Waves and Coastal Interactions

Stan Piotrowski

What is a wave?

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

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

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

Types of Breaking Waves

• Spilling• Plunging• Surging• Iribarren Number (surf familiarity parameter)• Breaker Depth Index

Spilling Breakers• ξ < 0.5 • Gently sloping coasts where waves break slowly and over a

long distance • Relatively gentle waves

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

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

Surging Breakers

• ξ < 0.5• Rapid drop offs – no shoaling zone

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

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

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

Sources

• www.noaa.org• www.meted.ucar.edu• www.oceanpowertechnologies.com