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Waves and Coastal Interactions
Stan Piotrowski
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What is a wave?
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Types of Waves
• Type of wave produced is dependent on the type of swell, wind direction, slope of sea bed, and bathymetric features (canyons, ridges)
• Groundswell- far off coast• Wind swell- formed from local winds (choppy)• Offshore/Onshore winds• Deep water waves- h/λ > 1/4• Shallow water waves- 1/20 > h/λ
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Winds
• Generate waves – wind speed, duration, and fetch
• Onshore – blow over wave crests, causing them to break in a rough froth
• Offshore – push the face of the wave up, generally steepen waves
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Why do waves break?
• Shoaling- amplitude increases, wavelength remains constant
• Deep water waves- break when wave steepness exceeds 0.17– H > 0.17λ
• Shallow water waves- individual waves break when their wave height H is larger than 0.8 times the water depth (h)– H > 0.8h
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Types of Breaking Waves
• Spilling• Plunging• Surging• Iribarren Number (surf familiarity parameter)• Breaker Depth Index
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Spilling Breakers
• ξ < 0.5 • Gently sloping coasts where waves break slowly and over a
long distance • Relatively gentle waves
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Plunging Breakers
• 0.5 < ξ < 3.3• Steeper coastlines• Form on reefs or sandbars• Wave face becomes vertical, then drops onto
the trough, releasing most of its energy at once
• Tube/Closeout
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Teahupoo
• No “back” of the wave• Parts of the reef are only 20 inches deep• Depths plummet to 1,000 feet 1/3 mile offshore
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Surging Breakers
• ξ < 0.5• Rapid drop offs – no shoaling zone
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Longshore Currents
• Currents running parallel to the shoreline• Occur most often when waves approach shoreline at an
angle• Larger waves – faster currents• Strongest currents generated on gently sloping beaches
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Rip Currents
• Strong seaward flowing channel• Typically flow at 1-2 ft/s, and can be as fast as 8 ft/s• Wave set-up • Generally only temporary, but can persist for long periods of
time due to man-made structures
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Wave Power
• Single buoy can generate enough electricity to power 40 homes in NJ
• Future wave power station in Oregon, comprised of 10 buoys, will generate 1.5 megawatts – enough electricity to power about 1,000 homes
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Sources
• www.noaa.org• www.meted.ucar.edu• www.oceanpowertechnologies.com