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EAS/BIOEE 154 Lecture 12 Introduction to Oceanography Waves Fundamental Principles Ideally, Waves Represent a Propagation of Energy, Not Matter

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EAS/BIOEE 154 Lecture 12 Introduction to Oceanography Waves Fundamental Principles Ideally, Waves Represent a Propagation of Energy, Not Matter EAS/BIOEE 154 Lecture 12 Introduction to Oceanography Waves Fundamental Principles Ideally, waves represent a propagation of energy, not matter. (but ocean waves are not always ideal). Three kinds: Longitudinal (e.g., sound wave) Transverse (e.g., seismic “S” wave) - only in solids Surface, or orbital wave Occur at the interfaces of two different densities. These are the common wind-generated waves. Some Definitions Wave Period: Time it Takes a Wave Crest to Travel one Wavelength (units of time) Wave Frequency: Number of Crests Passing A Fixed Location per Unit Time (units of 1/time) Frequency = 1/Period Wave Speed: Distance a Wave Crest Travels per Unit Time (units of distance/time) Wave Speed = Wave Length / Wave Period for deep water waves only Wave Amplitude: Wave Height/2 Wave Steepness: Wave Height/Wavelength Wave Interference Crossing waves can “interfere” to create either a bigger wave (constructive interference) or smaller wave (destructive interference). Generation of Waves Most surface waves generated by wind; therefore called wind waves Waves are also generated by Earthquakes, landslides — tsunamis Atmospheric pressure changes (storms) Gravity of the Sun and Moon — tides Height of Wind-Generated Waves depends on: Wind Speed Duration of Wind Event Fetch - the distance over which wind can blow without obstruction Waves and Swell Once generated, waves can propagate as swell without wind Dispersion: Large waves travel faster than small ones Because waves are not ideal, energy is dissipated and waves die out Small ones die out, or damp out, faster. Wave Speed and Water Depth Deep-Water waves travel in water that is deeper than 1/2 the wave’s wavelength; Depth > L/2 Speed is function of wavelength only – long wavelengths move faster Waves have nearly ideal shape and thus propagate energy but very little mass EAS/BIOEE 154 Lecture 2 Shallow-Water waves travel in water that is shallower than 1/20 of the wave’s wavelength; Depth <L/2 Wave speed is function of water depth only Waves are not ideal and propagate both Energy and Mass (Stokes Drift) Intermediate waves neither purely “deep” or “shallow”; L/20 < Bottom Depth < L/2 wave speed depends on both wavelength and water depth Breaking Waves As waves approach the shore, wavelength decreases, while height and steepness increase. As waves approach the shore, drag of the bottom slows the water motion near bottom Consequently, there is net forward transport at the surface as the waves steepens Once height reaches 1/7 wave length, the wave becomes unstable and breaks. Wave Refraction & Reflection Shallow-Water Wave change direction due to changes in speed; which are in turn due to changes in depth The net result is a rotation of wave fronts to become parallel with bottom depth contours. Wave may also be reflected Consequence of Wave Refraction: wave energy is focused on headlands and away from bays – nature likes a straight coast! Waves striking beach at an angle produce Longshore transport of sediment Transport Obstructed by Groins Other Waves Tsunamis Storm Surges & Sieches Internal Waves Planetary Waves Tsunamis Tsunamis (sometimes improperly called tidal waves) are large amplitude, long wavelength waves that propagate on the ocean surface Tsunamis can be generated by Earthquakes Landslides Volcanic Eruptions Meteorite/Asteroid Impact Properties of Tsunamis Tsunamis have long periods and long waves lengths (as long as 1 hour and 100 km respectively). For tsunamis, wavelength is always greater than twice the water depth. Therefore, tsunamis always behave as shallow water waves (speed 2 EAS/BIOEE 154 Lecture 2 depends on depth). In water of average depth (4000 m), a tsunami will travel at 700 km/hr. The December, 26 2004 Sumatran Earthquake and Tsunami Generated by a magnitude 9.3 earthquake as the Indian Plate thrust under the Sunda Plate Calculated vertical displacements were as much as 5 meters The fault ruptured along more than 1200 km One of the largest earthquakes in past 100 years. May have generated submarine landslides Extensive damage and loss of life throughout the Eastern Indian Ocean Can we predict tsunami’s and save lives? Generating a Tsunami Warning Earthquake occurs Seismic waves travel through the Earth (at about 8 km/sec) to seismometers Earthquake detected by seismometers Determine location - did it occur in the sea? Determine size - is it big enough to generate a tsunami? Pacific Tsunami Warning Center issues bulletin About 75% are false alarms - most earthquakes don’t generate damaging tsunamis Governments must then take action to warn & evacuate Detect tsunamis using seabed detectors - NOAA’s DART system: only 6 detectors deployed so far – most in the northeast Pacific; 32 planned for the Pacific by 2007 (at a cost of over $1 million a piece). What areas are vulnerable to tsunamis? Pacific Rim is most vulnerable Hawaii is particularly vulnerable Japan, Alaska, S. America Pacific Northwest The plate boundary system in the Pacific NW is behaves very similarly to the Indonesia one, with very large, very infrequent earthquakes. Geologic evidence and Japanese historical records indicate a very large tsunami generated there in 1700. Indian Ocean Atlantic and Caribbean Many of the Caribbean islands are subduction zone volcanoes - in addition to earthquakes, volcanic eruptions and volcanic landslides could generate tsunamis Eastern Mediterranean Both volcanically and seismically active Salt beds beneath Mediterranean are particularly subject to landslides. Other Waves Seiches – resonant oscillations Internal Waves – waves that propagate along density boundaries within 3 EAS/BIOEE 154 Lecture 2 the ocean Kelvin and Rossby waves: Low amplitude, long wavelength waves related to wind changes – notably El Niño Some Study Questions How deep must the water be for a wave with a 100 m wavelength to behave as a strictly deep water wave? How shallow must the water be for that same wave to behave as a strictly shallow water wave? What is the restoring force for capillary waves? If a wave has a period of 10 seconds, what is the frequency of that wave? Describe what happens as waves move out from a storm? How does the distribution of short and long wavelength waves change? Why would wave refraction cause headlands to erode, and sediment to be deposited in bays? Explain how waves can transport sediment along the coast in “longshore drift”. Why are tsunamis considered “shallow water waves”? About how long did it take the Dec. 26 tsunami to reach Sri Lanka? What role do NOAA’s DART buoys play in the Pacific Tsunami Warning System? How do the DART instruments detect a tsunami? How does water motion differ between tsunamis and wind-driven waves? 4 .
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