Waves, Swells, and Shallow Water

A Quick Refresher on Swells & Waves

• "Waves" are generated by local winds.
• "Swells" are waves generated, typically, by winds of distant weather systems where the waves have undergone dispersion and have transformed into groups of waves of roughly the same period and height by the time they reach your location.
• "Wave and Swell Heights" are measured from the crest to the trough (typically in feet or meters).
• "Wave and Swell Periods" are measured from the crest of a wave or swell to the crest of the following wave or swell (in seconds.)
• Finally, "Seas" are a combination of waves and swells superimposed on one another.

There is also a high degree of correlation between winds and wave heights. Since this correlation exists, the following can be deduced; when all other factors remain the same:

1. The higher the wind speed, the higher the waves.
2. The longer the wind blows, the higher the waves.
3. The greater the distance over which the wind blows (Fetch), the higher the waves.

The NOAA Marine Forecast

So just what is NOAA Weather Radio telling us when they give us wave heights during the marine forecast? The first thing when listening is to determine if they are talking about waves, swells, or seas. Often times all are used in the same area forecast and as noted above, there is a difference.

Wave Heights

When the National Weather Service (NWS) forecasts wave heights, these heights are the "Significant Wave Height." This is the average wave height (trough to crest) of the highest 1/3 of the forecast waves. But, and this is the kicker...

It does not mean that all waves encountered will be within the forecasted significant wave height; some will be less and some will be more, occasionally much more! The NWS presumes that individual wave heights can be described using a "Rayleigh Statistical Distribution Model." So what is that you ask? Well basically it is the continuous probability that a given random event will occur based on a given variable. Ok, I know, for most of us “Gobbly Gook” right? So here is what the Rayleigh Statistical Distribution Model means for the mariner:

If the significant wave height is forecast to be 10 feet:

• 1 in every 10 waves will be greater than 11 feet;
• 1 in every 100 waves will be greater than 16 feet;
• 1 in every 1000 waves will be greater than 19 feet;

Then there is the "extreme wave" of a ratio exceeding 2.0 to 2.5 times (20 to 25 feet) the significant wave height. Luckily these occur rather infrequently, but they do occur. Statistically, 1 in every 267,326 waves may reach this height

So, if the above Rayleigh Distribution Model is correct, and we presume a wave period of 8 seconds, for a significant wave height of 10 feet:

• A wave greater than 11 feet will occur every 80 seconds (1.3 minutes);
• A wave greater than 16 feet will occur every 800 seconds (13 minutes);
• A wave greater than 19 feet will occur every 8,000 seconds (2.2 hours).
• A wave greater than 20 to 25 feet will occur approximately (every 594 hours).

As you can see even with wave forecasts of a specific height, larger waves should always be expected.

Running in Shallow Water

So what does this have to do with anything? With increased wave height comes reduced water depth at the bottom of the wave trough. (See the figure below.)

Actual water depth can often be reduced by as much as half of the wave height. Let’s take a look at an example using a 6 foot wave in a water depth 10 feet.

An Example

In the case of a "significant wave height" of 6 feet, the bottom of the trough could reduce the normal water depth by as much as 3 feet. So what exactly does this mean for the mariner? Well, for a vessel drawing 5½ feet running 20 miles offshore in 300+ feet of water, it is likely of no consequence.

However, what about when you are in much shallower water say 10 feet deep? With the same 5½ foot draft under calm conditions you have 4½ feet of water under the keel. What happens if a 6 footer happens to roll by reducing the water depth by 3 feet at the bottom of the trough? In that case you are probably still good right? While these waves are probably breaking at this point, you still have 1½ feet under your keel.

Now, again presuming that the Rayleigh Distribution Model outlined above is correct, then 1 in every 100 waves is likely to be 10 feet or larger and that sir, changes things significantly. At the bottom of the trough the water depth is now reduced to 5 feet or less; and with a 5½ foot draft that equals "Not Good."

Something to consider when running in Shallow Waters and Heavy Weather.