3.6 Coastal Processes and Landforms

Coastal environment formation

Coastal processes shape shorelines by eroding rock and depositing sediment. Waves, tides, and currents constantly sculpt features like cliffs, beaches, and barrier islands. Understanding how these processes work matters for managing coastlines, protecting communities, and preserving ecosystems.

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Processes shaping coastal environments

Waves are the primary force shaping coastlines. Wind blowing across the ocean surface generates waves, and when those waves reach shore, their energy erodes rock and transports sediment. The stronger the wind and the longer the distance it travels over water (called fetch), the larger and more powerful the waves become.

Tides raise and lower sea levels in a regular cycle driven by the gravitational pull of the moon and sun. The tidal range (the difference between high and low tide) determines how much of the coast is alternately submerged and exposed. This creates the intertidal zone, a dynamic strip of coastline that experiences constant change.

Longshore currents move sediment sideways along the coast. They form when waves approach the shore at an angle, pushing water and sediment in a zigzag pattern down the beach. This process, called longshore drift, builds features like spits and barrier islands over time.

Rip currents are strong, narrow channels of water flowing away from shore. They typically form at low spots or breaks in sandbars and carry sediment offshore.

Weathering also contributes by breaking down coastal rocks and cliffs into sediment:

• Chemical weathering: salt crystal growth in rock pores, dissolution by seawater
• Physical weathering: abrasion from wave-thrown sand and gravel, freeze-thaw cycles cracking rock apart

Biological processes play a supporting role. Coral reefs build up limestone structures, mangroves and seagrass beds trap sediment and stabilize shorelines, and burrowing organisms mix and redistribute sediment (a process called bioturbation).

Factors influencing coastal environments

• Geology and rock type: Resistant rocks like granite form steep, lasting cliffs, while softer rocks like shale erode quickly. Structural features like joints and faults create weak points where erosion accelerates.
• Climate and weather: These control wave energy, storm frequency, and precipitation, all of which affect how fast erosion and sediment transport occur.
• Sea level changes: Rising sea levels submerge coastal features and increase erosion. Falling sea levels expose new land and shift where coastal processes act.
• Sediment supply: The grain size, composition, and availability of sediment determine what landforms can develop. Major sediment sources include rivers, cliff erosion, and offshore deposits on the continental shelf.

Coastal landforms

Erosional landforms

Erosional landforms develop where wave energy removes rock faster than sediment can accumulate. The specific features that form depend on rock type, structure, and the intensity of wave attack.

• Cliffs are steep rock faces along the coast, shaped by wave erosion undercutting the base and mass wasting (rockfalls, slumps) collapsing material from above.
• Wave-cut platforms are flat, rocky surfaces that extend seaward from the base of a cliff. Waves carve these platforms through abrasion at high tide. At low tide, they're often exposed, forming rocky intertidal habitats.
• Sea caves, arches, and stacks form in sequence. Waves exploit weaknesses (joints, faults) in a cliff face through hydraulic action and abrasion, hollowing out sea caves. When caves on opposite sides of a headland break through and connect, they create a sea arch. When the arch roof collapses, the remaining isolated rock tower is a sea stack.
• Blowholes are vertical shafts eroded through the roof of a sea cave. When waves surge into the cave below, compressed air and water shoot upward through the shaft. Examples include Halona Blowhole in Hawaii and Thor's Well in Oregon.

Depositional landforms

Depositional landforms develop where sediment accumulates faster than it's removed. Sheltered areas, changes in current speed, and obstacles that interrupt sediment transport all encourage deposition.

• Beaches are accumulations of sand, gravel, or shell fragments along the shoreline. Their shape and composition vary widely depending on sediment source and wave conditions.
• Dunes are mounds or ridges of wind-blown sand that form behind beaches. They act as natural buffers against storm surge and support specialized plant communities adapted to shifting sand and salt spray.
• Spits are elongated ridges of sand that extend from the shoreline into open water. Longshore drift builds them outward, and wave refraction often curves their tips. Farewell Spit in New Zealand stretches about 26 km into the sea.
• Barrier islands are long, narrow sand deposits running parallel to the mainland, separated from it by a lagoon or bay. The Outer Banks of North Carolina are a well-known example.
• Tidal flats are broad, flat areas of fine sediment exposed at low tide. They form in sheltered settings like estuaries and bays. The Wadden Sea along the coasts of the Netherlands and Germany is one of the largest tidal flat systems in the world.
• Salt marshes are coastal wetlands colonized by salt-tolerant plants. They develop where fine-grained sediment accumulates in sheltered, low-energy environments. Salt marshes provide critical habitat for wildlife and help buffer coastlines from storm damage.

Coastal erosion and deposition

Factors influencing coastal erosion

Several factors control how quickly a coastline erodes:

• Wave energy and storms: High-energy waves during storms can strip away beaches, flatten dunes, and undercut cliffs in hours. Erosion rates also shift seasonally as wave conditions change.
• Tidal range and currents: Higher tides let waves reach further inland. Tidal currents redistribute sediment and can concentrate erosion in specific areas.
• Rock type and structure: Softer rocks like shale and sandstone erode far more quickly than harder rocks like granite and basalt. Joints, faults, and bedding planes act as weak points that waves exploit.
• Sea level rise: As sea levels climb, waves and tides reach higher ground, accelerating erosion. Coastal features may migrate inland or become permanently submerged.
• Human activities: Coastal development, shoreline hardening (seawalls, riprap), and damming of rivers all disrupt natural sediment movement. Dredging removes sediment from the system entirely.

Processes of coastal deposition

Deposition happens when water or wind slows down enough that it can no longer carry its sediment load. The main depositional processes along coastlines include:

1. Longshore drift: Waves and currents carry sediment along the shore. Deposition occurs wherever that transport is interrupted, such as at jetties, inlets, or changes in coastline shape.
2. Wave and wind deposition: Waves push sediment onshore, building up berms and beach ridges. Wind then carries dry sand inland, forming dunes.
3. Tidal deposition: During high tides, fine sediment suspended in the water settles out in calm, sheltered areas. Over time, this builds tidal flats, salt marshes, and mangrove swamps.
4. River and delta deposition: Rivers carry enormous amounts of sediment to the coast. Where a river meets the sea, the current slows and sediment drops out, forming deltas. The Mississippi River Delta and the Nile River Delta are classic examples.
5. Biological deposition: Organisms contribute sediment directly. Coral reefs produce limestone, and accumulated shells build up deposits. The Great Barrier Reef in Australia is the world's largest coral reef system.

Human impacts on coastlines

Coastal development and infrastructure

About 40% of the world's population lives within 100 km of a coast. That concentration of people drives construction that often conflicts with natural coastal processes.

• Seawalls, groins, and breakwaters are built to protect property from erosion and storm damage. However, they frequently cause unintended problems. Groins trap sediment on one side while starving the beach downdrift, shifting erosion rather than stopping it. Seawalls reflect wave energy, which can scour the beach in front of them.
• Dredging of channels and harbors removes sediment from the coastal system and alters tidal flow, changing how nearby landforms evolve.
• Beach nourishment involves pumping sand onto eroding beaches to widen them. It's a temporary fix that typically needs to be repeated every few years and can disrupt marine habitats where the sand is sourced or placed.

Resource extraction and pollution

• Oil and gas production in coastal waters risks spills that devastate ecosystems. The 2010 Deepwater Horizon spill released roughly 4.9 million barrels of oil into the Gulf of Mexico, damaging marshes, fisheries, and wildlife across hundreds of kilometers of coastline.
• Sand and gravel mining removes sediment directly from the coastal system, accelerating erosion. This is a growing problem in regions where construction demand is high.
• Agricultural and urban runoff carries nutrients, pesticides, and pollutants into coastal waters. Excess nutrients cause eutrophication, triggering algal blooms that deplete oxygen and kill marine life.
• Marine debris and plastic pollution accumulates along coastlines and in ocean gyres. Microplastics now appear in coastal sediments worldwide, entering food chains and harming wildlife through ingestion and entanglement.

Climate change and sea level rise

Climate change intensifies many of the pressures coastlines already face.

• Thermal expansion of warming ocean water, combined with melting glaciers and ice sheets, is raising global sea levels. This increases coastal flooding and erosion, threatening low-lying islands and communities.
• Stronger storms: Warmer ocean temperatures fuel more intense hurricanes and typhoons. Hurricane Katrina (2005) and Typhoon Haiyan (2013) demonstrated the catastrophic damage these storms can inflict on coastal areas.
• Ocean acidification: Rising atmospheric $CO_2$ dissolves into seawater, making it more acidic. This stresses coral reefs by making it harder for corals to build their calcium carbonate skeletons. Warmer water temperatures compound the problem by causing coral bleaching, where corals expel the symbiotic algae they depend on for energy.
• Habitat loss: Rising seas and shifting conditions force coastal species to migrate. Wetlands and mangroves that would naturally shift inland are often blocked by roads, buildings, and other development, a situation sometimes called coastal squeeze.