The process of coastal transportation and longshore drift

Coastal transportation refers to the movement of eroded material along the coast, primarily driven by wave action. One of the key mechanisms of this movement is longshore drift, which plays a significant role in shaping coastal landscapes by redistributing sediment.

How longshore drift works

1. Waves are influenced by the prevailing wind direction, determining their approach to the shore.
2. These waves strike the coast at an oblique angle, meaning not directly head-on but at a slant.
3. The swash, which is the forward movement of water up the beach, pushes material in the same direction as the wave approach.
4. The backwash, the return movement of water down the beach, pulls material back towards the sea at a right angle due to gravity.
5. This results in a zigzag pattern of material movement along the coast over time, gradually shifting sediment from one area to another.

Different methods of material transportation in coastal environments

Beyond longshore drift, there are several other processes by which material is transported along coasts. These methods depend on the size of the particles and the energy of the water.

Types of coastal transportation processes

• Traction - Large, heavy particles such as boulders are rolled or pushed along the seabed by the force of moving water.
• Saltation - Medium-sized particles, like pebbles, are lifted and bounced along the seabed by the water's energy.
• Suspension - Fine particles, such as silt or clay, are carried within the water column, floating as they are moved along.
• Solution - Materials that can dissolve, like certain minerals, are broken down into the water and transported in a dissolved state.

The concept of deposition and beach formation

Deposition is the process where material carried by seawater is dropped or settled in a new location. This occurs when the water loses the energy required to keep the sediment in motion, leading to its accumulation in specific areas.

Understanding deposition and its outcomes

• Mechanism of deposition - Happens when the speed of water carrying sediment decreases below the level needed to sustain transport, causing the material to settle.
• Formation of beaches - Beaches are created in areas where the rate of deposition surpasses the rate of erosion, allowing sediment to build up over time.

Factors that increase material deposition on coasts

Certain conditions and processes enhance the likelihood of deposition occurring along a coastline, contributing to the growth of features like beaches and spits.

Conditions promoting deposition

• Abundant erosion nearby - Significant erosion in other coastal areas produces a large supply of material available for transport and eventual deposition.
• High transportation rates - Effective movement of sediment into a specific area increases the volume of material that can be deposited.
• Low-energy wave environments - Waves with reduced strength bring material to the shore but lack the power to carry it away, resulting in net deposition.

The role of constructive waves in shaping coastlines

Constructive waves are a specific type of wave that contribute more to deposition than erosion, playing a vital role in building up coastal features.

Characteristics of constructive waves

• Wave frequency - Typically low, with around 6-8 waves per minute reaching the shore.
• Wave shape - These waves are long and low in height, spreading their energy over a wider area.
• Powerful swash - The forward surge of water is strong, carrying sediment far up the beach.
• Weak backwash - The return flow of water is less forceful, unable to remove much of the deposited material.
• Net effect - Due to the imbalance between swash and backwash, more material is deposited than eroded, leading to the gradual build-up of coastal landforms.