Class The Oceans WAVES ALONG COASTS Wind-generated wavesWind-generated waves –Shallow water waves (more) –Changes as waves move onto shore –Longshore transport TsunamisTsunamis Seiches (omitted 2011)Seiches (omitted 2011)

1. Changes in wave characteristics -- Waves "feel bottom" in shallow water (D < L/2).Waves "feel bottom" in shallow water (D < L/2). "Deep-Water” waves --> "Shallow Water" waves"Deep-Water” waves --> "Shallow Water" waves –Shallower water --> Slower, closer spacing T remains constantT remains constant H increases (wave energy compressed into smaller area)H increases (wave energy compressed into smaller area) Changes in Wind-Generated Waves As They Move Onshore

Wave Theory: The shallow water wave model says: When D < L/20, S & L controlled only by depthWhen D < L/20, S & L controlled only by depth S = √[gD] = 3.13 √ DS = √[gD] = 3.13 √ D Top of wave can crash forward as the bottom of the wave slows downTop of wave can crash forward as the bottom of the wave slows down If H/L > 1/7 (D  H) --> "breakers," "surf"If H/L > 1/7 (D  H) --> "breakers," "surf" Changes in Waves As They Move Onshore

Changes in wave direction Changes in Wind-Generated Waves As They Move Onshore

(2) Changes in wave direction -- refraction If waves approach at an oblique angle... –Bending of wave "fronts" (crests) –Bending of wave movement direction

a. Wave loses speed as water gets shallower b. The part of wave that is in shallow water moves more slowly than the part in deep water Result: Wave movement direction turns toward shallow waterWave movement direction turns toward shallow water Waves crests become closer to parallel with shoreWaves crests become closer to parallel with shore Why does refraction occur?

Beach

Refraction + irregular shapes of coastlines causes:Refraction + irregular shapes of coastlines causes: a) Focussing of wave energy onto headlands or shallower parts of beaches

Refraction + irregular shapes of coastlines causes:Refraction + irregular shapes of coastlines causes: b) Quieter conditions on recessed beaches c) Erosion in headlands and deposition of sediment in recessed areas

Tsunamis video sea wall tsunami-damage-in-japan.html?ref=science Maps and block diagrams r.html?ref=science weight of water model

Tsunami: Very long wavelength waves caused by earthquakes, etc.

Example: Earthquake… Sea floor drops 6m T = min (600-1,200 s)T = min (600-1,200 s) L = km (100, ,000 m)L = km (100, ,000 m) H = m when water is deep -- not noticeableH = m when water is deep -- not noticeable Tsunami: Very long wavelength waves caused by earthquakes, etc.

L = km (100, ,000 m) How deep is the ocean- average??? The “Shallow-water wave” model applies The “Shallow-water wave” model applies The waves are REFRACTEDThe waves are REFRACTED Tsunami: Very long wavelength

2011 Honshu Tsunami Simulation- NOAA

Example: Earthquake… Sea floor drops 6m T = min (600-1,200 s)T = min (600-1,200 s) L = km (100, ,000 m)L = km (100, ,000 m) H = m -- not noticeable in open ocean!H = m -- not noticeable in open ocean! Behave like “Shallow-water waves”: L >> depth of oceanBehave like “Shallow-water waves”: L >> depth of ocean Thus, speed controlled by depth only (wave theory)Thus, speed controlled by depth only (wave theory) –S = √[gD] = 200 m/s (400 mph) Tsunami: Very long wavelength waves caused by earthquakes, etc.

Tsunami changes as it moves into shallower water

(Japanese Coast Guard vessel rides tsunami)

H = 1-2 m in open ocean, long L But as they move onshore: 1.S decreases 2.L decreases (compresses waves) 2. H increases to perhaps 20+ m, but wavelength is long, so these waves act like floods, not breakers 3. Energy focused by bottom topography and man-made barriers TSUNAMIS in coastal areas

Damage in Crescent City, CA, March 2011 Tsunami from Japan

Prediction of tsunamis Earthquake eventEarthquake event Large vertical displacement requiredLarge vertical displacement required Many large quakes --> no tsunamiMany large quakes --> no tsunami Can predict wave migrationCan predict wave migration Plenty of time to warn peoplePlenty of time to warn people

Prediction of tsunamis

“Standing” waves- they don’t move“Standing” waves- they don’t move –“Sloshing back and forth”- bathtub Occur in harbors, baysOccur in harbors, bays Natural resonance period (T) of a basinNatural resonance period (T) of a basin – Depends on depth and dimensions If entering wave has same T, it ”resonates" in basinIf entering wave has same T, it ”resonates" in basin –Water moves back and forth, up and down Winds, tides, and waves can cause seichesWinds, tides, and waves can cause seiches SEICHES (Omitted 2011)