waves - soest · waves wave speed the speed the disturbance, and wave crests, travel at. wave speed...
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Waves
Wave speedThe speed the disturbance, and wave crests,
travel at.
Wave speed = WavelengthPeriod
[m]
[s][m/s]
Wavelength0.017m 1m 10m 1000m
0.1s 1s 10s 30s 5min 12h 24h
10 1 10-1 10-2 10-3 10-4 10-5Wave frequency (s-1)
Period
Ener
gy
Wind
Tsunami
Seiche and storm surgeDisturbance: Sunand
moon
Surfacetension
Windwaves
Long periodwaves Tides
Gravity waves
Restoring:
Wave speed: Deep water wave✤ When ocean depth is greater than half the wavelength.
Direction of wave
Wave speedThe speed the wave crests travel at.
Deep water wave✤ Speed dependent on wavelength (or period). ✤ Speed (m/s) = (seconds).
✤ Speed (m/s) = 1.56 * wave period (seconds).✤ Longer waves travel faster.
gravity
2⇡Period
Wave speed: Deep water wave
✤ Speed = 1.56 * wave period✤ Longer waves travel faster
The speed the disturbance, and wave crests, travel at.
Broo
ks/C
ole
Storms produce waves of many different periods and heights. Wind wave periods can be up to 30 seconds.
Wind wave dispersionWavelength increases as period increases.
Longest waves travel fastest away from storm.
SwellLonger waves travel faster
(C=1.56*Period). They also decay slower.
Broo
ks/C
ole
Wave speed: Shallow water wave✤ When ocean depth is shallow compared to wavelength
(less than 1/20th of wavelength)
Direction of wave
Wave speed: Shallow water wave✤ When ocean depth is shallow compared to wavelength
(less than 1/20th of wavelength)✤ Speed dependent on ocean depth. ✤ Speed (m/s) = ✤ Speed (m/s) = 3.1 * square root(depth in meters)✤ Waves travel faster in deeper water.
pgravity ⇥Water depth
Wave refractionresults in waves being more parallel to shore.
Wave refraction
Wave refractionA shallow water wave’s speed is dependent
on ocean depth.
Wave refraction
If part of a wave is in shallower water then it will travel slower.
A shallow water wave’s speed is dependent on ocean depth.
Wave refraction
If part of a wave is in shallower water then it will travel slower.
A shallow water wave’s speed is dependent on ocean depth.
Wave reflection
Wave reflection
38cd_wavemotion.swf
Wave diffractionWhen waves pass through a small gap, they
reform and radiate outwards.
Wave groups
Deep water wave groups travel at half the
speed of individual waves.
New waves appear to form behind the wave
group.
Wave groups
matlab movie
Wind waves approaching shoreWaves ‘touch’ the bottom.
Wave period remains constant, wavelength decreases.
Wave speed =Wavelength
Period
and eventually becomeShallow water waves, so
Speed (m/s) = 3.1 * square root(depth in meters)
(Wavelength = Wave speed * Period)
Wind waves approaching shoreWaves ‘touch’ the bottom.
Wave period remains constant, wavelength decreases.
Friction slows the bottom of the wave.
Waves approaching shore✤ Bottom of wave slows even more as the depth
shallows, wave crest moves ahead of the base of the wave.
✤ Wave breaks when the ratio of wave height to water depth is ~3:4.
✤ How waves break depends on the beach slope.
Surf predictionNorth shore swells come from Northern
Hemisphere winter storms.
South shore swells often come from Southern Ocean storms.
Surf predictionNetwork of ocean buoys measure wave
height, period, and direction.
Then account for height attenuation and local shoaling effects.
hawaiibeachsafety.org
Tsunamis
Hilo, April 1946 after Alaskan earthquake (NOAA)
Tsunamis✤Tsunamis can be caused by Earthquakes, Landslide,
volcanoes, and asteroid impacts. All of these suddenly displace a large amount of water.
✤Have periods of about 20 minutes, and wavelengths of about 200 km.
✤Average ocean depth is 4 km, so always a shallow water water (depth < wavelength/20).
Earthquake generation
The March 11 2011 earthquake had a displacement of about 20m over an area ~500 km by ~200 km.
Open ocean detectionIn the deep ocean Tsunami waves have
small amplitudes, 10 to 50 cm.
Combined with their long wavelength, they are hard to detect, enter DART.
Why are they dangerous?Because of their long wavelength, the volume of the
displacement associated with a tsunami wave is huge.
When it shoals that volume piles up.
japan.mov
How to escape?✤ Get to higher ground.✤ Or go above the 3rd floor in a concrete building.✤ If possible don’t get into your car, traffic jams are
inevitable.✤ If the water recedes, don’t follow it. Run the other way.✤ Know the evacuation zones are your home and work.
Waikiki Evacuation map
International Market Place
Royal Hawaiian Shopping Center
DATE ST
ALA WAI BLVD
KUHIO AVE
PAKI AVE
S KING ST
KAPIOLANI BLVD
WINAM AVE
YOUNG ST
FERN ST
KAPAHULU AVE
MCCULLY ST
LIME ST
KALIA RD
LEAHI AVE
CITRON ST
KALAKAUA AVE
KANAINA AVE
MONSARRAT AVE
MAKINI ST
KONA ST
WAIOLA ST
HINANO ST
HAYD
EN ST
S BERETANIA ST
GEORG
E ST
DUVA
L ST
LEW
ERS
ST
KAUNAOA ST
PUNAHOU ST
ISENBERG ST
FRAN
CIS ST
ESTH
ER ST
OHUA
AVE
HAUSTEN ST
UNIVERSITY AVE
NOELA ST
CAST
LE ST
KUILEI ST
ALGAROBA ST
ALA MOANA
KAMO
KU S
T
BROKA
W ST
BINGHAM ST
HOBR
ON L
N
KAHEKA ST
9TH AVE
COOLIDGE ST
PALIULI ST
CATH
ERIN
E ST
8TH AVE
HERB
ERT S
T
ENA
RD
11TH AVE
7TH AVE
MOKIHANA ST
10TH AVE
ATKI
NSON
DR
KOA AVE
HOOLU
LU ST
PAOA
KALA
NI A
VE
UPPER ST
MARTH
A ST
PAOA
PL
KAAHA ST
OLOK
ELE
AVE
MOOHEAU
AVE
SARATOGA RD
KANUNU ST
LAAU ST
COYNE ST
LUKE
PANE
AVE
ARTESIAN ST
ALA MOANA PARK DR
BEAC
H W
ALK
KAIO
LU S
T
MALUHIA RD
HOLEI ST
EKEL
A AV
E
PAU
ST
NIU
ST
GAIL ST
DUKE
S LN
POHA LN
NANEA ST
OLOH
ANA
ST
LAUI
KI S
T
KALA
IMOK
U ST KAINA S
T
MALANAI ST
WELA ST
NAHU
A ST
CLEGHORN ST
JAMES
ST
LAUN
IU S
T
WAI
NAN
I WAY
WAL
INA
ST
MAHI
AI S
T
PONI ST
ALOHA DR
KUAM
OO S
T
KAPAAKEA LN
MAKA
LEKA
AVE
KANE
KAPO
LEI S
T
PALA
NI A
VE
MAHUKONA ST
LEMON RD
LAKIM
AU ST
KAHU
NA L
N
KEON
IANA
ST
KAIPUU ST
NOHO
NANI
ST
KAIOO DR
HOLOMOANA ST
NAMA
HANA
ST
OWENA ST
WAUKE ST
CARTWRIGHT RD
HOLL
INGER ST
NIKOLO ST
KEAL
OHIL
ANI A
VE
WILLIAM
S ST
DON HO LN
TUSITALA ST
KALAUOKALANI WAY
AINA
KEA
WAY
ALEXANDER ST VILLA LN
EDNA ST
ELSIE LN
KAHAK
AI DR
Pawaa
McCully
Kapiolani Medical Center
Honolulu Zoo
Iolani School
Kapiolani Park
Ala Wai Clubhouse
Ala Wai Golf Course
Ala Wai Yacht Harbor
Ala Moana State Rec Area
Kaimuki High
'Iolani School
Washington Middle
Waikiki Elementary
Ala Wai Elementary
Jefferson Elementary
Jefferson Orthopedic Unit
Kapiolani Beach
Park
Kuhio Beach
Park
Kapiolani Park
Beach Center
Duke Paoa
Kahanamoku Beach Park
E H1 FWY
Palama Market
Mccully Shopping Ctr
Waikiki Shopping Plaza
Royal Hawaiian Market Place
Waikiki (Hq.)
Lunalilo Elementary
Hawaii Center for the Deaf & Blind
Central Union Preschool and Kindergarden
LegendTsunami Evacuation Line
Tsunami Evacuation Zone
Fire Station
Ambulance
Hospital
Police
Public and Private School
Shopping Mall
Major Street
Street
Beach Park
TextText
Notes:
Remain at least 100 feet away from inland waterways and marinas
connected to the ocean due to wave surges and possible flooding.
If possible, remove or deploy vessels to deep water (at least 200
fathoms).
Structural steel or reinforced concrete buildings of six or more
stories provide increased protection on or above the third floor.
The evacuation zone is a guideline and should be considered the
minimum safe evacuation distance.
These maps do not consider the destructive effects of a locally
generated tsunami. If you feel shaking, move inland immediately,
well away from the evacuation zone.
0 400 800200
Scale in Feet
Map 1: Waikiki
Note: Data represented on this map is not intended to replace site survey. Copyright City & County of Honolulu
All Rights Reserved 2010Date Prepared: Aug. 25, 2010
Prepared by: DPP (HOLIS)
Data Source: City and County of Honolulu GIS
Some key ideas✤ Deep water wave, D > L/2, speed: c=1.56*T; Longer
waves travel faster than shorter waves; Wave groups travel at half the speed of individual waves.
✤ Shallow water wave, D < L/20, speed: c=3.1*sqrt(H); They feel the bottom; They refract.
✤ Period remains constant when wave approach the shore.✤ Swim across shore to get out of a rip current.✤ Tsunamis are shallow water waves. They have periods of
around 20 minutes. Get out of their path!