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Unit 51


Unit Overview

This unit explores the landscapes and associated landforms of coastal areas. The main sections are as follows:

  • Aggradational landforms
  • Degradational landforms
  • Coastal landscapes
  • Living shorelines

Coastal processes produce unique coastal landforms, and, like all other landforms, they can be characterized as either aggradational or de gradational. Most of the beach material in the coastal environment comes from rivers. A typical beach is comprised of a foreshore, near shore, and backshore.

The location and distribution of beaches is related to both sediment availability and wave energy. Eolian processes also operate in coastal environments; coastal dunes are eolian landforms. Longshore currents can deposit material and produce landforms such as sandspits, baymouth bars, and tombolos. Offshore aggradational landforms include sandbars and barrier islands. Degradational landforms include sea cliffs, sea caves, wave-cut platforms, sea arches, and stacks. The two general types of coastlines are emergent coasts and submergent coasts. Emergent coasts are uplifted, while submerged coasts are drowned.

Unit Objectives

  • To examine the characteristics of a beach
  • To relate beaches to the coastline's topographic and tectonic setting
  • To recognize related coastal landforms of aggradation, such as sand dunes, offshore bars, and barrier islands
  • To identify landforms typical of erosional coastlines
  • To relate erosional and depositional processes to a general classification of coastlines

Glossary of Key Terms

Atoll Ring-like coral reef surrounding empty lagoons; grew on the rims of eroded volcanic cones.
Backshore The beach zone that lies landward of the foreshore; extends from the high water line to the dune line.
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.
Baymouth bar A sandspit that has grown all the way across the mouth of a bay.
Beach A coastal zone of sediment that is shaped by the action of waves; constructed of sand and other materials, derived from both local and distant sources.
Berm A flat sandy beach that lies in the backshore beach zone; deposited during storms and beyond the reach of normal daily wave action.
Coral reef An aggradational reef formed from the skeletal remains of marine organisms.
Emergent coast A coastal zone whose landforms have recently emerged from the sea, either through tectonic uplift, a drop in sea level, or both.
Foreshore The beach zone that is alternatively water-covered during high tide and exposed during low tide; the zone of beach drift and related processes.
Longshore Bar A ridge of sand parallel to the shoreline that develops in the nearshore beach zone.
Nearshore The beach zone that is located seaward of the foreshore, submerged even during an average low tide; longshore bars and troughs develop here in this zone of complex, ever-changing topography.
Offshore bar An offshore sandbar that lies some distance from the beach and is not connected to land; a longshore bar is an example.
Sand spit An elongated extension of a beach into open water where the shoreline reaches a bay or bend; built and maintained by a longshore drift.
Sea arch A small island penetrated by the sea at its base; island originated as an especially resistant portion of a headland that was eroded away by waves.
Sea cave Cave carved by undercutting waves that are eroding the base of a sea cliff.
Sea cliff An especially steep coastal escarpment that develops when headlands are eroded by waves.
Stack A column-like island that is a remnant of a headland eroded away by waves.
Submergent coast A drowned coastal zone, more common than uplifted, emergent coasts; submergence caused in large part by the rise in sea level of the past 10,000 years.
Tombolo A sandspit that forms a link between the mainland and an offshore island.
Uplifted marine terrace In an emergent coastal zone, a wave-cut platform that has been exposed and elevated by tectonic uplift or a lowering of sea level (or both).
Wave-cut platform The abrasion platform that develops at the foot of a sea cliff, marking its recession; its nearly flat bedrock surface slopes seaward.

Unit Outline

  • Aggradational landforms
    • A beach is a coastal zone of sediment, shaped by wave action
      • begins at the foot of dunes and continues beneath the water
    • Beach dynamics (Figs. 51.1 & 51.2)
      • foreshore is the zone that is submerged at high tide and exposed at low tide
      • the nearshore (offshore) is seaward of the foreshore, and is submerged even at low tide
        • longshore bars (parallel ridges) form in this zone
      • the backshore is landward of the foreshore, extends from the high-water line to dunes
        • area contains berms, beaches that were deposited during storms or unusually high wave action
      • beaches have different seasonal profiles
      • beaches are open systems with inputs and outputs
      • location of beaches related to available sediment and wave energy
    • Coastal dunes
      • wind is the primary builder of dunes
      • strong sea breezes build up berms and dunes
    • Sandspits and sandbars (Fig. 51.7)
      • sandspits occur when longshore drift moves materials into an area with a bend or bay, where the material forms an extension into the water
        • baymouth bars are sandspits that grow all the way across the mouth of a bay
        • a tombolo may form if a sandspit connects an island to the mainland
    • Offshore bars and barrier islands
      • offshore (sand) bars are not connected to the land, interfere with wave action
      • barrier islands (Fig. 51.12) are permanent offshore bars, may have glacial origin
        • contain dunes and vegetation, such as grasses and mangrove
        • Miami Beach, Atlantic City, Galveston
  • Degradational landforms (Fig. 51.14)
    • Wave erosion is the dominant process
    • Headlands are eroded by waves, create sea cliffs
    • Sea caves often eroded at the bases of sea cliffs
    • Wave-cut platform can form at the base of the sea cliff, as it continues to be eroded
    • Last remaining remnants of sea cliff are sea arches and stacks, both temporary features
  • Coastal landscapes
    • Emergent coasts
      • elevated by tectonic forces
      • cliffs and wave-cut platforms are elevated above sea level
      • uplifted marine terraces form
    • Submergent coasts
      • lowered by rising sea levels over the past 10,000 years
      • tops of some submerged hills exposed as islands
  • Living shorelines (Fig. 51.17)
    • Corals, algae, and mangroves can shape a coastline
      • coral reef is built by tiny marine organisms, discharge calcium carbonate
      • can be attached to shoreline or free-floating
      • atolls are circular coral reefs that surround a lagoon
    • Mangrove is a shoreline builder, creates vegetated mud flats

Review Questions

  1. Define the terms sandspit and baymouth bar, and describe their formation.
  2. Name several of the degradational landforms associated with wave erosion, using Fig. 51.14.
  3. How are submergent coastlines formed?