Chapter 16 Oceans, Shorelines and Shoreline...
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11/19/2013 1 Chapter 16 Oceans, Shorelines and Shoreline Processes Surface currents are controlled by • Solar radiation • Coriolis Force Large scale, vertical currents controlled by: • temperature differences • density differences Oceanic currents are complex and controlled by water depths, locations of continents, etc. Exploring the Oceans Scientific study of the ocean basins began in the late 1700’s. Scientists discovered that the ocean floor was not flat, but had varied topography like the land. Today, research ships investigate the sea floor by drilling, echo sounding, and seismic profiling.
Transcript of Chapter 16 Oceans, Shorelines and Shoreline...
11/19/2013
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Chapter 16Oceans, Shorelines and
Shoreline Processes
Surface currents are controlled by• Solar radiation• Coriolis Force
Large scale, vertical currents controlled by:• temperature differences• density differences
Oceanic currents are complexand controlled by water depths,locations of continents, etc.
Exploring the Oceans
Scientific study of the ocean basins began in the late 1700’s.
Scientists discovered that the ocean floor was not flat, but had varied topography like the land.
Today, research ships investigate the sea floor by drilling, echo sounding, and seismic profiling.
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Seawater
Seawater contains more than 70 elements in solution.
Sodium and chloride make up 85.6% of the chemicals.
Sediments and Sedimentary Rocks of the Ocean Floor
Deep sea sediments are fine-grained.
Sediments and Sedimentary Rocks on the Seafloor
– Deep-ocean sediments mostly consist of:
• carbonate and siliceous skeletons of microscopic organisms (calcareous and siliceous ooze)
• pelagic clays, which are derived from continents and oceanic islands; mostly wind-blown dust and volcanic ash
Fig. 16.5, p. 391
Sediments and Sedimentary Rocks on the Seafloor
• Reefs are wave resistant structures built by the skeletons of corals and other marine organisms.
Fig. 16.6b, p. 392
Sediments and Sedimentary Rocks on the Seafloor
• Although commonly called coral reefs, they often contain clams, sponges and other marine organisms.
• Most coral reefs grow in shallow tropical waters, where temperatures do not drop below 20oC.
• Because corals depend on symbiotic algae that require photosynthesis, they rarely grow at depths greater than 50 m.
Sediments and Sedimentary Rocks on the Seafloor
– Three types of reefs:
– Fringing: up to 1 km wide, attached to a landmass, such as a volcanic island
– Barrier: separated from the landmass by a lagoon.
– Atoll: circular or oval reefs surrounding a lagoon
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Shorelines and Shoreline Processes
• A shoreline is the area from low tide to the highest level on land affected by storm waves.
• A coast is a much broader area. It includes the:
• Shoreline
• Nearshore sandbars and islands
• Sand dunes
• Marshes
• Sea cliffs
Fig. 16.7, p. 393Mt. Saint Michel, France
Tides
Tides occur because of the gravitational attraction of the moon andsun, and the rotational force on water provided by the rotation ofthe Earth. The water bulges towards the moon due to the greater
gravitational force there than at the center of the Earth. The waterbulges away from the moon due to the greater rotational force awayfrom the moon than at the center of the Earth. Therefore, two high
tides and two low tides on opposite sides of the Earth!
Spring tides occur when the Sunand Moon are aligned. How oftendoes this occur?
Neap tides occur when the Sunand Moon are not aligned.
New Brunswick, Canada
Tides do not occur at the same time every day because the Moontakes 24 hours and 50 minutes to get over the same part of theEarth each day, hence the tides come at different times each day.The cycle is predictable, however, and tide tables are readily available.The height of tides varies depending on the shape of the coast, locationon the Earth, etc.
Waves are formed by the interaction of wind on the surfaceof the water. The height of waves depends on three factors:
• Wind velocity• Amount of time the wind blows• The distance over which the wind blows (the fetch).
Wave motion decreases with depth.
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Shorelines and Shoreline Processes
• Wave Terminology– Crest – highest part of the wave– Trough – lowest part of the wave– Wavelength – distance from crest to adjacent crest– Wave height – vertical from the trough to the crest– Wave base ‐ a depth corresponding to one‐half wavelength
Fig. 16.10a, p. 395
Shorelines and Shoreline Processes
• Celerity (C) is the speed of an advancing wave. – C= L/T– L = wavelength– T = wave period, time it takes for two successive wave crests to pass a given point
Fig. 16.10a, p. 395
Shorelines and Shoreline Processes
• Waves are responsible for most erosion, sediment transport and deposition in coastal areas.
Wave Generation Most geologic modification
of shorelines is accomplished by wind-generated waves, especially storm waves.
Waves can also be produced by earthquakes, volcanic eruptions, and landslides.
Shorelines and Shoreline Processes
• Waves• Wave Generation
– Wave size in wind‐generated waves is controlled by fetch.
– Fetch is the distance the wind blows over a continuous water surface.
– Waves in the ocean have a larger fetch than waves in lakes and ponds.
Shorelines and Shoreline Processes
• Waves– Shallow‐Water Waves and Breakers
– As waves enter water shallower than their wave base (1/2 their wavelength), the waves hit the seafloor.
– The wave shape changes and water is displaced in the direction of wave advance.
– Broad, deep water waves become sharp crested.
– The waves then become oversteepened and plunge forward as breakers.
Shorelines and Shoreline Processes
• Nearshore Zone – an area that extends from the upper limit of the shoreline to just beyond the area of breaking waves.
– Includes the breaker zone and the surf zone
Fig. 16.10a, p. 395
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Shorelines and Shoreline Processes
• Nearshore Zone
– Includes:
• Longshore currents
• Rip currents
Fig. 16.10a, p. 395
As waves approach theshore, they bend orrefract. Why?
California
Wave Refraction Wave refraction also occurs at headlands, concentratingwave energy at the headlands, leading to more erosion.
California
Wave Refraction
Over time, theconcentration ofwave energy atthe headlandstends to erodethe coastline…
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… until the coastlinebecomes straight.
Shorelines and Shoreline Processes
• Nearshore Zone
– Includes:• Longshore currents• Rip currents
Fig. 16.10a, p. 395
Shorelines and Shoreline Processes
• Wave Refraction
– Most waves approach a shoreline at some angle.
– The forward end of the wave hits the shallow water first and slows down, while the trailing end races ahead, thereby bringing the wave more nearly parallel to the shoreline.
Fig. 16.11, p. 397
Shorelines and Shoreline Processes
• Longshore Currents
– Waves that hit the shore at an angle generate a current along the shore in the same direction as the approaching waves.
– Longshore currents can produce significant erosion, transportation and deposition.
Fig. 16.17a, p. 401
Shorelines and Shoreline Processes
• Rip Currents ‐ Narrow, surface currents which rapidly carry water from the nearshore zone seaward through the breaker zone. Extremely dangerous to swimmers
• Created by longshorecurrents
Fig. 16.12, p. 397
Beach Profile
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CaliforniaBerm
BackshoreForeshore
The Grand Strand,Myrtle Beach, SC
Erosion and Deposition Along Shorelines
• Seasonal Changes in Beaches
– Summer beaches usually have a wide berm, gently sloping beach face, and smooth offshore profile.
– Winter beaches tend to be steeper and made up of coarser materials than summer beaches because waves are more energetic during this season. Berms may be absent.
– The sand eroded from beaches during winter is stored in offshore bars until it is driven back shoreward by more gentle waves.
Beaches, orany shoreline,
will change overtime.
Winter
Summer
A longshore current is developed by some of thewave motion remaining parallel to the coastline.
Longshore currents move sand along the shore,Depositing sand in the form or baymouth bars or spits.
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California
A baymouth bar
Oregon
A spit
The shorelinebudget worksthrough a seriesof inputs & outputs.
Theories of Barrier Island Formation
The Outer Banks, NCNorth Carolina
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Near Pensacola, Florida
An erosional coastline has features such as these:
AustraliaAustralia
Wave-cut platforms or marine terraces form when waves erode rock.
California
Wave-cut platform
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California
Marine terrace
California
A marine terrace and sea stacks.
Barrier islandsmigrate as sealevel risesor falls.
The position of Galveston Island in Texashas changed significantly over the past 3500 years.
The evolution of Mononoy Point,Massachesetts, from 1830 to 1987.
1830 –1850
1870 –1890
1910 –1930
1950 –1970
1987
Cape May, NJ
An attempt atcontrolling Nature.Engineers haveconstructed groinsto help preserve the beach.
LongshoreCurrentdirection
How successfulwas this effort?
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Jetties were constructed to keep the channel open for shipping.
What direction is the longshore current here?
New Jersey
The construction ofjetties at Ocean City,Maryland to keep achannel open in thebarrier island after astorm in the 1930s hasled to an extreme caseof barrier island retreat.
The city of Miami Beach has opted to restore their beachesby pumping millions of tons of sand near the shore to forma new beach. What will happen to this sand with time?
New Beach Old Beach
Predicted rises inglobal sea levelswill cause barrierislands to migrate towards shore, and
the retreat ofcliffs along
emergent coastlines.
California
Massachusetts
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The big picture lesson for today:
Change along coastlines is inevitable.Any changes we make are temporary at best.
