Monterey Dive Infomation- Wave Dissertion

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Waves

Waves are the transfer of energy through a media. Waves on the ocean surface are transferring energy from the source, be it gravity (tides), earthquakes (tsunamis) or blowing wind, to a destination (the shore for example). Wind waves, are the primary type of wave that we are concerned with and play a major role on when and where we dive. These waves cause us to get sick on a boat (at least some of us), play havoc with visibility, and batter us in the surf. Having a better idea how these waves behave can help us pick your dive sites so that you can enjoy our outings more.
Wind waves are formed by wind blowing over water for a distance (fetch), a length of time (duration) and at a certain speed and direction (velocity). Imagine sitting on the shore of a lake reading a book, enjoying the sun streaming down with a cool breeze on your back. You look out at the water and see small ripples on the surface lapping on the shore. A gust comes by and you close your eyes to keep the dust out. After a moment, you look out on the lake and other than seeing a few more ripples at the far side of the lake, nothing has changed. Why? Because the duration of the wind was short and did not have time to increase the wave size. Now, imagine that instead of sitting by a lake, you are sitting on an ocean beach. The same wind is blowing onto the beach. Even though the wind isn’t strong, the waves coming onshore are bigger than at the lake. Some of the waves could be from a storm thousands of miles offshore, but many are from the cool breeze you are enjoying. Why are the waves larger here than at the lake? Because the fetch is greater; the wind has more effect because of the longer distance it has travelede along the water's surface. And as the day progresses at both locations, the wind usually increases as the day warms and the waves, therefore, grow due to both wind speed and duration.
On a world scale, this can be seen in the 2 pictures below. Figure 1 shows an image of the Pacific taken from a weather satellite and Figure 2 shows the wave height during this time. The clouds (signifying storms with strong winds) are in the area where the large wave heights are.


Figure1: A satellite picture of the Pacific Ocean	Figure 2 : The 'WAM' report- wave height for the Pacific

As the waves travel away from the storm, they are now called swell. The small waves have dissipated and transferred their energy to the longer ones. The swell travel away from the storm in groups or trains. We commonly see this phenomenon as we watch the waves break on shore. There are ‘sets’ of large waves and then a lull.
As the waves travel into shallow water, its energy starts to interact with the bottom. This will slow the wave down, changing its length and height but its period will remain the same.
Wavelength is an important parameter and affects wave height and surge (back and forth water movement at depth). As the wave travel into shallow water, the wavelength decreases. Figure 3 shows this relationship.

Figure3: Wavelength vs Depth

Wave height also changes as depth changes. As the wave approaches the shore, water depth (usually) decreases. This, as illustrated above, causes the wavelength to decrease. This in turn causes wave height to decrease and then peak as shown in Figure 4.

Figure 4 : Wave height vs Depth

As the wave interacts with the bottom, its movement turns from a circular motion on the surface (watch a small object bobbing on the surface) to a to-and-fro motion at the bottom (surge). Below is a graph (Figure 5) and how wave period affects surge. On thing to keep in mind is that the distance shown is for small particles suspended in the water. Since we are much bigger (and have more inertia) we will not move nearly as far as is shown. But this can be used as a relative guide on the amount of surge you can expect as you change depth.

Figure 5 : Surge vs Depth

Waves can reflect, refract, and diffract, all affecting the wave’s strength (and height) in a given area. As a wave hits a hard surface, a portion or its energy is reflected back, as in a mirror. On a day with defined swell, watch a hard wall like a breakwater or large rocks on the shore. You will see the wave crash but if you look closely you will see a small wave traveling back out to sea.
Refraction and Diffraction both bend the waves; refraction is caused by the slowing of the wave and diffraction is caused by the wave passing by an object. As described above, as a wave moves into shallow water, the wavelength decreases. This implies that the wave speed also decreases. If the wave approaches the shore at an angle (assuming the bottom is changing regular to the shore), the wave in the shallow water will slow as the wave in the deeper water is continues to traveling faster rate. This has the effect of turning the wave so that it breaks parallel to shore. This helps explains why if you draw the exact wave shadow on the coast, it does not match with observed shadow. Figure 6 is the Monterey peninsula with the wave shadow shown for waves coming in at 300º (a typical direction during the winter). Figure 7 shows actual wave heights for a 300º wave front. Figure 7 does not follow the ideal wave shadow shown in Figure 6. Refraction, diffraction are the main reasons for this.


Figure 6: Ideal wave front on Monterey Peninsula	Figure 7: Wave height for Monterey Bay and Area

How all this ties into reading weather reports.

1 Day Wave Model and 3 day Wave forecast- gives you and idea where to ‘hide’ during big waves or if it is even worth going. On the 1-day forecast, you can also get the height and period for deep water swell, helping you predict the surge you may encounter.
Current wave height is only part of the story. This may help you decide where you have the least chance of being tossed around but it won’t tell you how the visibility is. Past height gives some insight into that.

Swell Graph- Show how big the waves have been. If they have been big for a while, visibility may be marginal as a lot of stuff may be churned and ripped up.

Buoy 46042 – A lot of open ocean data is provided at this website but it can be confusing. For example, there are 3 different measurements for wave height and 4 for wave period. I use this site to get an idea of the winds and general wave conditions at sea.

As you start to learn to read all of this data, it’s a good idea to go back and review these sites immediately after your dive. This will allow you to validate (or modify) your conclusions you drew so that you have a better idea how to interpret all of this data for your next dive.

References:
Willard Bascom, “Waves and Beaches”. No longer in print but you can buy it used on Amazon.com.
US Army Corps of Engineers, “Coastal Engineering Manual”

last updated 13 Dec 2008