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

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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 © 2011 Pearson Education, Inc.

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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:

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– Tidal range smaller for lakes – Water level change differences – Reefs only in oceanic water © 2011 Pearson Education, Inc.

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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

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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 © 2011 Pearson Education, Inc.

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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

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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

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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 © 2011 Pearson Education, Inc.

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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

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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

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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

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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

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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

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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 © 2011 Pearson Education, Inc.

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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

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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

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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 © 2011 Pearson Education, Inc.

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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

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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

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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

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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

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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

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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 © 2011 Pearson Education, Inc.

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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

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Coastal Landforms • Distribution of coral coasts worldwide

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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 © 2011 Pearson Education, Inc.

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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 © 2011 Pearson Education, Inc.

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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 © 2011 Pearson Education, Inc.

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