Coastal Erosion and Deposition Notes

LEARNING INTENTION

Understand and name three coastal erosion processes.

Explain each coastal erosion process in detail with examples.

COASTAL EROSION PROCESSES

Hydraulic Action

Definition: The force exerted by waves colliding with shores and cliffs, resulting in changes to the landscape.

Mechanism: Waves crash onto shorelines, creating pressure as they force air and water into cracks and crevices within the rock formations. This rapid influx of force generates a phenomenon where the trapped air compresses and expands, leading to explosive releases that break apart the rock.

Outcome: This process causes breaks and fragmentation in the rock, significantly contributing to erosion over time. The cumulative impact leads to substantial alterations in coastal geography, with potential impacts on marine habitats.

Abrasion

Definition: Erosion caused by rocks and sediment carried by waves colliding forcefully with cliffs and shorelines.

Mechanism: As waves transport rocks and pebbles, these materials strike against the cliffs with considerable force. The repeated impacts chip away parts of the rock face, effectively sandblasting the surface and leading to the gradual wearing down of the cliffs.

Outcome: This process results in the reduction of cliff height and the formation of various coastal landforms, such as wave-cut notches and platforms. Over time, this can lead to more significant erosion and destabilization of the coastal area, increasing the risk of landslides and rockfalls.

Solution

Definition: A chemical erosion process whereby minerals in certain types of rock dissolve in seawater.

Mechanism: The salts and other chemical components in seawater interact with minerals in the rock, particularly limestone and chalk, causing a chemical reaction that weakens the rock structure. Over time, the dissolved minerals are carried away, leading to significant decomposition of the rock mass.

Outcome: Parts of the rock disintegrate and break apart, contributing to the overall erosion of the coastline. This process often results in unique geological features and can lead to the formation of underwater caves and channels.

FORMATION OF COASTAL FEATURES

Wave-Cut Platforms

4. Formation Stages:

Stage 1: Found where cliffs meet the sea and experience hydraulic action, particularly in areas with strong wave activity.

Stage 2: Waves trap air and water into rock cracks, causing extensive erosion and forming a wave-cut notch. The notch is typically a small recess cut into the base of the cliff.

Stage 3: As waves continuously crash against the cliff face, the size of the notch increases, eventually undercutting the cliff. The undercutting leads to a collapse of the cliff's overhanging section, leaving behind a flat area known as a wave-cut platform, which may be strewn with debris from the collapsed rocks and becomes more prominent during low tide.

Headlands and Bays

4. Formation Stages:

Stage 1: Erosion occurs along cliffs where softer rocks, such as sandstone or clay, erode more rapidly than harder rocks, such as granite, leading to differential erosion.

Stage 2: Hydraulic action, abrasion, and solution work together on the cliffs. The softer rock on the coastline erodes, generating an indentation that forms a bay while the harder rock persists, forming a headland that juts out into the sea.

Stage 3: This differential erosion creates a coastal landscape characterized by the presence of headlands that extend into the sea, providing protection to the adjacent bays which develop over time due to the greater erosion of softer rock sections.

Caves, Arches, Stacks, and Stumps

4. Formation Process:

Stage 1: Waves attack the weakest points of headlands, initiating the formation of cracks in the rock. These cracks occur at points where the rock is already structurally compromised, typically due to pre-existing geological features.

Stage 2: The cracks widen through hydraulic action and abrasion, eventually forming a cave. The continual erosion from wave action inside the cave causes it to deepen and widen over time.

Stage 3: Erosion continues inside the cave, eventually leading to the formation of an arch when the opposite side of the cave becomes eroded enough for the cave to break through the headland completely.

Stage 4: Over time, the arch may collapse due to natural forces such as weathering or excessive erosion, forming a stack, which is a steep, relict column of rock.

Stage 5: The stack undergoes further erosion at its base from wave action, eventually leading to the formation of a stump, which is the remnant left from the stack that appears during lower tidal conditions.

COASTAL TRANSPORTATION AND DEPOSITION

Transportation: The movement of materials, including sand, pebbles, and rocks, by waves along the coastline, primarily influenced by prevailing wind direction and wave approach, primarily through the process of longshore drift.

Deposition: Occurs when the sea loses energy and deposits materials along the coastline, typically happening when swash (the forward movement of water) exceeds backwash (the backward flow of water), allowing sediment to accumulate.

Longshore Drift

Mechanism: Waves approach the shore at an angle due to prevailing winds, allowing materials to be transported up the beach face at an oblique angle during the swash, while returning straight down during backwash.

Result: This continuous swash and backwash move material in a zigzag pattern along the beach, facilitating the redistribution of sediment and contributing to the formation of coastal features.

Coastal Deposition Features

Sand Spit: An extended stretch of beach material that is connected to the mainland at one end, formed by the process of longshore drift, offering a sheltered environment behind it which can lead to the formation of marshlands or estuaries.

Sand Bar: A bar of sand that forms as waves deposit materials over subaqueous areas, often submerged at high tide, creating shallow water zones that can be crucial for marine ecosystems.

Tombolo: A ridge of sand connecting an island to the mainland, formed from deposited materials due to wave action, typically providing vital habitat for wildlife and contributing to unique coastal landscapes.