Coastal Geomorphology

sediment_cycle0002Shorezone

  • high-energy geomorphic system where wave energy is dissapated
  • a landscape of small height and width but circumscribes the continents
  • accounts for less than 10% of the earth’s land surface but about 2/3 of the population

Swell

  • waves in the offshore
  • represent the transmission of energy but not mass, ie the ocean surface swells as the crests of waves pass by
  • created as wind shear stress is applied to the ocean surface
  • wave size (height and wave length) is a function of: wind velocity, duration of wind from a constant direction, and fetch: the length of water over which the wind is passing.
  • therefore the large, long duration storms produce the largest waves, typically 15 m in height , but up to 34 m
  • low rounded symmetrical waves move 1000s km with little energy loss, as the potential energy (PE = mgh) is transmitted with the wave form
  • the kinetic energy (KE = mv 2 /2) is the circular orbit of the water molecules

Surf

  • breaking waves in the shorezone (foreshore)
  • as waves drag on the sea floor in the foreshore, the waves decelerate becoming asymmetric (circular orbit becomes elliptical), shorter and higher ( ie increasing PE); waves become higher and steeper until they are unstable and break to form surf

Wave erosion and sediment transport

Erosional proceses

  • hydraulic action: force of waves and trapped air and rock is plucked from headlands
  • attrition: reduction of particle size as clasts bounce in the surf
  • abrasion (corrasion): erosion as clasts as cast against headlands

Longshore drift

Controls on wave erosion

  1. sea floor geometry
    • determines where waves break relative to the shore
    • profile geometry
      • maximum waver erosion occurs where the sea floor profile causes waves to gain height in the foreshore and break at the shoreline
      • if the sea floor slopes gradually surf will occur seaward of the shoreline
      • if the sea floor slopes steeply, surf may not form and the swell will contact the headlands
    • planimetric geometry: wave refraction
      • waves conform to the sea bottom topography, that is, decelerate first opposite the headlands and bend around them into the bays
      • wave energy is concentrated on headlands and diverges in the bays
      • wave refraction also generates a longshore drift of water and sediment from the breaking waves at the headlands to the lower water in the adjacent bays
      • erosion on headlands and deposition in bays causes coastlines to become less irregular over geologic time

2. storm surges 

  • water levels rise by 1 cm for every 1 mb drop in atmospheric pressure
  • thus during storms (high wind shear), large waves pass through elevated sea levels and break in the backshore
  • low pressure is common over the oceans in winter (warmer than land), so winter wave erosion can remove the sand from beaches, where it is stored offshore and returned to shore with lower wave energy that favours deposition

3. seismic (not tidal) waves 

  • tsunamis (Japanese; tsu: harbour, namis: wave): very long low waves induced by the tectonic disturbance of the sea floor
  • tsunamis travel at speeds up to 800 km/hr and reach heights of 15 m or more in the shorezone

4. tides 

  • the diurnal rise and fall of sea level in response to the gravitational interaction between the earth and its moon
  • distributes wave energy over a range of shoreline elevations between low tide and high tide
  • also tidal bores move up the mouths of rivers at 15-30 km/hr and currents can be generated between basins with different tidal periods

Sea (lake) level change

  • with eustatic and isostatic change over geologic time (eg Pleistocene ice ages) coastlines emerge with falling sea level and are submerged with rising sea level
  • thus relict coastal landforms occur at various elevation relative to present sea level

Subaerial weathering and erosion

  • various aspects of coastal environments favour other geomorphic processes
  • eolian: beaches and mud flats at low tide are unprotected from onshore winds, thus coastal sand dunes are common landforms
  • mass wasting
    • wave erosion is concentrated at the water surface producing sea caves and wave-cut notches
    • headlands therefore retreat as sea cliffs are undermined by wave erosion, and therefore fail by sliding, flowing or falling
    • the mass wasted debris can initially protect the base of a sea cliff, but then contributes to erosion as the material is exported and reduced in size (attrition) by waves and also contributes to abrasion of the sea cliff
  • weathering
    • chemical and mechanical weathering are accentuated at the shoreline by the presence of water and salt
    • water-level weathering contributes to the planing of shore platform

Erosional Landforms

  • in resistant rock, erosional landforms tend to relict, because of significant Quaternary sea level change
  • thus most contemporary erosional landforms are in relatively weak rock or on coastlines with considerable wave energy

1. shore (wave-cut) platform

  • gentle rock slope that extends from high tide to low tide
  • the remnant of erosion of headlands, because erosion occurs at and above the water level
  • abrasion and water-level weathering have a planing as the shoreline diurnally transgresses and regresses over the platform
  • the platform geometry reflects an equilibrium between wave energy and rock resistance
  • because the platform slopes, the sea cliff becomes progressively lower and eventually is replaced by a long shore platform, given enough time and a stable sea level

2. sea caves and wave-cut notches : the common products of wave erosion at the base of sea cliffs

sea stacks and arches

  1. sea stacks and arches 
  • the subaerial remnants of headlands that project above the shore platform
  • with wave refraction, headlands are eroded on three sides, causing sections of headland to be isolated as sea stacks
  • the erosion of a cave(s) in a sea stack creates a sea arch

Depositional Landforms

landforms in the sediment (mostly sand) delivered by rivers and, to a much lesser extent, generated by headland erosion

beach 

  • average wave energy is sufficient to transport sand from the shallow sea bed and move it onshore
  • higher gradient gravel, boulder or shingle beaches occur at the base of headlands and behind sandy beaches , that is, where there is higher wave energy capable of removing sand
  • sand is carried onshore in the swash ; the backwash seeps into the beach and flows seaward with has sufficient energy to suspend and remove only the finer sediments (silt and clay)
  • thus beaches are a lag deposit of sand drifting along the shore, as the swash commonly is oblique to the shoreline whereas the backwash is always directly seaward (down the beach)
  • beaches adjust quickly to changes in wave and tidal energy resulting in a change in beach mass balance = sediment inputs (fluvial + from offshore + eolian + mass wasting sea cliffs) – sediment outputs (eolian + to offshore)

barrier bars 

  • ridges of sand up to a km wide and 100 m high that lie parallel to about 13% of the world’s coasts
  • consist of sand blow seaward onto tidal flats during low tide and sand in the backwash which comes out as suppension as the rip current meet the incoming surf
  • interupted by tidal inlets so that logoons behind the bars are subject to tides

beach ridges

  • berm (beach crest) usually exist just above high tide
  • other beach ridges represent storm deposits or emerged offhsore bars

spit 

extension of a beach from a headland in lower energy environment (bay or laggon) in the direction of the longshore drift

baymouth bar 

  • spits merge to create bars that extend across the mouths of bays
  • waves energy is dissipated on the bar and the bay becomes a logoon
  • logoons fill with sediment, supporting salt marshes
  • thereby, sediment progrades towards retreating headlands and coastlines become less irregular

tombolo : a beach that extends between a headland, or other part of the mainland, and an island

intertidal mud flats and marshes 

  • at low tide, especially during the slack water interval before the tide reverses, fine sediment is stirred by the surf and deposited by streams on the intertidal flats
  • the fine sediment flocculates in the salt water (cemented by Na + ) and resist erosion during the rising tide
  • mudflats are be colonized by salt-tolerant plants that reduce wave and tidal energy and trap sediment
  • mangrove swamps along tropical deltaic coats make the shoreline hard to define; the mangrove trees tolerate tidal immersion

source: http://uregina.ca/~sauchyn/geog323/coastal.html

About nandi

Lecturer of Department of Geography Education

This entry was posted in GeoIssues and tagged , , , , . Bookmark the permalink.