Lab_09_Ocean_Waves

In this expedition you will investigate the movement of deep water waves, and how their patterns and characteristics are modified by the entrance in shallow water (water that is less than ½ their wavelength). A. DESCRIBING WAVES (WAVE CHARACTERISTICS) The main characteristics used by oceanographers to describe ocean waves are: • Wave length , L (distance between adjacent crests, or troughs) • Wave height, H (vertical distance between a crest and trough) • Wave steepness (H/L) • Wave amplitude (vertical distance between the highest crest or deepest through and the equilibrium line) • Wave period, T (time between two successive crests, or troughs) • Wave speed (L/T) • Wave frequency , the number of wave crests or troughs that pass a point in a given period of time. 1. Using the waves in Figure 1 below, • Calculate the wave length (round to the nearest 5 m) __________ • Calculate wave height (round to the nearest 0.5 m) __________ • Calculate wave steepness _____________ • Knowing that waves break when steepness is equal to or greater than 1/7, are the waves in Figure 1 breaking? (yes or no) Figure 1

ESSC 320, LAB 10 2 2. Using the waves in Figure 2 below, • Calculate the wave length (round to the nearest 5 m) __________ • Calculate wave height (round to the nearest 0.5 m) __________ • Calculate wave steepness (within two decimal digits) _____________ • Knowing that waves break when steepness is equal to or greater than 1/7, are the waves in Figure 2 breaking? (yes or no) Figure 2 B. DEEP WATER WAVES Recall from your textbook that deep water waves are waves that travel in water depth greater than half of their wavelength. The water molecules of a deep-water wave move in a circular orbit, and the diameter of the orbit decreases with the distance from the surface. The motion is felt down to a distance of approximately one half wavelength, where the wave's energy becomes negligible. The wave animation posted online shows the movement of water particles at and below the ocean surface as a series of swells is passing by, moving from left to right. 3. Using a ruler and the horizontal and vertical scales, estimate the following (you may need to stop the animation to measure and read time; use the orange arrow above the animation as reference point when counting wave crests): • wave length (round to the nearest 10 m) __________ • wave height (round to the nearest 5 m) __________ • wave period (round to the nearest second) _____________ • wave speed (within two decimal digits) _____________ • wave frequency (per minute) __________________ 4. At what depth should a diver descend to not feel water motion? _____________ meters.

ESSC 320, LAB 10 4 7. Describe the relationship between wind strength and direction, and the height and direction of movement of the waves. Can you conclude that the highest waves are produced by storms? Why or why not? 8. Copy and paste the three maps to the file you created. There should be four (4) maps now. Submit the file. D. WIND SPEED, FETCH, AND FULLY DEVELOPED SEAS: FORCASTING WAVE HEIGHTS Recall from your reading that in oceanography the fetch is the area of ocean or lake surface over which the wind blows in an essentially constant direction, thus generating waves . When the wind has imparted its maximum energy to the waves, the sea is said to be fully developed. A fully developed sea has the maximum wave size theoretically possible for a wind of a specific strength, duration, and fetch . Knowing wind speed and fetch, it is possible to forecast maximum wave heights. We will investigate wind-generated waves under a variety of circumstances starting with the Southern California scenario in Figure 3. Tables 1, 2, and 3 provide data for various sets of conditions about which you are questioned. Figure 3 The distance from San Pedro, California, to Avalon, on Santa Catalina Island, is about 25 nautical miles (nm). The island is almost due south (180°) from the Los Angeles Harbor at San Pedro (see Figure 3). 9. Using Table 1, determine the minimum northerly (from the north) wind speed needed to set-up a fully developed sea in this channel. (HINT: Interpolate between fetch distances 10 nm and 70 nm in Table 1; if you do not recall how to interpolate, see link posted online.) a. 12.5 knots c. 31.4 knots b. 22.3 knots d. 32.7 knots 10. How long must a northerly wind blow in order to generate a fully-developed sea in this channel? (HINT: A Table 1 interpolation between fetch distances 10 nm and 70 nm is required here as well.) a. 1 hour. c. 3 hours b. 2 hours d. 4 hours 11. Determine the average wave height that would result in the Catalina Channel from the winds that you determined would set-up a fully-developed sea. (Use Table 2). a. 0.33 m b. 0.58 m c. 0.74 m d. 1.5 m High waves can be dangerous to boaters because they can cause excessive rolling. As a general rule, dangerous wave heights depend on the length of a boat. Specifically, waves are dangerous when: Wave Height = or > Boat Length x 30%

ESSC 320, LAB 10 5 Table 1. Minimum fetch and duration required for selected wind speeds to set up fully developed seas. Wind Speed (knots) Fetch (nautical miles) Duration (hours)* 10 10 2 20 70 10 30 280 23 40 710 42 50 1420 69 *Duration times rounded off to the nearest hour Table 2. Characteristics of waves resulting from selected wind speeds in a fully developed sea Wind Speed (knots) Average Height Average Length Average Period (seconds) Height of 10% of waves (feet) (meters) (feet) (meters) (feet) (meters) 10 0.9 0.27 28 8.5 2.9 1.8 0.55 20 5 1.5 111 33.8 5.7 10.2 3.1 30 13.6 4.1 251 76.5 8.6 27.6 84 40 27.9 8.5 446 135.9 11.4 56.6 17.2 50 48.7 14.8 696 212.2 14.3 98.9 30.2 Table 3. Conditions necessary for a fully developed sea at given wind speeds and the parameters of the resulting waves. Wind Speed (knots) Fetch (nautical miles) Duration (hours) Average Height Average Length Average Period (seconds) (feet) (meters) (feet) (meters 10 10 2 0.9 0.27 28 8.5 3 12 18 4 1.4 0.43 40 12.2 3.4 14 28 5 2 0.61 55 16.8 4 16 40 7 2.8 0.85 71 21.6 4.6 18 55 8 3.8 1.2 90 27.4 5 20 75 10 4.9 15 111 33.8 5.7 22 100 12 6.3 1.9 133 41.2 6.3 24 130 14 7.8 2.4 160 48.8 7 26 180 17 9.3 2.9 188 57.3 7.4 28 230 20 11.4 3.3 218 66.4 8 30 280 23 13.6 4.1 251 76.3 8.6 32 340 27 16 4.9 285 86.9 9 34 420 30 18.6 3.7 322 98.2 9.7 36 500 34 21.4 6.3 361 110.1 103 38 600 38 24.5 7.3 102 122.6 10.9 40 710 42 27.9 8.5 446 136 11.4 42 830 47 313 9.6 491 149.7 12 44 960 52 33.4 10.8 540 164.6 12.6

ESSC 320, LAB 10 7 Figure 4. Depth Contour Map Figure 5. Water Surface 17. In Figure 4 the shallowest seafloor is located in the ______ of the figure, the deepest seafloor is located in the _____ of the figure, and the Jaws ridge extends from the ______ of the figure. a. upper right part; lower part; upper right to the lower left. b. upper left part; lower part; upper left to the lower right. c. lower part; upper right part; upper right to the lower left d. lower part; upper left part; upper left to the lower right. Figure 5 is a map of the water surface obtained from the REF/DIF model. You should view it as an aerial photograph of the site, where the high parts of the waves are red and the low parts are blue. The waves are moving from the lower right corner to the top left corner of the figure. 18. Use a ruler to estimate wavelengths (the distance between consecutive crests (red color) or consecutive troughs (blue color)). As the waves enter shallower water, the wavelength ____ and the wave height ______. This is due to the fact that waves _____ when interacting with the seafloor. a. increases; decreases; speed up b. increases; increases; speed up c. decreases; decreases; slow down d. decreases; increases; slow down 19. Compare Figure 4 and Figure 5. The highest waves in Figure 5 (darker red areas) are recorded at a depth of about _____ . Past this point, as the waves move further towards the coast, the wave height ______ due to the wave breaking (towards the upper left in the figure). a. 5 ft.; decreases b. 20 ft.; increases c. 40 ft.; decreases d. 70 ft.; increases 20. Figure 5 also shows that as the waves enter shallower water they _____ around the underwater ridge in a process known as _____. a. bend; wave refraction b. becomes more straight; wave diffraction c. bounce back; wave reflection d. bounce back; wave refraction

ESSC 320, LAB 10 8 A group at the Scripps Institution of Oceanography uses computer models to predict how deep water swell waves will change direction and height as they enter shallow water along the California coast. In southern California, islands and an irregular coastline cause large changes in the swell waves from place to place. In this part, we will look at examples from their web site. Figure 6 online is a map of predicted wave heights during the winter along the coast between Point Conception (just west of Santa Barbara) and the Mexican border (south of San Diego). In this and following images, land is shown in black and the colors in the ocean correspond to different wave heights in shallow water. Blue colors indicate small waves and yellow to red colors indicate large waves; green colors indicate intermediate-sized waves (see scale at the bottom of the image). The inset compass diagram is a wave spectrum (swell intensity as a function of period and direction). For the purpose of this expedition, the only information you will need is the wave direction , or where the waves are coming FROM . In the compass, like on the map, north is at the top and east is to the right; for example, and a colored area located at the very bottom of the compass indicates waves coming from the south, and a colored area at the top of the compass indicates waves coming from the north. 21. Examine Figure 6 (posted online). On December 3, 1996 (the date of this image), the primary deep- water swell was approaching the coast in southern California from the a. southeast b. southwest c. northwest d. northeast 22. Notice the high waves (red color) along the northwest sides of some of the offshore islands. How large are these waves (in feet) and why are the waves so large in these locations? a. 9 ft; as waves enter shallower water, they slow down and their height increases b. 9 ft; as waves enter deeper water, they slow down and their height increases c. 6 ft.; as waves enter shallower water, they slow down and their height increases d. 4 ft.; as waves enter deeper water, they slow down and their height increases. 23. Now examine the California coastline. What is the approximate latitude and longitude of the area where the highest waves are predicted to occur, and how large are those waves? a. between 34.5-35 degrees N and 121-120.5 degrees W; about 8 ft b. between 34-34.5 degrees N and 120.5-118.5 degrees W; about 2 ft c. between 34.5-35 degrees N and 121-120.5 degrees W; about 6 ft d. between 34-34.5 degrees N and 120.5-118.5 degrees W; about 4 ft The graph in Figure 7 online shows predicted wave height along the Southern California coast, from north of Point Conception, west of Santa Barbara, to the Mexican Border. Examine the graph and answer the questions below. 24. Where are the waves highest, and what height (in feet) are they predicted to be? a. Pt. Loma; 2 ft. b. Long Beach; 3 ft. c. Santa Barbara; 4 ft. d. south of Point Conception; 9 ft. and higher 25. How do you explain the waves being highest in that location? a. the seafloor is deeper; b. the seafloor is shallower; c. waves are approaching the shore at higher speed; d. waves are approaching the shore at lower speed;