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!