PHYSICAL OCEANOGRAPHY

Waves

GEOL 1033

(Lesson 30)

ANATOMY OF A PROGRESSIVE WAVE

V depends on wave properties

V = L/P

V depends on wave properties and water depth, so it is mathematically complex.

Maximum V depends on water depth

THREE TYPES OF PROGRESSIVE WAVES

STAGES OF WAVES

• 1) Sea– Waves in area effected by wind

– Tend to be very irregular waves

– Composed of many waves superimposed

• 2) Swell– Far from origin (=storm area)

– Larger wavelength & period waves

– Travel faster than smaller waves

– Travel great distances (1000`s km)

– Are deep-water waves

• 3) Surf– Nearshore where depth decreases to L/20

– Swells that shoal and break

FORMATION OF SURF WAVES

WAVE REFRACTION• Note the bend of almost 90 degrees as the larger swell

waves refract onshore (rocky Maili Point, Oahu, Hawaii).

clouds

BEACH DRIFT & LONGSHORE CURRENT• Refracting wave comes in at an angle and breaks

– Swash moves grains & water obliquely up beach face– Backwash, being controlled by gravity, moves grains & water

“straight” down slope of beach face– result = “zig-zag” movement of grains & water on beach face

– Similar action

– in surf zone– produces the– longshore– current

Beach DriftLongshoreCurrent

BEACH DRIFTWaves approaching obliquely along a gravelly shoreline set up a

longshore current and beach drift. Sediment moves from left to right (toward the viewer) (East coast, Lake Michigan).

A TYPICAL BEACH PROFILE

beach driftlongshore current

LONGSHORE CURRENT• Refracting, breaking waves approach shore obliquely

– Lifting action under breakers as they pass over a spot• Lifts• Displaces obliquely (laterally)• Drops more or less vertically

– Moves grains and water along shore in a “zig-zag” manner

liftup

over

down

The oblique approach of wave trains to a shoreline results in wave refraction and leads to the development of longshore currents, beach drift and other phenomena. Sediment is being transported toward the bottom left (Ringarooma Bay, Tasmania, Australia).

beach drift longshore current

USE & EFFECTS OF GROINS & SEAWALLS

EFFECTS OF GROINS & JETTIES

groin jetty jettygroin

groinjetties

erosion erosionerosiondeposition

depositiondeposition

depositiondeposition

erosion

erosion

Rip-Rap to retard erosion affecting cemetary above

Second attempt at Rip-Rap to retard erosion affecting cemetary above

Groin being buries by updrift sands

A closer look at groin

EFFECTS OF BREAKWATERS

IRREGULAR, ROCKY COASTLINES• Very irregular, rocky, cliffed shorelines do not have well-

developed beach drift and a longshore current.

• Rocky coastline on an active continental margin. These folded rocks form high cliffs with only small pocket beaches. The extensive offshore rocks and irregular underwater topography preclude or greatly retard formation of longshore currents (Point Reyes, California).

ORIGIN OF TSUNAMIS

• Shallow-focus earthquakes– Primary cause– Especially associated with subduction zones of the margins of

the Pacific Ocean Basin

• Volcanic Eruptions• Landslides

– Into water from land– Submarine landslides (=slumps)

• Man-made explosions– Nuclear bombs– WWI (1917) and WWII explosions in Halifax caused small

tsunamis in the Halifax Harbour (about 1 m high)

msl

shelfslope

SLUMP

PROCESSES OF TSUNAMI FORMATION

WHAT HAPPENS AT THE COASTLINE?• At the coastline:

– First, there is a great withdrawal of water– Water concentrates a short distance offshore– Then, water returns to shore as a high wave– H is commonly 10’s of feet high (possibly up to 200’)– Usually get 3 to 5 oscillations before reduction of H

Before

Withdrawal

Surge

TSUNAMI WAVE CHARACTERISTICS

• Tsunamis are “shallow-water” type waves!– L = up to 100’s km, (~100 to 200 km common)– Ocean averages 3.8 km deep.

• H = relatively low out at sea, (typically 1 to 2 m)• T = usually about 10 to 30 minutes

• V = up to 790 km/h, (~400 miles/h)

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