Waves. Parts of a wave. Insert wind_wave.wmv. Shark attack

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1 Waves Recall: Waves = transmitted energy What causes waves? Wind gravity Earthquakes We will talk about all of these, but first Insert wind_wave.wmv Shark attack Parts of a wave Wavelength = L Height Amplitude = crest to sea level = 1/2 H Period (T) = time between crests Frequency (f) = 1/T This is constant for any single wave - it does not change The frequency of a wave is a function of it s energy You are standing in water on the beach as a wave approaches you. Which direction is the water moving? A. Toward the beach B. Away from the beach 1

2 Motion in an Orbital Wave Orbital motion Ahead of crest particle speeds-up in backward direction At crest forward motion Behind crest particle moves down, slows and reverses At trough only backward motion This motion defines an orbit. Wave Orbitals and Speed Notice diameter of orbit Decreases with depth (inefficient E transfer) No motion at depth > 1/2L Speed (C) = L/period Speed and L can change, but T cannot. Period (T) is a function of force that generated wave Deep Water Waves: D>1/2 L Deep water waves are not influenced by seafloor L = (g/2π)t 2 = 1.56T 2 (Long L waves have long period) C = 1.56T (The longer the wavelength, the faster the wave!) Result... 2

3 Dispersion: Storms generate waves of mixed wavelength ( and thus mixed C) Dispersion = long wavelength waves outrun short waves Thus avoiding destructive interferences Swells = long L waves (travel across oceans) When Deep water waves approach shore... Intermediate waves: 1/20L < Depth < 1/2L Speed and wavelength are function of both period and depth Orbits feel bottom or interact with seafloor Wave slows (recall C =L/T, T is fixed so L decreases) Compressed wave causes H to increase Waves in shallow water... Shallow water waves: D < 1/20 L Speed and wavelength depend only on depth Waves slow and build rapidly (increase H) Notice particle motion Orbits flatten w/ depth Horizontal at seafloor When waves become too steep... 3

4 Maximum Steepness Steepness (S) S = H/L If S > 1/7 (max steepness) The wave is gravitationally unstable It will break or fall surfing fun Largest wave surfed Have you noticed the area of the Surf Zone (Area where shallow water waves break) changes from day to day? Why? A. The seafloor changes daily resulting in depth changes that influence the width of the surf zone. B. The forces generating waves change, thus periods and wavelengths vary over time and so the width of the surf zone changes On a given beach the width of the surf zone changes as period (T) changes Surf Zone = Area where shallow water waves break Slope changes between different beaches and so does the width of the surf zone for period (T) 4

5 Waves & Coastline Interaction Wave approaches at angle near end slows causing bending REFRACTION Result - wave fronts swing to approach nearly parallel to beach Refraction, Headland & Bays Waves refracted (focused) on headlands (high E) Wave E dissipated in bay (low E = deposition of beach) Example... Headland vs. bay 5

6 Wave Diffraction Wave passes through gap (inlet) Energy dissipates (like in bay) as waves bend Tsunami Cause = seafloor displacement Earthquake or slump (~avalanche into ocean) Generates huge wave Characteristics L > 10s km - given that D oceans ~4000 m, is this a shallow water wave? T > 10 minutes C ~ 200 m/sec (400 m/hr) H ~ 2 m (hardly noticeable at sea) Result (if not diffracted by continental shelf) Build height well off-shore Approach beach at great speed Possible devastation December, 2004 Earthquake and Tsunami: M = 9, fourth largest EQ since Earth rang like a bell for more than a month afterward. EQ lasted 7 minutes, displacement of up to 20 m along a 1200 km rupture. Released strain = to sum of all EQs between 1976 and >443,000 people killed by Tsunami (most lethal tsunami, ~5th largest natural disaster in human history). Bbc Animation 6

7 Review Questions How does water move as a wave passes? What controls the wavelength of a deep water wave? Why does wavelength change as waves enter intermediate water depths? Compare and contrast Episodic Waves and Swells. Describe how interferences prevent the production of waves of many different wavelengths during a storm. How is wave energy focused on headlands and dissipated in bays? How are Tsunami generated and why are they destructive? When waves collide (interact): If waves collide at high angle Confused Sea If wave fronts are parallel and in phase, then constructive interference results Waves combine their motion (resonate) and wave heights grow out of phase, then destructive interference results Wave heights diminish Episodic wave... Destructive Interference" Constructive Interference" 7

8 Episodic (giant or rogue) waves: Large waves in phase Either two sets of windgenerated waves or wind generated waves and a current H ~ 10s m, T ~ 10s sec, C ~ 30m/s (60 mph) Video 1 Greener Video 2 tanker Video 3 cruise ship Video 4 dude on deck 8