast of East Yorkshire.

Feature	Detail
Length	Approximately 5.5 km
Width	As narrow as 50 m in places
Direction	Extends southwards, curving inwards towards the estuary
Source of material	Eroding Holderness boulder clay cliffs to the north — longshore drift carries material southward
Age	The current spit has been building for approximately 250 years; previous spits have formed and been destroyed in cycles
Features behind it	Salt marshes and mudflats in the sheltered Humber Estuary
Human use	RNLI lifeboat station; birdwatching; was a military base in both World Wars
Threats	Vulnerable to breaching during storms; tidal surges (e.g., December 2013); reduced sediment supply if Holderness cliffs are protected

Exam Tip: Spurn Point is an excellent case study for Edexcel B because it links coastal erosion (Holderness cliffs) with coastal deposition (the spit) and coastal management (protecting the cliffs reduces sediment supply to the spit). This shows you understand the coast as a connected system.

---

Bars and Lagoons

A bar forms when a spit grows all the way across a bay, completely enclosing the water behind it. The enclosed body of water is called a lagoon.

How a Bar Forms

1. A spit begins to grow across a bay or inlet.
2. If there is no strong river current to keep a channel open, the spit continues to grow.
3. Eventually, it extends from one headland to the other, sealing off the bay.
4. The water trapped behind the bar becomes a lagoon — a shallow, enclosed body of water.
5. Over time, the lagoon may fill with sediment and become a freshwater lake, marsh, or dry land.