Class 16Spring 2014Geology 101

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

14.03.a

How Waves Are DescribedHow Waves Are Described

How Waves Propagate Across the WaterHow Waves Propagate Across the Water

Wave characteristics and terminology

• Wavelength (L)

• Still water level

• Orbital motion

• Crest

• Trough

• Wave height (H)

Calm water

• Orbital size decreases with depth to zero at wave base

• Depth of wave base = ½ wavelength, measured from still water level

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

Wave Dissipation Wave Dissipation

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

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

10

Wave Forecasting Diagram (Gröen and Dorrestein, 1976

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?

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

Second basic type of wave generated largely by this Bad Boy

Tidal forces

Tides enhanced during full Moon and new MoonSun-Moon-Earth closely aligned

                                                                                                                       

         

Influence of Perigee, Apogee, Perihelion and Aphelion on the Earth’s Tides

Stronger for perigee and perihelion

Annapolis Tidal Power Generating Station

Gravity the driver but shape of shorelinesMake variable the local affects of tidal waves

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

"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.

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

Most tsunami originate from underwater fault movement

Figure 8-21a

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

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?

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

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

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

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

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

The Dynamic Beach, dunes and offshore bars

Dune vegetation – South Padre Island

Storm Scarped Dunes on Padre Island August 2005

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

Protected “dunes” at Miami

Copyright © Kathleen Walling Fry

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

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

http://sofia.usgs.gov/virtual_tour/ecosystems/index.html

Red Mangrove with arching and drop roots

Black Mangrove with

http://www.finexpeditions.com/BirdsMangrove.htm

Pneumatophores that allow it to breathe

Miami Beach

Before beach replenishment

After beach replenishment

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

http://www.lib.utexas.edu/maps/national_atlas_1970/ca000060.jpg

New Orleans

Mobile

Houston

Muddy waters

Louisiana Gulf Coast

Mud Coast

Louisiana Gulf Coast Near Grand Isle

Miami

Key West

West Palm Beach

Tampa

Jacksonville

Daytona Beach

Tallahassee

Orlando/Kissimmee

Cape Canaveral

Mobile Pensacola

Barrier Islands

http://images.google.com/imgres?imgurl=http://lh3.google.com/_ylmDBLJtgmc/RwLv4c3zSLI/AAAAAAAAARI/Wk9pMF4Y0Kw/s800/IMG_0346.JPG&imgrefurl=http://picasaweb.google.com/lh/photo/8LRle9SeVS11vZ5LEjR_5g&h=600&w=800&sz=110&hl=en&start=5&tbnid=VnorDMHMr-ASSM:&tbnh=107&tbnw=143&prev=/images%3Fq%3Dbeach%2Bkey%2Bwest%26gbv%3D2%26ndsp%3D20%26hl%3Den%26sa%3DN

Crushed coral beach in Key West

Panama City – no dunes left

http://www.threebestbeaches.com/florida/

Spring Break – Daytona Beach

http://www.threebestbeaches.com/florida/uploaded_images/tbb-daytona04-780348.jpg

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