Ch20

Chapter 20: Coastal Processes and Terrain

McKnight’s Physical Geography: A Landscape Appreciation,

Tenth Edition, Hess

Coastal Processes and Terrain

• The Impact of Waves and Currents on the Landscape

• Coastal Processes• Coastal Landforms

2© 2011 Pearson Education, Inc.

The Impact of Waves and Currents on the Landscape

• Coastal processes affect a tiny portion of Earth’s landscape

• Waves agents of erosion

• Currents agents of transportation and deposition

• Beaches mark transition between land and water and are highly variable


Figure 20-4

Coastal Processes

• Interface of three major components of Earth’s environment

• Highly energetic due to constant motion of waters

• Importance of wind• Ocean and lake formations

similar except for:– Tidal range smaller for lakes

– Water level change differences

– Reefs only in oceanic water


Figure 20-1

Coastal Processes

• Waves—transfer of energy through cyclical rising and falling of a substance

• Most are wind generated over oceans

• Wind stress generated waves, forced waves

• Swells


Coastal Processes

• Waves of oscillation and translation– Waves that move in a circular

or oscillatory fashion with little forward movement, waves of oscillation

– Wave crests and troughs

– Wavelengths and wave heights

– Wave amplitudes

– Shallow water waves influenced by ocean floor, gain forward progress, called waves of translation; wave breaks


Figure 20-2

Coastal Processes

• Wave refraction – Change in wave direction as

they approach shore

– Uneven coastline and irregular water depth

– Waves bent due to uneven slowing of waves from irregular water depth

– Wave action focused on headlands, much gentler in adjacent bay areas


Figure 20-5

Coastal Processes

• Wave erosion– Consistent pounding of small

waves results in erosion

– Large storms significantly enhance coastal erosion

– Air forced into cracks in coastal rocks when water moves inland; air released as water recedes and enhances erosion

– Chemical action of seawater

– Notches cut in the bases of cliffs


Figure 20-6

Coastal Processes

• Tsunamis– Waves triggered by disruptions

in ocean floor

– When fault rupture on ocean floor generates tsunami, entire depth of ocean above rupture is displaced

– Inconspicuous in open ocean with long wavelengths and low heights

– Can travel over 400 mph

– Significant withdrawal of up to 40 meters before a significant surge of water 9© 2011 Pearson Education, Inc.

Figure 20-8

Coastal Processes

• Tides– Alterations of ocean level

from gravitational pull of Sun and Moon

– Two high tides and two low tides per day

– Topographic effects generally small

– Significant agents of erosion only in narrow bays, around shallow seas, and in passages between islands


Figure 20-9

Coastal Processes

• Changes in sea level and lake level• Two primary causes of sea level changes

– Rising or sinking of landmass (tectonic change)

– Increase or decrease in amount of ocean water (eustatic sea-level change)

– Emergent versus submergence land characteristics

• Global warming and sea-level change– Thermal expansion of water and melting of ice caps increasing

water volume (eustatic)

– Sea level rise of up to 0.5 m by the end of the century


Coastal Processes

• Ice push– Bodies of water that freeze in winter, resulting in expansion

and subsequent contraction

– Ice pushes onto land, significantly modifying land surface, similar to small glacial advance

– Most common in Arctic and Antarctic regions


Coastal Processes

• Organic secretions– Many aquatic animals form

calcium carbonate shells

– Animals cluster together and form enormous masses of reefs, platforms, and atolls

• Stream outflow– Streams important sources

of sediment to oceans and lakes


Figure 20-11

Coastal Processes

• Currents and coastal sediment transport

• Longshore currents– Water moves parallel to

shoreline (“along” shore)

– Develop just offshore and set up by waves striking coast at an angle

– Wind direction reflected in longshore currents


Figure 20-12

Coastal Processes

• Currents and coastal sediment transport (cont.)

• Beach drifting– Short distance shifting of

sand by breaking waves and retreating water

– Zigzag pattern of particle movement downwind and parallel to coast

– Affects of tides on debris movement

– Dune formation on coasts


Figure 20-13

Coastal Processes

• Coastal deposition– Results when energy of

moving water is diminished

– Maritime deposits more ephemeral than noncoastal deposits due to composition and lack of vegetative cover

– Sediment budget must be in balance to allow for deposit to persist


Figure 20-14

Coastal Landforms

• Depositional landforms• Beaches

– Beaches relatively homogeneous

– Mark transition between land and ocean

– Backshore contains berms; foreshore regularly covered and uncovered by tides

– Offshore is zone that is permanently submerged


Figure 20-15

Coastal Landforms

• Spits– At mouth of a bay, sediment

moved into deeper water

– Deposit attached to land at one end and extends to open ocean in downcurrent direction is a spit

– Spits that extend across a bay, bay barriers or baymouth bars

– Tombolos: waves converge on each side and deposit sand so the bar connects to land 18© 2011 Pearson Education, Inc.

Figure 20-16

Coastal Landforms

• Barrier islands– Long, narrow sand bar built up

in shallow offshore waters

– Oriented approximately parallel to shore

– Only rise a few meters above sea level, but some extend to great lengths

– Lagoon formation; mudflats

– Life cycle of a lagoon


Figure 20-20

Coastal Landforms

• Human modification of coastal sediment budgets– Dams act as sediment traps,

allowing less sediment to reach oceans and resulting in shrinking beaches

– Use of groins to help impede the downcurrent flow of sediment

– Jetties used to keep water moving and reduce sediment deposits in navigation channels


Figure 20-23

Coastal Landforms

• Shorelines of submergence– Most oceanic coastline shows

evidence of submergence at some time within last 15,000 years

– Ria shorelines• Submergence results in

drowning of previous river valleys, producing estuaries

• Coast with numerous estuaries is called a ria shoreline


Figure 20-25

Coastal Landforms

• Shorelines of submergence (cont.)– Fjorded coasts

• Extensive glaciation gouges out troughs by glaciers or ice sheets

• Troughs far below sea level, eventually fill with sea water

• Deep coastal indentations are called fjords

• Create extraordinarily irregular coastlines


Figure 20-26

Coastal Landforms

• Shorelines of emergence and erosion– Shoreline features raised well above

current sea level

• Wave-cut cliffs and platforms– Constant pounding of waves at base

of landforms cuts a notch at the high water level

– Broad erosional pattern called a wave-cut bench or wave-cut platform

– Most cut debris shifted just beyond wave-cut bench to wave-built terrace


Figure 20-27

Coastal Landforms

• Marine terraces– Wave-cut platforms uplifted

along tectonically rising coasts

– Several instances of marine terraces indicate several episodes of marine terrace formation

– Can be used to deduce history of the water levels in a region


Figure 20-28

Coastal Landforms

• Coral coasts– Most continents and islands fringed with coral reefs or

another coralline structure

– Critical element is a group of anthozoan animals

– Calcium carbonate skeletons from coral polyps

– Have a blossomlike appearance similar to plants

– Have strict requirements for their survival

– Fringing reefs: those built right onto a volcano

– Barrier reefs: coral that appears to float around a volcano

– Atolls


Summary

• Coastal regions make up a very small percentage of the Earth’s landscape, but have unique structure and processes

• The coasts are the interface between three of the four primary spheres of the Earth

• The most energetic coastal processes are observed by wave motions

• Tsunamis are significant dangerous waves that result from underwater earthquakes, not from winds

• Tides play a small role in the sculpting of landforms of coasts


Summary

• Numerous processes are involved that modify the lake level and sea level of bodies of water

• Many other coastal processes, including ice push, organic secretions, and stream outflow, help structure coastal landforms

• Currents are primarily responsible for the transport of coastal sediment

• Coastal deposition takes place in areas where ocean water moves more slowly, and is typically ephemeral

• The most widespread coastal landform is called a beach


Summary

• Spits and barrier islands result from deposition of sedimentary material by the longshore currents

• Lagoons result when barrier islands cut off one small region of ocean water from the remaining ocean

• Humans have modified the structure of shorelines through damming and the building of groins and jetties

• Shorelines can be divided into two categories, emergence or submergence

• Coral coasts consist of organic material and typically surround volcanoes


Ch20

Technology

Transcript of Ch20