MAR 110: Lecture 14 Outline Ocean Waves 1 MAR 110 LECTURE #14 Ocean Waves Figure 19.1 Do Ocean Surface Waves Present a Hazard?...a picture is worth a thousand words Tsunamis - giant shallow water waves this one devastating the city of Lisbon Portugal in 1755, which was already suffering from fast spreading fires caused by the earthquake that triggered the wave. Most of the city and much of its population had been completely destroyed. (NG) Figure 19.3 Ocean Swells Ocean swells from distant ocean storms grace a California beach surfer s paradise!. (ItO)
MAR 110: Lecture 14 Outline Ocean Waves 2 Figure 19.4 The Destructive Power of Coastal Ocean Waves (Santa Catalina Island California) (???) Figure 19.5 Open Ocean Chaotic Waves In the open ocean waves are coming from all directions with varying intensities, creating a chaotic sea surface (left). Waves with different wavelengths (rights) when combined form an irregular wave profile. (ItO, LEiO)
MAR 110: Lecture 14 Outline Ocean Waves 3 Figure 19.6 A Simple Surface Wave A wave is a periodic (or repeated) sea surface distance that is defined by its wave length L and its wave height H. (LEiO) Figure 19.7 Ocean Waves a Constant Competition Gravity restores a sea surface disturbance most often caused by wind towards flat-ocean equilibrium. Overshoots keep the cycle going in which the potential energy of elevation is converted to kinetic energy of motion and back again. (??)
MAR 110: Lecture 14 Outline Ocean Waves 4 Figure 19.8 The Wave Form Propagates Waves move horizontally they propagate away from their generation sites. (LEiO) Figure 19.9 Wave Mechanics Surface waves In the open ocean propagate horizontally (carrying their energy), HOWEVER, the water parcels associated with the passing wave DO NOT propagate! Rather they move in relatively small circles in place. (LEiO)
MAR 110: Lecture 14 Outline Ocean Waves 5 Figure 19.10 Wave Propagation Speed The time it takes for a full wave with a wave length L to pass a fixed spot is called the wave period T. The wave speed C p is the ratio of the wave length to the period as shown above. (LEiO) Figure 19.11 Wave Spectrum Waves are classified or sorted in a spectrum according to their wave periods, from the very smallest period capillary waves ( witht less than 0.1 sec) to ocean swell (T about 15 sec) to tsunamis (T about 20 min). The amount of energy in the particular wave classes is determined by the typical wave heights H in the class. The forces that cause the particular waves are indicated above. (ItO)
MAR 110: Lecture 14 Outline Ocean Waves 6 Figure 19.13 Deep Water Waves A wave is classified as a Deep Water Wave, if the water depth D is greater than half of its wave length L. Below the depth = wave length, the water motion associated with this wave virtually disappears. Note the decreasing size of the wave parcel orbits. (LEiO) Figure 19.14 Deep Water Wave Speed Deep water wave speed depends on wavelength. (LEiO)
MAR 110: Lecture 14 Outline Ocean Waves 7 Figure 19.15 Shoaling Waves As deep water waves propagate into water shallower (depths D less than their wave length L), the ocean bottom interacts with the wave and the waves are transformed, as they shoal. The shoaling waves slow down and wave heights increase. (NH) Figure 19.16 Shallow Water Waves Shallow water waves do not have the sinusoidal shape of deep water wave because the particle orbits are forced into narrowing ovals (bottom) as they move towards shore instead of the normal circular orbits. As the water gets shallower the wave height increases until gravity forces it to break (top) (LEiO)
MAR 110: Lecture 14 Outline Ocean Waves 8 Figure 19.17 Shallow Water Wave Speed Shallow water wave speed depends on depth. (LEiO) Figure 20.2 Wave Energy Transmission & Wave Groups Deep ocean waves with slightly different wavelengths superimpose to form wave groups with speeds (C g ) that are half that of individual wave speeds (C p ). Because the individual waves move faster than the wave group and energy (C p = 2 C g ), individual waves are seen to (a) appear at the back of the wave group (b) propagate through the slower-moving wave group; and finally (c) disappear at the head of the wave group.
MAR 110: Lecture 14 Outline Ocean Waves 9 Figure 20.3 Deep Water Wave Energy Transmission Deep ocean waves with slightly different wavelengths form wave groups which are distinguished by their wave height envelope; which contains the wave energy. The wave group speed for deep water waves (C g ) is the wave speed C p. The form of the deep water wave group speed - given by the relation above - indicates that the longer wave length, deep water waves travel faster than shorter wave length waves. Figure 20.4 Shallow Water Wave Energy Transmission The shallow water wave group speed C g is equal to the wave speed C p and thus depends on only the local water depth D, according to the above.
MAR 110: Lecture 14 Outline Ocean Waves 10 Figure 20.6 Open Ocean Wind Wave Generation and Dispersion (Upper) In the wind-generation region, the ocean is rough and chaotic because waves with all different wavelengths superimpose and interfere with each other (lower left). As the waves propagate toward the right and away from the generation zone, the waves sort according to wave group speed. The longer wavelength/longer period waves known as swell (lower right) race out ahead of the shorter wavelength/shorter period waves.. (ItO) Figure 20.7 The strong winter storms in the Southern Ocean generate waves that have a long time (& distance) to sort themselves out according to wave length before they strike the southwest facing shores of California. (NH, ItO)