101 spring 2014 lecture 15
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Class 16Spring 2014Geology 101
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Objectives – Chapter 19
Explain the origin of the three kinds of ocean waves (Gravity, tidal, tsunami)
Discuss how waves and tides shape coastline features
Explain how beaches and dunes dissipate wave energy hand help protect the coastland.
Describe the evidence and consequences of rising sea levels
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14.03.a
How Waves Are DescribedHow Waves Are Described
How Waves Propagate Across the WaterHow Waves Propagate Across the Water
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Wave characteristics and terminology
• Wavelength (L)
• Still water level
• Orbital motion
• Crest
• Trough
• Wave height (H)
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Calm water
• Orbital size decreases with depth to zero at wave base
• Depth of wave base = ½ wavelength, measured from still water level
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Wave characteristics and terminology (continued)
If wave steepness exceeds 1/7, the wave breaks
Period (T) = the time it takes one full wave—one wavelength—to pass a fixed position
)( wavelength)(height wave
steepness WaveLH
Tf 1 )(Frequency
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Wave Dissipation Wave Dissipation
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Ocean waves can be classified in various ways:
Disturbing Force- the forces which generate the waves.
1. Gravity or Wind waves - sea and swell belong Short wave length .
2. Earthquakes; they generate tsunamis, which are shallow water or long waves.
3. Tides (astronomical forcing); they are always shallow water or long waves.
Wave Classification
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Wind or Gravity Waves Wind passing over water – 2%-3% of energy
transferred to water in form of waves Longer wind blows, bigger waves get. Time and fetch or length of run determines Wave velocity (Celerity) approaches wind
speed Wave motion almost frictionless – energy
can travel great distance without much loss
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10
Wave Forecasting Diagram (Gröen and Dorrestein, 1976
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14.03.c
Breaks where Breaks where wave base = ½ wavelengthwave base = ½ wavelength
Wind blows Wind blows across across surfacesurface Waves increase Waves increase
in sizein sizeWave Wave collapses if collapses if becomes too becomes too steep steep
How do waves form?How do waves form?
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The “sea” and swell
Waves originate in a “sea” area
Swell describes waves that: Have traveled out
of their area of origination
Exhibit a uniform and symmetrical shape
Figure 8-9
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Second basic type of wave generated largely by this Bad Boy
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Tidal forces
Tides enhanced during full Moon and new MoonSun-Moon-Earth closely aligned
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Influence of Perigee, Apogee, Perihelion and Aphelion on the Earth’s Tides
Stronger for perigee and perihelion
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Annapolis Tidal Power Generating Station
Gravity the driver but shape of shorelinesMake variable the local affects of tidal waves
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Tide wave
These are shallow water waves because wavelength is so long.
Entire water column involved Speed determined by rotation of
earth and water depth Sea basins complicate tides, huge in
some places very slight in others
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"Tsunami" - a Japanese word meaning "great wave in harbor". It is a series of ocean waves commonly caused by violent movement of the sea floor by submarine faulting, landslides, or volcanic activity. A tsunami travels at the speed of nearly 500 miles per hour outward from the site of the violent movement.
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Tsunami
Tsunami terminology Often called “tidal waves” but have nothing to do
with the tides Japanese term meaning “harbor wave” Also called “seismic sea waves”
Created by movement of the ocean floor by: Underwater fault movement Underwater avalanches Underwater volcanic eruptions
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Most tsunami originate from underwater fault movement
Figure 8-21a
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Tsunami characteristics
Affect entire water column, so carry more energy than surface waves
Can travel at speeds over 700 kilometers (435 miles) per hour
Small wave height in the open ocean, so pass beneath ships unnoticed
Build up to extreme heights in shallow coastal areas
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14.03.c
Breaks where Breaks where wave base = ½ wavelengthwave base = ½ wavelength
Wind blows Wind blows across across surfacesurface Waves increase Waves increase
in sizein sizeWave Wave collapses if collapses if becomes too becomes too steepsteep
How do waves form?How do waves form?
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BayBay
Promontory
Promontory
14.04.a-b
How Waves Interact with the How Waves Interact with the ShorelineShoreline
Swirl away loose pieces of bedrock Swirl away loose pieces of bedrock or break off new piecesor break off new pieces
Grind and break clasts, Grind and break clasts, making sand and stones making sand and stones
that promote erosionthat promote erosionWaves bend (refract) if Waves bend (refract) if approach shore approach shore at an angleat an angle
Bays protected Bays protected from largest wavesfrom largest waves
Waves break directly on Waves break directly on promontory promontory from several from several sidessidesPart that Part that
encounters encounters bottom bottom slows, slows, bending bending wavewave
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SlumpSlumpMovement of sand
Movement of sand
14.04.c
How Sand and Other Sediment get Moved on a BeachHow Sand and Other Sediment get Moved on a Beach
Sand washed back Sand washed back and forth by wavesand forth by waves
Water flows Water flows downslope carrying sedimentdownslope carrying sediment
Wind blows Wind blows sand on, sand on, off, or along off, or along beachbeach
Sediment Sediment moves moves laterally along laterally along coast if wave coast if wave at angle to at angle to beachbeach
Washes up Washes up at angle, at angle,
but washes but washes directly directly
down slopedown slope
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RiverRiver
DunesDunes
DeltaDelta
Offshore Offshore islandisland
ReefReef
Long
shore cu
rrent
Long
shore cu
rrent
Currents Currents transport transport
sediment sediment along coastalong coast
14.04.d1
Consider what determines whether a shoreline gains Consider what determines whether a shoreline gains or loses sand with timeor loses sand with time
Rivers provide influx Rivers provide influx of sedimentof sediment
Sediment largely from Sediment largely from erosion on landerosion on land
Dunes with sand Dunes with sand mostly from beach mostly from beach
or riveror river
Reefs erode, adding Reefs erode, adding material to the systemmaterial to the system
Beach erosion and Beach erosion and slumping rocks add slumping rocks add sediment to shoresediment to shore
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The Beach
In some ways, the beach is nothing more than a great energy dissipation system to protect the land from the stormy sea.
Others – rock cliffs, mangrove, coral reefs, and rarely - mud Beaches –very rare SW Louisiana
Sand in beaches always moving –onshore-offshore in response to storm energy
Sand moves alongshore in response to longshore drift
Waves break when they reach shallow water and start interacting with the bottom
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The importance of the dunes The dunes’ particularly the foredunes are
directly interacting with the beach and near shore sand bars.
Wind (Aeolian) and waves move sand around but where beaches exist, they work to keep the sand in place over time.
Importance of vegetation in anchoring dune – grasses and forbs can not be understated
The importance of dunes to a stable beach environment also cannot be under stated
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The Dynamic Beach, dunes and offshore bars
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Dune vegetation – South Padre Island
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Storm Scarped Dunes on Padre Island August 2005
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Sand moved from dunes during tropic storm to the offshore sand bars
Normal waves slow moves back onshore where the wind willBlow it back into the dunes where it is trapped by vegetation
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Protected “dunes” at Miami
Copyright © Kathleen Walling Fry
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Disappearing beaches Sand moves from a source (a river mouth or
eroding seaside cliff to a sink, either and inlet or an offshore canyon
Waves and physics keeps most of the sand in the surf zone
Dunes and dune sediment keep sand from blowing inland
If sediment supply gets reduced (dam on river, new inlet, protection of eroding cliff) sand supply washes away, erode width of beach
Groins capture some sand but cause greater erosion downdrift
Beach nourishment projects the only way out for some beaches – Miami the most famous
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In the northern Gulf of Mexico, the currents move Counter clockwise, Bringing silt and mud from the Mississippi to Galveston and leaving Alabama beaches sandy
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http://sofia.usgs.gov/virtual_tour/ecosystems/index.html
Red Mangrove with arching and drop roots
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Black Mangrove with
http://www.finexpeditions.com/BirdsMangrove.htm
Pneumatophores that allow it to breathe
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Miami Beach
Before beach replenishment
After beach replenishment
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Barrier Islands
Protection for areas behind them from waves.
Extensive on the east coast of North America
Sea-level rise is forcing barrier islands landward. Many of the low-lying ones in Louisiana are disappearing.
Occasional washover in storms normal. Galveston, Padre, Mustang, Matagorda
Cape Canaveral, Miami Beach, Pensacola Beach, Sea Islands of Georgia
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http://www.lib.utexas.edu/maps/national_atlas_1970/ca000060.jpg
New Orleans
Mobile
Houston
Muddy waters
Louisiana Gulf Coast
Mud Coast
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Louisiana Gulf Coast Near Grand Isle
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Miami
Key West
West Palm Beach
Tampa
Jacksonville
Daytona Beach
Tallahassee
Orlando/Kissimmee
Cape Canaveral
Mobile Pensacola
Barrier Islands
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Crushed coral beach in Key West
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Panama City – no dunes left
http://www.threebestbeaches.com/florida/
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Spring Break – Daytona Beach
http://www.threebestbeaches.com/florida/uploaded_images/tbb-daytona04-780348.jpg
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Objectives – Chapter 19
Explain the origin of the three kinds of ocean waves (Gravity, tidal, tsunami)
Discuss how waves and tides shape coastline features
Explain how beaches and dunes dissipate wave energy hand help protect the coastland.
Describe the evidence and consequences of rising sea levels