Modul 7 Ocean Waves(AMF)

7/29/2019 Modul 7 Ocean Waves(AMF)

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Module 6 Ocean Waves

Ahmad Mukhlis Firdaus

Ocean Engineering ITB

[email protected]


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Wave characteristics andterminology

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 atwave base

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


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Deep- and shallow-waterwaves

Deep-water waves

Water depth > wave

base

Shallow-water

waves

Water depth < 1/20 of

wavelength

Transitional waves

Water depth < wave

base but also > 1/20 of

wavelength

Figure 8-6a & b


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Classes of Waves

Ripples, Wind Waves, and Swell

(Effect of Wind on the air/water interface)

Internal Waves

Occur when density variations are present caused by current

shear and surface disturbances Tsunamis

Seismic disturbance of sea bottom

Rossby or Planetary Waves

Gyroscopic-Gravity Waves

Caused by wind stress change, Atmospheric Pressure Change- Coriolis

Tides

Gravitational Forces Moon -- Sun


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Ocean waves can be classified in various ways:

Disturbing Force- the forces which generate the

waves.

1. Meteorological forcing (wind, air pressure); seaand swell belong to this category.

2. Earthquakes; they generate tsunamis, which are

shallow water or long waves.

3. Tides (astronomical forcing); they are alwaysshallow water or long waves.

Another Wave Classification


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Definition: Waves are the

undulatory motion of a watersurface.

Two general wave categories:

Progressive waves

Surface waves

Internal waves

Tsunamis Standing waves

Seiches

7-1 Properties of Ocean Waves


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Wind Waves- gravity waves formed bythe transfer of wind energy into water

Wave ht- usually


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Free Waves, Forced Waves

Free waves- a wave that is formed by adisturbing force such as a storm. Waves

continue to move without additional wind

energy

Forced wave- a wave that is maintainedby its disturbing force, e.g., tides


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Restoring Force

Force necessary to restore the water surface

to flatness after a wave has formed in it

Capillary waves- wavelength < 1.73 cm

Gravity waves- wavelength > 1.73 cm


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Deep-water, Transitional, & Shallow-water waves

Wavelength- determines the size of the orbits of

water molecules within a waveWater depth- determines the shape of the orbits

Deep-water waves

Water depth > wave base

More circular orbits

Shallow-water waves

Water depth 1/20 of wavelength

Intermediate-shaped orbits


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The sea and swell

Waves originate ina sea area

Swell describeswaves that:

Have traveled out

of their area oforigination

Exhibit a uniformand symmetrical

shape

Figure 8-9


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7-3Ocean Waves : From sea waves to oceanwaves


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August 29, 2005

Hurricane Katrina Hurricane Iniki

September 1992
http://www.fws.gov/home/hurricane/katrina-nasa.jpg


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Swells: wave type found outside thefetch


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Internal Waves- at thermocline/pycnocline layer


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Internal waves form within the water

column along the pycnocline.

Because of the small density difference

between the water masses above and below

the pycnocline, wave properties are different

compared to surface waves. Internal waves display all the properties of

surface progressive waves including reflection,

refraction, interference, breaking, etc.

Any disturbance to the pycnocline can generateinternal waves, including: flow of water related

to the tides, flow of water masses past each

other, storms, or submarine landslides.

7-5


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Internal waves form within the water

column along the pycnocline.

7-5


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Internal Waves- surface view


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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 500miles per hour outward from the site of the violent movement.


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Tsunami

"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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Generation of a Tsunami


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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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Fault displacement

under water

displaces water,

water moves to fill

vacuum,

generating largewaves.

Tsunami


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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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Surface Water Waves


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Wave Spectrum (Moskowitz)


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Wave Height and Period


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Idealized Wave Spectrum


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Idealized Wave Spectrum

101

100

10-1

10-2

(m)


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Most of the waves present on the

oceans surface are wind-generatedwaves.

Size and type of wind-generated waves are

controlled by:

wind velocity

wind duration

Fetch

original state of the sea surface.

As wind velocity increases wavelength, period

and height increase, but only if wind duration

and fetch are sufficient.

7-1Properties of Ocean Waves


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A fully developed sea is a sea state wherethe waves generated by the wind are as

large as they can be under current

conditions of wind velocity and fetch.

Significant wave height is the average of

the highest 1/3 of the waves present.

Good indicator of potential for wavedamage to ships and for erosion of

shorelines.

7-1 Properties of Ocean Waves


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Progressive waves are waves

that move forward across asurface.As waves pass, wave form and wave

energy move forward, but not the

water. Water molecules move in an orbital

motion as the wave passes.

Diameter of orbit increases withincreasing wave size and decreases

with depth below the water surface.

7-2 Wave Motions


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7-2 Wave Motions

O bi Di d S k D if


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Orbit Diameter and Stokes Drift


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Wave base is the depth to which a surfacewave can move water.

If the water is deeper than wave base:

orbits are circular

no interaction between the bottom and the

wave.

If the water is shallower than wave base

orbits are elliptical

orbits become increasingly flattened

towards the bottom.

7-2 Wave Motions

Deep- and Shallow-Water


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Deep- and Shallow-WaterMotion


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There are three types of waves defined bywater depth

Deep-water wave (d>or=1/2 of L)

Intermediate-water wave (d>1/20 and


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Fetch is the area of contact

between the wind and the waterand is where wind-generated

waves begin. Seas is the term applied to the sea

state of the fetch when there is achaotic jumble of new waves.

Waves continue to grow until the sea

is fully developed or becomes limited

by fetch restriction or wind duration.

7-3 Life History of Ocean Waves

Wave interference is the momentary interaction


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ybetween waves as they pass through each other.Wave interference can be constructive ordestructive.


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Chaotic Sea exhibiting complex surface wave forms.


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Dispersion: Gradual separation of wavetypes based on their relative wavelengthsand speeds

Because celerity increases aswavelength increases:

-long waves travel faster than short waves.

-This causes dispersion outside of the fetchand regular ocean swell.


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Swells: wave

type foundoutside the fetch.

Chaotic seasinside fetch area.


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A Rogue wave occurs when there is a momentary

appearance of an unusually large wave formedby constructive interference of many smallerwaves.


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The shallower the water, the

greater the interaction between

the wave and the bottom alters

the wave properties, eventually

causing the wave to collapse.

Wave speed decreases as depth

decreases. Wavelength decreases as depth decreases.

Wave height increases as depth decreases.

Troughs become flattened and the wave

profile becomes extremely asymmetrical. Period remains unchanged. Period is a

fundamental property of a wave.

Celerity equation of shallow water wave.


Wave refraction is the bending of a wave crest into


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Wave refraction is the bending of a wave crest intoan area where it travels more slowly.


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Wave steepness is a ratio of wave

height divided by wavelength (H/L).

In shallow water, wave height increases and

wave length decreases. When H/L is larger than or equals 1/7 (H/L

1/7), the wave becomes unstable and

breaks. There are three types of breakers: spilling

breakers, plunging breakers, and surging

breakers.



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Spilling, Plunging and Surging Breakers

Spilling breaker: Top of wave

crest spills over wave. Energyreleased gradually across entiresurf zone.

Plunging breaker: Crest curlsover front of wave. Energydissipates quickly. Common atshorelines with steep slopes

Surging breaker: Never breaksas it never attains critical wavesteepness. Common alongupwardly sloping beach faces orseawalls. Energy releasedseaward.


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Storm surge is the rise in sea

level resulting from low

atmospheric pressure and the

accumulation of water driven

shoreward by storm winds. Water is deeper at the shore area,

allowing waves to progress fartherinland.

Storm surge is especially severe

when superimposed upon a springhigh tide.



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Storm surge damage


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Storm surge damage


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Standing waves or seiches

consist of a water surfaceseesawing back and forth.

Node : The line about which the

surface oscillates.

Located in centers of enclosed basins

and toward the seaward side of open

basins.

Antinodes: Points where there are themaximum displacement of the surface

as it oscillates.

Antinodes usually located at the edge

7-4 Standing Waves


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Natural Period of Standing Waves


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7 5Other Types of


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Internal waves form within the

water column along thepycnocline. Because of the small density difference

between the water masses above and

below the pycnocline, wave properties are

different compared to surface waves.

Internal waves display all the properties of

surface progressive waves including

reflection, refraction, interference, breaking,

etc.

Any disturbance to the pycnocline can

generate internal waves, including: flow of

water related to the tides, flow of water

7-5 Progressive Waves

Internal waves form within7 5

Other Types of


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the water column along the

pycnocline.


7 5Other Types ofP i W


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Tsunamis were previously

called tidal waves, but areunrelated to tides. Tsunamis consist of a series of long-period

waves characterized by very long

wavelength (up to 100 km) and high speed

(up to 760 km/hr) in the deep ocean.

Because of their large wavelength,

tsunamis are shallow-water to intermediate-

water waves as they travel across the

ocean basin.

They only become a danger when reaching

coastal areas where wave height can reach

10 m.




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Modul 7 Ocean Waves(AMF)