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Unit 50

COASTAL PROCESSES

Unit Overview

This unit examines various coastal processes and the subsequent impacts of those processes on coastal landscapes. The main sections are as follows

  • Coasts and shores
  • Waves and their properties
  • Waves against the shore
  • Degradation and aggradation by waves
  • Tides and shore zone currents
  • The role of storms
  • Crustal movement

Coastal processes operate at the interfaces of landmasses and oceans. These processes are critically important, primarily because of the propensity for human settlement in coastal areas. Landforms and landscapes of coastal zones result from the complex interaction of many processes - the most important of which are ocean waves.

Most waves are generated through the frictional drag of winds. Wave properties depend on wind velocity, wind direction, wind duration, and fetch. The characteristics of a wave change dramatically when the wave approaches the shore; a wave of oscillation becomes a wave of translation. Waves of translation perform erosional work in the littoral zone. The refraction of waves as they approach the shore produces a longshore current, which flows approximately parallel to the shoreline. The movement of material by the longshore current and its associated processes is known as littoral drift.

Unit Objectives

  • To establish the importance of coastal zones as areas of interaction between physical processes and human settlement
  • To examine the physical properties of waves and their significance in the operation of coastal processes
  • To discuss other sources of energy in the coastal zones and their erosional and depositional significance


Glossary of Key Terms

Backwash The return flow to the sea of the thinning sheet of water that slid up the beach as swash.
Barrier island A permanent, offshore, elongated ridge of sand, positioned parallel to the shoreline and separated by a lagoon from it; may have originally formed as an offshore bar during the last glaciation, migrated coastward, and grew as it shifted.
Beach drift The larger process associated with longshore drift that operates to move huge amounts of beach sand downshore in the direction of the longshore current.
Coast General reference to the strip of land and sea where the various coastal processes combine to create characteristic landscapes.
Corrasion The mechanical coastal-erosion process whereby waves break off pieces of rock from the surface under attack; the rock- fragment-loaded water, now a much more powerful erosive agent, continues to be hurled against the surface.
Littoral zone Coastal zone.
Longshore current A shore-paralleling water current, similar to the longshore drift of sand, that is generated by the refracted, oblique- angled arrival of waves onshore.
Longshore drift The movement of sand along the shoreline in the flow of water (longshore current) generated by the ref acted, oblique- angled arrival of waves onshore.
Rip current Narrow, short-distance stream-like current that moves seaward from the shoreline cutting directly across the oncoming surf.
Shoaling The near-shore impact of ever shallower water on an advancing, incoming wave.
Shore Has a more specific meaning the coast; denotes the narrower belt of land bordering a body of water, the most seaward portion of a coast.
Storm surge The wind-driven wall of water hurled ashore by the approaching center of a hurricane, which can surpass normal high tide levels by more than 5 m (16 ft): often associated with a hurricane's greatest destruction.
Surf The water zone just offshore dominated by the development and forward collapse of breaking waves.
Swash The thinning sheet of water that slides up the beach after a wave reaches shore and has lost its form.
Tide The cyclical rise and fall of sea level controlled by the Earth's rotation and the gravitational pull of the moon and sun: daily, two high tides and two low tides occur within a period slightly longer than 24 hours.
Wave height The vertical distance between the wave crest (top) and the wave trough (bottom).
Wave length The horizontal distance between one wave crest (or wave trough) and the next.
Wave refraction The near-shore bending of waves coming in at an oblique angle to the shoreline; shoaling slows part of the wave, which progressively bends as the faster end "catches up".
Waves of oscillation Waves that move water particles in a circular up-and-down path; their depth is one-half their length.


Unit Outline

  • Coasts and shores
    • Land meets sea in the littoral zone
    • Coast refers to a strip of land and sea where coastal processes occur
    • Shore refers to a narrower strip of land bordering the water
    • Shoreline is the actual contact boundary between land and sea
  • Waves and their properties (Fig. 50.1)
    • Waves are the real erosional agents of the coast
    • Large waves form when several criteria are met
      • high wind velocity
      • persistent wind direction
      • fetch is long
      • wind duration long
    • Wave height is the distance between a wave crest and a wave trough
    • Wave length is the horizontal distance between two wave crests (or two troughs)
    • Wave period is the time between the passing of two successive crests
    • Waves of oscillation move water particles in a circular up-and-down motion
      • the depth of this type of wave is half of its length
  • Waves against the shore (Fig. 50.3)
    • As waves enter shallow water near a coastline, become waves of translation
      • water particles do not return to their original positions in orbital motion, begin to erode
      • wave becomes so steep that its crest collapses, becomes a breaker continual breaking of waves against the coastline
      • swash is the thin flow of water that slides up the beach when a wave breaks
      • backwash is when swash flows back into the water
    • Wave refraction (Fig. 50.5)
      • shoaling is when an advancing wave is affected by shallow water
      • wave refraction is the process by which waves approach the coast at an oblique angle, the leading edge is slowed, and the wave is bent to an angle parallel with the coastline
    • Longshore drift (Fig. 50.6)
      • sand is moved along the beach by arriving waves bringing material to shore at an angle, and the backwash returning materials to sea at a right angle
      • beach drift is the movement of huge amounts of sand along the shore
      • longshore drift can create ridges that eventually form barrier islands
  • Degradation and aggradation by waves
    • Erosion by hydraulic action
    • Corrasion is the mechanical process of wave erosion
    • Corrosion is the breakdown of rock by solution or chemical methods
    • Low relief dominates most coastlines
      • beaches
      • dunes
      • sandy islands
  • Tides and shore zone currents
    • Effects of tides
      • high tide
      • low tide
      • tidal range is the average vertical distance between low and high tide
      • tidal currents refer to the changing water levels associated with tides
      • tidal bore created when rapid high tide creates a wave front that runs into a river or bay
    • Shore zone currents
      • longshore current parallel to shore
      • rip currents are stream-like, high velocity currents that move materials toward the ocean (Fig. 50.11)
  • The role of storms
    • Storms create powerful winds, creating waves called the storm surge when colliding with the shoreline
  • Crustal movement
    • Coastlines rise and fall in the long run due to isostatic rebound and sea level fluctuation


Review Questions

  1. Define the term longshore drift, and describe how it occurs.
  2. What is a storm surge?
  3. Describe the conditions necessary for the formation of large waves.