Ch 9: Waves1. Features of Waves2. Deep-water,

shallow water and transitional waves

3. Breaking Waves4. Wind Waves5. Tsunamis

Waves are created by a ‘disturbance’. * wind (wind waves, L= 60-150 m), where most of ocean’s wave energy is located.* earthquakes (seismic waves, L = 200 km)* sun/moon (tidal waves, planetary scale).

Restoring force is *gravity (gravity waves).*surface tension (capillary waves)

Cf. Fig. 9-2

1. A wave starts out as a ‘ripple’= capillary wave, named after the restoring force(surface tension, or capillary force) with very short L < 1.74cm.

2. As wind continues to blow, larger waves will build up that are restored by gravity =gravity wave.

3. H, L, S begin to build up until H/L > 1/7, the wave breaks, ‘white caps’ form.

Formation of a wind wave

9.8

Wind waves

• Factors that increase wave height:– Increasing wind speed– Increasing duration (time) of wind– Increasing fetch (distance)

• A fully developed sea is the maximum height of waves produced by conditions of wind speed, duration, and fetch (white caps form)

See Table 9-2 for fetch and duration required to produce a fully developed sea

9-10. Global wave height acquired with radar altimeter (TOPEX/Poseidon) Highest waves occur in the Southern Ocean (up to 6 m).Lowest waves were found in the tropics and subtropics.

The “sea” and swell• Area where wind

generated waves originate is called the “sea”

• Swell describes waves that:– Have traveled out of their

area of origination– Exhibit a uniform and

symmetrical shape– Are sustained not by wind

but by energy obtained in the sea

– Waves with longer L leave sea area first, followed by slower ones

= wave dispersion (= sorting of waves by wave length)

Figure 9-9

• Merchant ship laboring in heavy seas as huge wave looms astern. Huge waves are common near the 100-fathom curve on the Bay of Biscay. Published in Fall 1993 issue of Mariner's Weather Log

• http://www.photolib.noaa.gov/historic/nws/wea00800.htm

Wave trains

Swells often propagate as wave trains (groups of waves)

Leading wave keeps disappearing,and is replaced by additional wave in the back.

Wave train speed is ½ the speed of a single wave.

Interference patterns

• Constructive– Increases

wave height• Destructive

– Decreases wave height

• Mixed– Variable

patternFig. 9-14

In phase-constructive

Out of phase,destructive

Mixed,most common

Interference patterns

• Constructive– Increases

wave height• Destructive

– Decreases wave height

• Mixed– Variable

patternFigure 9-15

Fig. 9-16. Example of a rogue wave that is created where storm-drivenwaves meet strong ocean currents at a current boundary.

Rogue waves

Rogue waves: Largest wind-generated waves authentically recorded

• In 1935, the vessel USS Ramapo experienced a large wave while crossing the Pacific Ocean

• Wave height was measured at 34 meters (112 feet)

Rogue wave

Ch 9: Waves1. Features of Waves2. Deep-water,

shallow water and transitional waves

3. Breaking Waves4. Wind Waves5. Tsunamis

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 volcanic eruptions– Land slides

A tsunami is created by an abrupt vertical movement along a fault in the earth’s crust which pushes up the ocean water column above thefault. Massive long, low waves are created (L>200km, H=0.5m) whichtravel very fast (up to 700 km/h). Tsunamis cause strong flood surgesup to 40 m above normal sea level.

Fig. 9-23

Tsunami characteristics

• Affect entire water column, so carry more energy than surface waves (shallow-water waves everywhere

• 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

Coastal effects of tsunami

• If trough arrives first, appear as a strong withdrawal of water (similar to an extreme and suddenly-occurring low tide)

• If crest arrives first, appear as a strong surge of water that can raise sea level many meters and flood inland areas

• Tsunami often occur as a series of surges and withdrawals

• Most tsunami are created near the margins of the Pacific Ocean along the Pacific “Ring of Fire”

Tsunami from Chile earthquake 1960 Magnitude 9.5

4) Earthquake destructionMagnitude 9.5 1960 Chile earthquake,

Tsunami damage in Hawaii :From 1960 Chile earthquake,15 hours later

Tsunami warning system

• Pacific Tsunami warning system center (PTWC) stationed in Hawaii

• seismic listening stations track underwater earthquakes that could produce tsunami

• once a large earthquake occurs, the tsunami must be verified at a nearby tide-measuring station

• If verified, a tsunami warning is issued• Successful in preventing loss of life (if people

heed warnings)• But damage to property has been increasing

Indian Ocean Earthquake and Tsunami (Dec 26, 2004)

The earthquake took place in a region with previous earthquake activity as shown in the map prepared by the TheGlobal Seismic Hazard Assessment Program (GSHAP) in the framework of the United Nations International Decade for Natural Disaster Reduction (UN/IDNDR). The present earthquake took place in a seismically active region at the plate boundary separating the Indian-Australian and East-Asian Plates. There are 12 plates in the world and earthquakes occur when these collide. A 13th plate was created by the breakup of the Indo-Australian plate was documented in 1995. This breakup has set up compression zone near Northern Sumatra.

Indian Ocean Earthquake and Tsunami (Dec 26, 2004)

http://iri.columbia.edu/~lareef/tsunami/#Tsunami_Animation:_National_Institute_of

Indian Ocean Earthquake and Tsunami (Dec 26, 2004)

• The 9.0 Earthquake at 6.58 hours at the epicenter (and in Sri Lanka) led to a sequence of 15 quakes across the Andaman region.

• The initial eruption happened near the location of the meeting point of the Australian, Indian and Burmese plates. Scientists have shown that this is a region of compression as the Australian plate is rotating counterclockwise into the Indian plate.

• Tsunamis are rarer in the Indian Ocean as the seismic activity is less than in the Pacific. There have been 7 records of Tsunamis set off by Earthquakes near Indonesia, Pakistan and one at Bay of Bengal in the last century.

• While earthquakes could not be predicted in advance, once the earthquake was detected it was possible to give about 3 hours ofnotice of a potential Tsunami. Such a system of warnings is in place across the Pacific Ocean but not in the Indian Ocean. In addition, coastal dwellers are educated in the Pacific littoral to get to high ground quickly following tremors and waves.

http://iri.columbia.edu/~lareef/tsunami/#Tsunami_Animation:_National_Institute_of

1. Landforms and terminology in coastal regions

2. Interaction of waves with shores

3. Longshore current and longshore drift

4. Beach modification by protective structures

5. Wave refraction along an irregular shoreline and features of erosional shores

6. Features of depositional shores

7. Changes in sea-level

Ch. 9 and 11: The beach and shoreline processes

Landforms and terminology in coastal regions

Figure 11-1

Ch. 9 and 11: The beach and shoreline processesInteraction of waves with shore

Summertime beach Wintertime beach

Fig. 11-12

Perpendicular movement of sand on the beach

Movement perpendicular (↕) to shorelineCaused by breaking wavesLight wave activity moves sand up the beach face toward the bermHeavy wave activity moves sand down the beach face to the longshore barsProduces seasonal changes in the beach

Summertime and wintertime beach conditions

Summertime beach Wintertime beach

Wave refraction along a straight shoreline

Figure 9-19

Movement parallel to shoreline

Most waves approach shore at an angleThe part of the wave in shallow water slows downThe part of the wave in deep water continues at its original speedCauses wave crests to refract (bend)These waves cause a longshore currentthat moves water (and particles) along a zigzag line downstream.

Longshore current and longshore drift (Ch. 11)

• Longshore current= zigzag movement of water in the surf zone

• Longshore drift = movement of sediment caused by longshore current

Swash-Backwash

11-3

Longshore currents and rip currents

Movement of sand on the beach

• Movement parallel (↔) to shoreline– Caused by wave refraction (bending)– Each wave transports sand either upcoast or

downcoast– Huge volumes of sand are moved within the

surf zone – The beach resembles a “river of sand”

Modification of beaches by hard stabilization: Jetties and Groins

• Jetties are always in pairs

• Groins can be singular or many (groin field)

• Both trap sand upstream and cause erosion downstream

Figure 11-22

Types of hard stabilization

• Hard stabilization perpendicular to the coast within the surf zone:– Jetties—protect harbor entrances– Groins—designed to trap sand

• Hard stabilization parallel to the coast:– Breakwaters—built beyond the surf zone– Seawalls—built to armor the coast

Example for hard stabilization interference of longshore drift: Breakwater at Santa Barbara

Harbor, California

• Breakwater causes deposition in front and in harbor and erosion downstream

• Sand must be dredged regularly to keep Santa Barbara Harbor free

Figure 11.23

Seawalls and beaches

• Seawalls are built to reduce erosion on beaches

• Seawalls can destroy recreational beaches

• Seawalls are costly and eventually fail Figure 12-14

Seawall damage in Leucadia, California

Figure 10-25

Beach compartments in southern California

• Beach compartments include:– Rivers– Beaches– Submarine

canyons

Fig. 11-12

Wave refraction along an irregular shoreline and features of erosional

shores• Wave energy

is concentrated at headlands and dispersed in bays

• Causes erosion of headlands and creation of erosionalfeatures

Figure 10-14b

Orthogonal lines run perpendicular to wave crests and indicate equal wave energy

Features of erosional shores

• Headland• Wave-cut

cliff• Sea cave• Sea arch• Sea stack

Uplifted, ancient wave-cut benches exposed in southern California

Sea stack and sea arch, Oregon Features of depositional shores

• Spit• Bay barrier• Tombolo• Barrier island• Delta

Figure 11-7

Barrier coast along coast of Martha’sVineyard

Tombolo in the Gulf of California

Barrier Islands along North Carolinas Outer Banks

Barrier Islands along south Texas coast

Fig. 11-9

Barrier island, New Jersey Formation of barrier islands

Figure 11-10• Sea level has been rising since the last Ice Age

• Caused barrier islands to roll toward shore like a tractor’s tread

Figure 11B

Relocation of the Cape Hatteras lighthouse, North Carolina

Evidence of emerging and submerging shorelines

• Emergent features:– Marine terraces– Stranded beach

deposits• Submergent

features:– Drowned beaches– Submerged dune

topography– Drowned river

valleys (Chesapeake Bay!)