Chapter 19

The Work of Waves and Wind

OVERVIEW

This chapter examines the processes and landforms that result when wind energy moves surface materials, either directly as wind or indirectly as waves. Both waves and wind are driven by the rotation and unequal surface heating of the Earth. Unlike fluvial action and mass wasting, wind and waves can move material uphill against the force of gravity.

·  Shoreline refers to the line of contact between water and land, while coastline refers to the zone of influence of coastal processes.

·  Wave action is the most important agent shaping coastal landforms. Waves approaching a shoreline are slowed by the drag of the bottom, become steeper, and eventually collapse to form breakers.

·  The energy of breakers expended along a coastline causes erosion and transportation of shoreline materials.

·  Weak or soft shoreline materials are eroded rapidly by wave action to form marine scarps, while more resistant materials are eroded slowly to form marine cliffs.

·  Beaches are thick wedge-shaped accumulations of sand shaped by the swash and backwash of water along the shoreline.

·  Waves striking a shoreline at an oblique angle cause littoral drift: the movement of sediment along the shore by the processes of beach drift and longshore drift.

·  The sands transported by littoral drift can form spits, bars, and pocket beaches along a coast.

·  Variations in sediment transport along a coast can lead to progradation (building out) and retrogradation (cutting back) of the shoreline.

·  Tides are regular fluctuations in sea level caused by the gravitational forces of the sun and moon.

·  Tides drive ebb and flood currents which redistribute fine sediments within bays and estuaries.

·  Coastlines of submergence result from partial drowning of the coast by a rise in sea level or sinking of the land. Ria coasts and fjord coasts are deeply embayed with bold relief.

·  Coastlines of emergence, where submarine deposits become exposed, usually have gentle relief and slope gently toward the ocean. Repeated crustal uplift produces barrier island coasts while rapid emergence can produce raised shorelines and marine terraces.

·  Coastlines created when new land is built out into the ocean include delta and volcano coasts as well as coral reef coasts.

·  Global sea level has fluctuated substantially in the past and is presently rising slowly. The present rise may be due to the melting of glacier ice and thermal expansion of the oceans caused by global warming.

·  Wind action is capable of moving dry, fine, loose sediments that are not protected by a vegetation cover and is an effective landforming agent in deserts, semiarid regions, and along coasts.

·  Wind erosion includes abrasion and deflation. The wind drives loose sand and dust particles against exposed surfaces in a process of abrasion.

·  The removal of loose particles from the ground surface by wind is called deflation. Deflation can produce blowouts, desert pavements, and dust storms. Clay and silt particles are lifted high into the air and carried long distances, while sand particles are lifted only with moderately strong winds and are only carried one to two meters above the ground.

·  Sand dunes form where there is an abundant source of sand available for movement by wind.

·  Variations in wind conditions, vegetation cover, and sand abundance produce a wide variety of dune types. Some of the more important are:

·  Barchan dunes - individual, crescent-shaped dunes with arms pointing downwind

·  Transverse dunes - wave-like dunes with a crest aligned perpendicular to the wind direction

·  Parabolic dunes - crescent-shaped dunes with the dune crest bowed outward in a downwind direction.

·  Longitudinal dunes – long, narrow ridges oriented parallel with the prevailing wind direction

·  Loess is a sheet-like deposit of wind-transported silt. Extensive loess deposits in North America were derived from fresh glacial deposits exposed at the end of the last ice age.

·  Human disturbance of the vegetation cover in semiarid regions can expose the soil to induced deflation or wind erosion.

KEY TERMS

shoreline
coastline
coast
bay
estuary
marine cliff
beach
littoral drift
beach drift / longshore drift
ocean tide
progradation
retrogradation
submergence
emergence
delta
coral reef
atoll / deflation
barchan dune
transverse dune
parabolic dune
blowout
desert pavement
dust storm
sand dune
loess

STUDY QUESTIONS

1.   What is the most important energy source driving wind and wave action?

2.   Sea caves, sea arches, and stacks are landforms often associated with marine cliffs. How do these features develop?

3.   How do the processes of swash and backwash move sediment particles along a beach?

4.   What is a pocket beach, and how is it formed?

5.   Why can the construction of a dam upstream of the mouth of a river induce retrogradation of the shoreline near the mouth?

6.   What role do tidal currents play in the formation of a salt marsh?

7.   Ria coasts and fjord coasts are both coastlines of submergence. How are they similar and how do they differ?

8.   How do deltas form and how do they affect shoreline shape?

9.   Distinguish between fringing reefs, barrier reefs, and atolls.

10.   What landforms would you expect to find along a bedrock coast that had been subjected to a several episodes of rapid emergence or uplift?

11.   How might global warming affect global sea level, and what implications might this have for human societies?

12.   What is deflation, and under what conditions is it effective?

13.   Parabolic dunes are often found adjacent to beaches. Why do parabolic dunes develop at these sites?

14.   What is the angle of repose? Which part of a sand dune would have a slope inclined at the angle of repose? Why?

15.   What is loess, and what are the primary sources of material for loess deposits?

CHAPTER QUIZ

Multiple Choice Questions

1.  A steep slope formed by the rapid erosion of soft shoreline materials by wave action is called a:

a)  marine scarp

b)  marine cliff

c)  marine terrace

d)  marine delta

2.  Which of the following is most indicative of coastline emergence?

a)  ria coast

b)  fjord coast

c)  barrier-island coast

d)  delta coast

3.  Sand dunes that are aligned parallel to the prevailing wind direction are known as:

a)  barchan dunes

b)  parabolic dunes

c)  transverse dunes

d)  longitudinal dunes

4.  Which of the following sites has the thickest deposits of loess?

a)  northern China

b)  central Europe

c)  eastern United States

d)  western Argentina

5.  A landform resulting from littoral drift is:

a)  a lagoon

b)  a sandspit

c)  an atoll

d)  an estuary

True/False Questions

1.   Wave action usually moves sediment from bayheads to headlands. (T/F)

2.   Fjord coasts rarely have beaches. (T/F)

3.   When the tide begins to rise, an ebb current sets in. (T/F)

4.   Global sea level is rising at the present time. (T/F)

Short Answer Questions

1.   What conditions are necessary for wind to be an effective agent of erosion? Where do these conditions most commonly occur?

2.   What are ebb currents and flood currents, and how are they related to tide levels?

3.   What are breakers, and how do they develop? Use a diagram to illustrate your answer.

Short Essay Questions (1 - 2 paragraphs)

1.  Identify and describe the pathways of flow in a coastal sediment cell. Your answer should consider the sediment inputs, outputs, and storage locations in the system.

2.  Describe the processes involved in littoral drift, and discuss how they would be affected by the construction of groins along a beach.

3.  Why do wind-transported sand and wind-transported silt form different kinds of landforms?

Internet Resources

1.  An introduction to wind erosion control: <http://www.agric.gov.ab.ca/agdex/500/72000002.html>

2.  Wind and water erosion: <http://huron.lre.usace.army.mil/shore.protection/wndwtr.html

3.  Geomorphology from space – eolian landforms: <http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_8/GEO_CHAPTER_8_TABLE.HTML

4.  Geomorphology from space – coastal landforms: <http://daac.gsfc.nasa.gov/DAAC_DOCS/geomorphology/GEO_6/GEO_CHAPTER_6_TABLE.HTML

5.  A review of coastal processes: <http://129.252.37.27/hses/coastal/frames.htm

6.  A huge repository of desert landform images and text: <http://www.tec.army.mil/research/products/desert_guide/terrain/desert/lguide.htm

7.  Geology of the Loess Hills, Iowa: <http://pubs.usgs.gov/info-handout/loess/

8.  Coastal erosion and management at Reculver in Kent, U.K.: <http://www.georesources.co.uk/recintro.htm >