Coastal Morphology and Types
Coastal Morphology
Coastal morphology is the study of the relief caused by waves.
Wave Dynamics
- Swash (Uprush): After a wave breaks, turbulent, foamy water rushes up the beach. This powerful energy source moves sand and gravel inland.
- Backwash: As the swash loses energy, water flows back down the beach. Some water disappears underground, while the rest carries gravel and sand back towards the sea.
Types of Waves
- Giant Waves (Tsunami): These waves, caused by earthquakes or volcanic eruptions, can reach over 30 meters in length and travel at speeds of approximately 700 km/h. Their impact on land can be devastating.
- Oscillation Waves: These waves appear to move water, but only the wave’s shape travels. They are driven by wind and change in height and direction.
- Translation Waves: As oscillation waves approach shallow coastlines, their lower portions encounter resistance from the seabed. The crest moves forward rapidly, causing the wave to shorten and increase in height, eventually breaking.
- Ripples: These small surface waves occur in lakes or ponds, with water advancing and receding.
Wave Erosion
The forces of waves moving up and down cause the release and transport of particles, resulting in erosion through:
- Hydraulic Action: The force of water pounding against rocks.
- Corrosion: The action of rock fragments carried by the water wearing away at other rocks.
- Abrasion: The wearing down of fragments as they collide with each other.
Littoral Drift
Waves striking the shore at an angle create a current parallel to the coastline, known as littoral drift. This current, primarily within the surf zone, transports sediment along the shore.
Beach Formation
A beach is a deposit of sand, gravel, or pebbles formed in the surf zone by wave action.
- Progradation: When sand is deposited faster than it’s removed, the beach widens.
- Retrogradation: When sand is removed faster than it’s deposited, the beach narrows, and the shoreline recedes.
Wave Refraction, Reflection, and Diffraction
As waves approach the shore, they change direction, slow down, and become steeper due to the seabed topography. When the water depth is less than half the wavelength, three phenomena occur:
- Refraction: Wave crests become parallel to isobaths (contour lines below sea level), causing waves to break parallel to the coast.
- Reflection: Waves encountering an obstacle bounce back. Perpendicular incidence creates standing waves, while oblique incidence generates additional waves.
- Diffraction: When a wave encounters a barrier (e.g., a cape or island), the wave crest splits, causing it to diverge and attenuate due to energy dissipation.
Formation of Sea Cliffs
Steep slopes where land meets water are ideal for sea cliff formation.
Initial Stage
Waves erode the rock, creating a small cliff with a rocky platform below the water level. Rock fragments are dispersed offshore, preventing beach formation.
Intermediate Stage
Waves carve out softer rock, forming sea caves and arches. The cliff becomes irregular and continues to erode.
Mature Stage
The abrasion platform expands, reducing wave energy at the cliff base. Weathering and rain transform the cliff into a gentle slope.
Sandbars and Spits
- Littoral Spit: A sandbar extending from the coastline into a bay. Curved spits are called hook spits.
- Baymouth Bar: A spit that connects to the opposite side of a bay, separating it from the open sea.
- Tombolo: A sandbar connecting an island to the mainland.
- Curved Bar or Cuspate Foreland: Sand deposition from opposite directions creates a pointed sand deposit. Continued deposition can form a larger sand cape.
Tidal Deposits and Marshes
Tidal currents can carve channels and transport sediment, leading to the formation of tidal flats and marshes in bays and estuaries.
Classification of Coasts
1. Submergent Coasts
Formed by land subsidence or rising sea levels.
- Ria Coasts: Characterized by irregular coastlines with drowned river valleys forming estuaries and headlands becoming peninsulas.
- Fjord Coasts: Formed by the retreat of glaciers, featuring steep walls, deep water, and inland penetration.
2. Emergent Coasts
Formed by land uplift or falling sea levels.
- Coastal Plains: Gently sloping plains with simple coastlines.
- Steep Cliff Coasts: Characterized by deep water near the shore and a relatively narrow coastal plain.
3. Neutral Coasts
Formed by material deposition.
- Alluvial Fan Coasts: Curved coastlines formed by streams depositing sediment.
- Delta Coasts: Formed by river sediment accumulation.
- Volcanic Coasts: Formed by volcanic eruptions or lava flows.
4. Fault Coasts
Rare coastlines formed by faulting, resulting in steep cliffs.
5. Compound Coasts
Formed by a combination of the above types.
Coastal Development and Geographic Aspects
Coastal morphology significantly influences human activities. Submergent coasts, with their deep bays and natural harbors, often support fishing, shipbuilding, and maritime commerce. Emergent coasts may offer agricultural opportunities or resources like forests and hydroelectric potential. Barrier islands and coral reefs present unique challenges and opportunities for human settlement and resource utilization.
Coral Reef Coasts
Neutral coasts formed by the growth of coral and algae.
- Fringing Reefs: Platforms connected to the coast, thriving in areas with strong wave action and clear water.
- Barrier Reefs: Offshore reefs separated from the mainland by a lagoon, often with channels for water exchange.
- Atolls: Circular reefs enclosing a lagoon, without any landmass inside.
Geographical Aspects of Atolls
Atolls are composed entirely of coral limestone, limiting vegetation and freshwater resources. Coconut palms are a crucial resource, providing food, materials, and fibers. Fishing is a primary food source, and the lagoon offers a calm environment for navigation.