Transcript Stan-Waves
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