10.2 Depositional coastal landforms

Coastal landforms are shaped by the constant interplay of water, wind, and sediment. Beaches, spits, and tombolos form as waves and currents deposit materials along shorelines. These features are dynamic, constantly evolving as environmental conditions change.

Longshore drift plays a crucial role in shaping coastal landscapes. This process moves sediment parallel to the shore, creating and maintaining features like barrier islands. Coastal dunes form above the high tide line, providing natural protection against storms and flooding.

Formation of Coastal Landforms

Beach Components and Formation

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• Beaches accumulate sediment along the shoreline through wave action and longshore currents
  • Deposited materials include sand, gravel, and shells
• Beach profiles consist of distinct components
  • Foreshore extends from low tide line to high tide line
  • Backshore lies above high tide line, only affected by waves during storms
  • Berm forms a raised platform above normal high tide level
• Beach cusps create crescentic patterns along the shoreline
  • Form due to complex interactions between waves, currents, and sediment transport
  • Typically spaced at regular intervals (10-100 meters apart)

Spits and Tombolos

• Spits extend from the mainland into open water as elongated depositional features
  • Often form where coastline direction changes
  • Growth depends on consistent sediment supply and longshore drift
  • Examples include Spurn Point (England) and Sandy Hook (New Jersey)
• Tombolos connect islands to the mainland or other islands with sand or gravel bars
  • Form when waves refract around islands and deposit sediment in their lee
  • Size and stability influenced by wave energy, sediment supply, and island-mainland distance
  • Notable examples include Chesil Beach (England) and Monte Argentario (Italy)

Longshore Drift and Deposition

Mechanics of Longshore Drift

• Longshore drift moves sediment parallel to the shoreline
  • Driven by waves approaching coast at oblique angles and longshore currents
• Swash-backwash process contributes to net sediment movement
  • Waves push sediment up beach at an angle
  • Gravity pulls sediment back perpendicular to shore
• Rate and direction influenced by various factors
  • Wave energy
  • Coastal orientation
  • Sediment characteristics (size, shape, density)

Impact on Coastal Landforms

• Longshore drift shapes depositional landforms
  • Crucial for formation and maintenance of spits, bars, and barrier islands
• Interruptions to drift patterns affect sediment distribution
  • Groins or jetties cause accretion on updrift side
  • Erosion occurs on downdrift side of structures
• Contributes to zetaform beach development
  • Curved planform shape results from gradual changes in sediment transport rates
• Essential for coastal management strategies
  • Informs beach nourishment projects
  • Guides design of coastal protection structures (seawalls, breakwaters)

Types of Coastal Dunes

Primary Dune Formation

• Coastal dunes accumulate wind-blown sand above high tide line
  • Serve as natural barriers against storm surges and coastal flooding
• Embryo dunes initiate dune formation
  • Develop when sand traps around pioneer vegetation or obstacles on upper beach
  • Examples of pioneer plants include sea rocket (Cakile maritima) and sand couch grass (Elytrigia juncea)
• Foredunes form primary dune ridge parallel to shoreline
  • Sand accumulates around vegetation like marram grass (Ammophila) or sea oats (Uniola paniculata)
  • Provide first line of defense against coastal erosion and storm impacts

Advanced Dune Systems

• Parabolic dunes exhibit U-shaped formation
  • Form when blowouts occur in established dunes
  • Arms point upwind, often stabilized by vegetation
  • Examples found in coastal regions of Australia and New Zealand
• Transgressive dune sheets create large, mobile sand masses
  • Move inland over existing landscapes
  • Form in areas with high wind energy and abundant sand supply
  • Extensive examples in the Namib Desert coast and parts of coastal Brazil
• Backdunes represent older, more stabilized dunes
  • Located further inland from foredunes
  • Support diverse vegetation and ecosystems
  • Often found in coastal dune systems of the Netherlands and Denmark

Stability of Coastal Features

Natural Factors Affecting Stability

• Sediment supply balance critical for long-term maintenance
  • Equilibrium between erosion and deposition required
• Sea-level changes impact coastal landform stability
  • Rising sea levels potentially increase erosion and landward migration
• Wave energy and storm frequency shape depositional landforms
  • Extreme events (hurricanes, nor'easters) can cause significant changes
• Vegetation cover contributes to dune and feature stability
  • Traps sand and reduces wind erosion
  • Examples include beach grass (Ammophila breviligulata) and sea grape (Coccoloba uvifera)

Anthropogenic and Climate Influences

• Human activities disrupt natural sediment transport processes
  • Coastal development alters sediment pathways
  • Sand mining reduces available sediment supply
• Feature orientation relative to prevailing winds and waves affects stability
  • Alignment with dominant forces can enhance or reduce erosion potential
• Climate change impacts alter long-term stability of depositional systems
  • Changes in storm patterns affect wave energy and frequency
  • Altered precipitation regimes influence sediment supply from rivers
  • Examples include increased erosion rates in Arctic coastal areas due to permafrost thaw