On November 29th we had “a big swell event” at Morro Bay. I had noticed a very large low pressure (storm) up by the Aleutian Islands for several days so it was not a surprise when the forecasts said that large swells would arrive at our coast. Actually, they were only 11 to 13 feet high which isn’t exceptional but the swell period (distance from crest to crest) was very long, 21 seconds. At Morro Bay the effect was very large explosions of spray as these swells hit the breakwater. 
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
This photo is the best I could get on the morning of the 30th. The day before the spray was quite a bit higher than this photo shows.
-
I just had to look up the formulas concerning waves to learn more about what was going on. It was quite amazing! The wavelength of these swells was 2260 feet, more than 1/3 of a mile from crest to crest (wavelength is directly related to their period). Their speed was 63 knots, or 73 miles per hour. When a wave is moving in deep water the wave crest moves forward but the water in the wave hardly moves at all; the water just goes back and forth, up and down. But when a wave enters shallow water and breaks, the water at the crest is moving forward at the speed of the wave (73 miles/hour). No wonder these things were hitting the breakwater so hard and sending up so much spray! [Common swells might have a period of 12 seconds and would be moving at only 42 miles per hour.] Actually, swells slow down as they get into shallow water but the point is that these were moving much faster than usual.
-
Why were the periods so long? It seems that the farther a swell travels, the more it stretches out and the longer the wavelength. This storm was a long ways away. Another phenomenon of interest was that since the long wavelength swells travel the fastest, they reached our shore first. The period stayed at 21 seconds for 15 hours, by 24 hours it was down to 17 seconds, and by 48 hours it had decreased to 15 seconds and the swell height was down to 8 feet.
