A. Wind B. Tides C. The Moon D. Boats E. Landslides. What is responsible for most of the waves in water?

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2 Waves Waves aka Revenge of the Oceanographers Why do we care? Q di and the Harley harley-9p.photoblog900.jpg 2

3 Learning Goals Identify key properties of waves Explain how waves move energy (can be carried large distances) and matter (not moved far, just back and forth) Use these properties to determine wave speed and behaviour in either shallow or deep water Define wave breaking, and determine when a wave will break. Determine how two waves will interact, and explain constructive and destructive interference. Relate wave interference to marine hazards. (rogue waves)

4 A. Wind B. Tides C. The Moon D. Boats E. Landslides What is responsible for most of the waves in water?

5 Wave Anatomy: Shape Crest wavelength Distance Trough wavelength Crest = highest point Trough = lowest point Wavelength (L or ) = distance for one full cycle (e.g. crest-tocrest, or trough-to-trough)

6 Wave Anatomy: Shape height Distance amplitude wavelength What units are used to describe wavelength? Amplitude? Height? Steepness? Wave Height (H) = vertical distance from crest to trough Amplitude (a) = H/2 (one-half the height) Steepness = H/L (height divided by wavelength)

7 Wave Motion Distance OR Time! Period, (T) = time for one wavelength to pass a point (e.g. seconds per cycle) Frequency (f) = number of waves per time (e.g. cycles per second) Celerity (c) = L/T (distance per time, e.g. meters per second). Often just called SPEED.

8 How are wave period and frequency related? A. Period = Frequency B. Period = Frequency/Wavelength C. Period = 1/Frequency D. Period = 1/Wavelength E. Period = 1/celerity

9 Wave & Energy Waves require: A Medium (e.g. the rope) A generating force (e.g. the hand) The generated waves carry ENERGY through the medium

10 Water Movement & Waves Water moves back & up, then and down & forward CIRCULAR motion (orbits) ENERGY passes through the water, but there is NO NET MOVEMENT of the water. Seagull goes (almost) nowhere

11 Activity Put notebooks and computers away Fold down your desk 11

12 Orbital motion vs. Depth d > L/2 Wave orbits are largest at the surface Orbits decrease (attenuate) with depth Orbits disappear below d > L/2 At wave base, orbit is ~1/23rd of orbit at surface At d > L/2 essentially NO movement

13 Orbital motion vs. Depth In open water, water particles have an orbital motion that decreases with depth Water remains (more-or-less) in place and is the medium for the wave energy to travel through (except near shore)

14 Orbital motion vs. Depth Deep water wave: d L 2 Transitional/ Intermediate waves: L 20 d L 2 Shallow water wave: d L 20

15 Deep Water Waves Wave does not feel the bottom speed of the wave can be measured using celerity (wavelength divided by period) c L T In deep water, do longer waves travel FASTER or SLOWER than shorter waves? (in meters/second) NOTE: Only works using units of meters and seconds

16 Shallow Water Waves Wave feels ocean bottom Speed is determined by DEPTH (d) only c 3.1 d (in meters/second) In shallow water, do waves travel FASTER or SLOWER over shallower depth? NOTE: Only works using units of meters and seconds

17 Two waves are travelling through 2m deep water; one has a wavelength of 100m, and the other has a wavelength of 120m. Which of the following is true? A. The 100m wave is moving faster than the 120m wave B. The 120m wave is moving faster than the 100m wave C. The waves are moving at the same speed D. There is not enough information to solve this problem.

18 As waves SHOAL (move into shallower water), their characteristics change Deep water waves approach shallow water (circular orbits) become transitional waves then shallow water waves. Orbits flatten. Waves slow down Wavelengths shorten Wave height increases Wave period STAYS THE SAME

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20 When do waves break? Waves Break when: 1) Steepness (H/L) >= 1 / 7 Wave is too high for it s wavelength becomes unstable & collapses 2) Height/Depth 3 / 4 Base of wave is restricted by ocean floor, crest is not crest collapses forward height amplitude wavelength

21 Breaking Waves and Energy Becomes Surf Energy : - Turbulent mass of agitated water rushing onshore - Energy lost as: 1) Heat (mixed back into ocean water) 2) Motion of rocks, sediments - Some reflected back to sea

22 Types of Breakers SLOPE of surf zone determines SHAPE & SIZE of breakers Case 1: Gentle slope: Small, gentle SPILLING breakers Wave energy lost over a wide area 15m 10m 5m Large distance between depth contours

23 Spilling breakers

24 Types of breakers Case 2: Steep slope: Large, violent PLUNGING breakers Sudden release of energy potentially deadly! 15m 10m 5m Short distance between depth contours

25 Plunging breakers

26 Waves are the result of energy travelling across the ocean, but what ultimately happens to that energy? A. The energy reflects off coasts, creating new waves traveling in the opposite direction B. The energy is absorbed by the coast C. Friction causes the waves to slowly lose energy and die D. Waves grow until they break in the open ocean E. The wave energy turns to sound

27 Wave Interference: Hilo Bay Tsunami Video 1 : Hilo Bay tsunami M9.5 earthquake

28 Hilo Bay Tsunami Tsunami damage is often greatest in bays and harbors (e.g. Hilo Bay, Hawaii) Interactions between waves is part of the reason Question: how do different waves interact?

29 What do you think happens when two different surface waves run into each other? A. This can t happen, because ocean waves all move in the same direction B. The bigger wave absorbs the smaller wave s energy, and gets even bigger C. The smaller wave removes some of the bigger wave s energy D. The effect of the two waves is added together, making a more complex wave E. The waves crash together and break

30 Wave Interference Wave 1 Wave 2 Wave 3 Wave 1 + Wave 2 + Wave 3 Three waves, with different amplitude, wavelength, and speed Combination gives PACKETS with higher or lower energy (height) than any wave alone Packets move together Surf Beat

31 Constructive and Destructive Interference Constructive interference: Crests, Troughs line up BIGGER WAVES High Energy Packet Destructive interference: Crests line up with troughs SMALLER WAVES Low Energy Packet

32 Constructive and Destructive Interference BIGGER WAVES

33 Constructive and Destructive Interference SMALLER WAVES

34 Worksheet wave 1 + wave 2 = wave 3

35 Rogue Waves Generated by CONSTRUCTIVE INTERFERENCE Can be very dangerous, and unpredictable especially if oversteepened! Also called monster wave, freak wave Can be 3-4x larger than other waves in same area Can appear, disappear very quickly

36 ROGUE WAVES Most hazardous for ships at sea but they ve also hit the coast: 1992 calm Florida beach hit by a 5.5m high wave (75 people injured)

37 Rogue Waves

38 Where do you find ROGUE WAVES? Notorious off the Cape of Good Hope, South Africa Common where a strong current flows against wind-driven waves from storms Antarctic Circumpolar current