### Re: What is the frequency of ocean waves hitting the shore?

Date: Mon Nov 29 17:10:06 2004
Posted By: Michael Richmond, Faculty, Physics, Rochester Institute of Technology
Area of science: Earth Sciences
ID: 1101239040.Es
Message:

I don't think that there is a single answer like "8 waves per minute" to this question. The frequency with which waves wash up on a shoreline depends on quite a few factors, among which the wind is very important. People who design ships and piers and docks and other structures which must encounter waves study wave properties carefully, so there is quite a lot of data on wave frequency.

For example, check out a nice textbook called "Introduction to Physical Oceanography", written by Robert H. Stewart of the Department of Oceanography at Texas A&M University. You can find a copy of this text on-line, at

If you go to Chapter 16, "Ocean Waves", you'll find some models which attempt to reproduce actual measurements made by instruments on board ships. Figure 16.7 shows the spectrum of waves produced by winds of several different strengths. The horizontal axis shows the frequency of waves (how many waves pass each second), and the vertical axis shows (roughly) the number of waves with each frequency. You can see that a light wind of 12.9 meters per second will produce mostly waves with a frequency of about 0.1 waves per second. We can convert that to "waves per minute" by multiplying by 60: it turns into about 6 waves per minute. On the other hand, the model predicts that under stronger winds of 20.6 m/s, the frequency of waves decreases to about 0.07 waves per second, or about 4 waves per minute.

Now, I am just quoting the peak of the spectrum shown in this figure. The fact that the spectrum under any wind condition has a broad peak means that the number of waves per minute fluctuates quite a bit. The peak for a strong wind of 20.6 meters per second shows roughly equal power at 0.06 waves per second (3.6 per minute) and 0.10 waves per second (6 per minute). In any case, then, this model predicts a range in the number of waves which would reach shore each minute.

Figure 16.8 may be easier for you to read: it shows the increase in period between waves as a function of wind speed. The faster the wind blow, the longer you have to wait between waves. It shows typical periods between perhaps 4 and 15 seconds, which corresponds to frequencies of 15 to 4 waves per minute.

Later on in the chapter, you can find another spectrum showing properties of waves produced under a slightly different model. The peak frequency ranges from about 0.2 waves per second (12 per minute) up to about 0.4 waves per second (24 per minute).

As a final -- and much less meaningful -- check, I asked my brother to count the number of waves per minute which reached the shore near his house (he lives on the coast near Boston). He told me that one minute he counted 8, and the next minute he counted 11.

So, your memory of "8 waves per minute" does indeed seem to be in the range of typical values. However, the real value does have quite a large range, and it would appear that it decreases as the wind grows stronger.

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