Hobie Surf: Surf Science 101: Reading Buoy Data

For this week’s science blog, I thought it would be useful to take what we have learned in the past two blogs, about wave height and period, and apply that to how we read buoy observations, in-turn allowing us to do our own wave forecasting. Realtime buoy data is abundantly available via the internet through many sites. I find that the National Data Buoy Center (NDBC) online database offers the most helpful wave-data sets. Their database can be found at;  The National Data Buoy Center.

When you are at the NDBC site you will want to choose relevant buoy data that correlates to the location of the beach you plan on surfing at. I will be looking at the Dana Point buoy (#46223) for the following examples because this buoy is close in proximity to my local beaches and it is well exposed to swell from all directions. Some hints when choosing a buoy to use for forecasting; make sure it is not located on the leeward side of an island or tucked in too close to shore where points along the coastline might block swell from the buoy.

The Dana Point buoy collects wave and temperature data and records them every ten minutes.

By looking at this snapshot of the Dana Point buoy observations, we can see that the wave height is 2.6ft with a dominant period of 17 seconds coming from a south-southwest direction (195°).

So what exactly can we gather about surf conditions from this report? Well, I believe that everyone has their own way of interpreting buoy data and coming to their own conclusions and I will take you through how I like to analyze this info.

The most important pieces of information for me are the swell height and period. The swell height should not be interpreted as the wave face height of a breaking wave. We must remember that these observations are being collected, in this case, from a stationary buoy located   about 2.5 miles offshore where the ocean depth is about 1200 feet. At this location, swell groups are still deep-water waves meaning that the wavelengths are relatively long.

Now picture in your mind a stretched out accordion. When deep-water swell approaches the coast it will remain at this long (stretched out) wavelength. When the swell moves into shallow water, the wave will begin to “feel the bottom.”

Shallow water is defined as water depth of 1/2 the wavelength. This is where a little math and a calculator will come in handy. It is known, by using the deep-water wave speed formula mentioned in the previous blog, that 2.5ft waves travel at about 26mph in deep water. Now we can simply convert the miles to feet (26mph = 140,400 feet per hour) and then convert the time from hours to seconds (140,400 feet per hour =  2340 feet per minute  = 39 feet per second.) So now, by knowing the wave speed as 39 feet per second, we can insert the swell period, 17- seconds (39 feet per second multiplied by 17 seconds) gives us a wavelength of 663 feet.

The shallow water depth of this swell is (663/2) 331.5ft, equal to half the wavelength. So now, going back to the stretched out accordion analogy, when the waves move into water less than 331.5 feet in depth, the waves will begin to “feel the bottom,” or, where water particle motion interacts with the ocean floor. This interaction causes the waves to “drag” along the bottom and slow down. When the wave train begins to slow down, due to “drag,” the wavelengths become shorter, in-turn increasing wave height. Think about the accordion coming together, as it is being squeezed, the length decreases and the height increases.

Now, hopefully, we can all have a slight understanding of why long-period swells can equate to higher wave heights at the beach compared to shorter period swells.

Remember, longer period equals longer wavelength meaning that the wave begins to slow down and gain height much farther out in deeper water.

Now back to the buoy data, it is best to check the wave heights daily, preferably just before you plan on heading down to the beach. What I like to do is jot down the height, period and direction, then physically observe the waves at my local beach. This way I know I have a solid mental note of how the buoy reports translate to actual wave heights at the beach.

Gary Larson

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