12.3 Coastal processes and landforms

Coastal Processes and Landforms

Coastlines are some of the most dynamic environments on Earth. Waves, tides, and currents constantly reshape shorelines by eroding rock, moving sediment, and building new landforms. Understanding these processes helps explain why beaches grow and shrink, why cliffs retreat, and why human attempts to control the coast often create new problems.

Coastal Processes

Coastal Erosion and Sediment Movement

Waves are the primary force driving coastal change. They erode rock and sediment through three main mechanisms:

• Hydraulic action occurs when waves slam into cracks in rock, compressing the air trapped inside. That repeated pressure wedges the rock apart over time.
• Abrasion happens when sediment carried by waves acts like sandpaper, grinding against coastal rock and wearing it down.
• Attrition is the process of sediment particles colliding with each other in the surf, gradually becoming smaller and more rounded.

Beyond wave erosion, several types of currents move sediment along and away from the coast:

• Longshore drift is the most important sediment transport process at most coastlines. When waves approach the shore at an angle, they push sediment diagonally up the beach with each swash, but gravity pulls it straight back down the slope with each backwash. The result is a zigzag movement of sediment parallel to the shoreline.
• Tidal currents erode and redistribute sediment as water flows in during flood tides and back out during ebb tides.
• Rip currents are strong, narrow flows of water moving seaward from the shore. They carry sediment offshore and can cut channels through sandbars.

All this sediment movement builds depositional landforms:

• Beaches form where wave action and longshore drift deposit sand or gravel along the shoreline.
• Spits are elongated ridges of sand or gravel that extend from the coast into open water, built by longshore drift continuing past a change in the coastline's direction.
• Deltas form where rivers deposit their sediment load as they enter a larger, slower body of water like an ocean or lake.

Formation of Coastal Landforms

Beaches are shaped by the balance between sediment supply and wave energy. A typical beach profile has two key zones: the berm, which is the flat, elevated area above normal wave reach, and the foreshore, which slopes down toward the water and is regularly washed by waves.

Barrier islands are long, narrow sand islands that run parallel to the mainland coast. They form through a combination of longshore drift and wave deposition. Barrier islands serve as natural buffers, absorbing wave energy and storm surges before they reach the mainland. The Outer Banks of North Carolina and Padre Island in Texas are well-known examples. These islands are not permanent features; they migrate and reshape over time, which creates challenges for development built on them.

Sea cliffs form where high wave energy meets resistant rock. Waves undercut the base of the cliff, creating a notch. Eventually the unsupported rock above collapses, and the cliff face retreats landward. This cycle of undercutting, collapse, and retreat repeats over time, producing dramatic features like the White Cliffs of Dover in England and the Cliffs of Moher in Ireland.

Coastal Morphology and Management

Factors in Coastal Morphology

Three main factors control what a coastline looks like:

• Wave energy determines how much erosion and sediment transport occurs. Wave energy depends on wind speed, wind duration, and fetch, which is the distance of open water over which the wind blows. Longer fetch and stronger winds produce larger, more powerful waves.
• Tidal range is the vertical difference between high and low tide. Macrotidal coasts (tidal range greater than 4 m) tend to have wide tidal flats and strong tidal currents. Microtidal coasts (tidal range less than 2 m) are more wave-dominated, with tides playing a smaller role in shaping the shoreline.
• Sediment supply controls whether a coast is building outward or eroding. Sediment comes from rivers, cliff erosion, and offshore deposits. When supply is abundant, beaches widen and barrier islands grow. When supply is limited or cut off, erosion dominates and the shoreline retreats.

Human Impact on Coastal Systems

Human activities frequently disrupt the natural balance of coastal processes:

• Coastal development (buildings, roads, infrastructure) removes natural vegetation and dunes that stabilize shorelines, often accelerating erosion.
• Dredging and sand mining remove sediment from the coastal system, altering sediment budgets and starving nearby beaches of material.
• Engineering structures like seawalls, groins, and breakwaters are built to protect specific stretches of coast, but they interrupt longshore drift. This often causes increased erosion downdrift of the structure, essentially shifting the problem to neighboring areas.
• Pollution and nutrient runoff degrade water quality and damage coastal ecosystems like coral reefs and salt marshes that naturally protect shorelines.

To address these issues, several management strategies are used:

1. Integrated Coastal Zone Management (ICZM) takes a holistic approach, balancing ecological, social, and economic factors when making planning decisions for coastal areas.
2. Setback lines and building restrictions keep new development a safe distance from erosion-prone areas.
3. Beach nourishment involves pumping or trucking sand onto eroded beaches to rebuild them. It's effective short-term but requires repeated application since waves continue to move the added sand.
4. Marine Protected Areas (MPAs) restrict activities like fishing and development in designated zones to conserve coastal and marine ecosystems.
5. Climate change adaptation strategies include managed retreat (relocating development away from vulnerable coasts), nature-based solutions (restoring wetlands and dunes), and building more resilient infrastructure.