Question

The letters following each Review Question refer to the corresponding Learning Objective from the Chapter Opener. How does beach sand migrate as a result of longshore drift? Explain the sediment budget of a coast. (F)

Short answer

Beach sand migrates as a result of longshore drift, a geological process where sediments are transported along the shoreline due to wave action. Longshore drift occurs when waves approach the shore at an angle, causing sediment to move up the beach (swash) and retreat down the beach (backwash) in a zigzag pattern, following the longshore current's direction. The sediment budget of a coast refers to the balance between sediment input and output in a coastal area, influenced by natural factors and human activities. Longshore drift plays a significant role in the sediment budget, as the rate of sediment transport can impact the balance between sources and sinks, leading to beach growth or erosion depending on the positivity or negativity of the sediment budget.

Step-by-step solution

Define Longshore Drift

Longshore drift is a geological process by which sediments, like sand, are transported along the shoreline due to the action of waves and currents. This process is essential in the formation and movement of various coastal features such as beaches, sandbars, and barrier islands.

Mechanics of Longshore Drift

Longshore drift occurs due to the angle at which waves approach the shore, known as the wave angle. When waves approach the shoreline at an angle, they carry sediment particles up the beach with them (known as swash), and then retreat back down the beach (known as backwash). The backwash typically occurs at a 90-degree angle with the shoreline due to gravity. This continual process results in the sediment gradually moving along the shoreline in a zigzag pattern, in the direction of the longshore current - the water moving parallel to the shore. Factors that influence the rate and direction of longshore drift include the size and angle of the waves, the shape of the coastline, and the composition of the beach material.

Sediment Budget of a Coast

The sediment budget of a coast refers to the balance between the amount of sediment being added to (sources) and removed from (sinks) a particular coastal area. The sediment budget can be either positive, negative, or balanced. A positive sediment budget indicates a net gain in sediment, resulting in beach growth. A negative sediment budget indicates a net loss in sediment, resulting in beach erosion. A balanced sediment budget means that the amount of sediment entering and leaving the area is equal, maintaining a stable beach. The sediment budget is influenced by natural factors (e.g., river input, sea-level changes) and human activities (e.g., coastal engineering, mining).

Interaction between Longshore Drift and Sediment Budget

Longshore drift plays a significant role in the sediment budget of a coast. The rate at which sediment is transported along the shoreline by longshore drift can impact the balance between sources and sinks of the sediment. For example, if longshore drift moves sand from one beach to another at a faster rate than sediment can be replenished from other sources (e.g., rivers), this can result in a negative sediment budget and subsequent beach erosion. Conversely, if longshore drift moves sand into an area where there is a net gain in sediment from other sources, this can lead to a positive sediment budget and beach growth. Understanding and managing the interaction between longshore drift and sediment budget is essential for coastal management and the preservation of coastal environments.

Key concepts

Longshore Drift

Longshore drift is a fundamental coastal process that moves sediment along the coastline. This movement occurs due to the action of waves approaching the shore at an angle. When a wave crashes onto the beach, it pushes sediment such as sand up the shore in a process called "swash." The backwash follows, pulling the sediment back towards the ocean, usually at a right angle due to gravity. This zigzag motion gradually shifts the sand down the coast.

The phenomenon of longshore drift influences how beaches and other coastal features are formed and reshaped. Several factors affect this process, including:

• The angle and force of incoming waves
• The shape of the coastline
• The size and type of beach material present

Understanding longshore drift is crucial for predicting changes along the coast, especially in areas prone to erosion.

Sediment Budget

The sediment budget of a coast is a measure of the balance between sediment being added to and removed from a coastal region. A positive sediment budget means more sediment is added than lost, typically resulting in the expansion or growth of a beach. A negative sediment budget indicates a loss of sediment, usually leading to beach erosion. When the inputs and outputs balance, the beach is considered stable.

Different sources and sinks contribute to the sediment budget. Sources include river deposits, cliff erosion, and material brought by tides. Sinks, on the other hand, can be offshore transport, tidal erosion, or mining activities. Human-induced actions like coastal construction and dredging also significantly impact the sediment balance.

Monitoring and managing the sediment budget are key to understanding how beaches change over time and how to protect them from severe erosion or excessive accretion.

Beach Erosion

Beach erosion is a natural process where wave action removes sediment from the beach, leading to its deterioration over time. Factors contributing to this phenomenon include strong wave activity, storms, and a negative sediment budget. When more sediment is transported away than is replenished, the beach starts eroding.

Beach erosion can have significant impacts, such as loss of habitat for wildlife and increased vulnerability of coastal structures to storm damage. Human activities such as construction near shorelines and interference with natural sediment flow exacerbate the problem.

• Construction of jetties and groynes can alter sediment transport, accelerating erosion in some areas.
• Removing natural vegetation for development reduces the stability of sand dunes and enhances erosion.

Addressing beach erosion often involves a combination of natural and engineered solutions, like replenishing beaches with sand or installing structures that mimic natural processes.

Coastal Management

Coastal management refers to the strategies and actions applied to preserve and protect coastal environments from erosion and other environmental impacts. It involves understanding the complex interactions of coastal processes like longshore drift and sediment budgets.

Effective coastal management includes a range of approaches:

• Soft engineering methods, such as beach nourishment, which involves adding sand to eroded beaches.
• Hard engineering solutions, like sea walls and groynes, which provide physical barriers against waves and sediment movement.
• Regulations and planning to control development and land use in coastal areas.

Integrating sustainable practices into coastal management helps maintain the balance between human needs and environmental protection. It is crucial to employ a proactive approach, involving monitoring, continuous assessment, and adaptation to changing environmental conditions.