September 20, 2015

Freak waves and tide rips

MOST OF US by now have come to realize that freak ocean waves are not really freak at all. They are real and quite regular, though the chances of meeting one in a small yacht are quite remote. They are the result of waves in regular trains riding on each others’ backs.

In any sea state, there are many wave trains present, each with its own speed, height, and direction. They constantly get into and out of step with each other, and every now and then it just happens that a number of these components will get into synch and produce an exceptionally high wave. The same conditions may also produce an unusually low trough, incidentally, and large ships have often reported dropping into huge holes in the ocean without warning, mostly with severe damage.

As a matter of interest, the probability of occurrence of exceptionally high waves may be calculated mathematically:

Ø It has been shown that one wave in 23 is over twice the height the average wave in a sea state.

Ø One wave in 1,175 is over three times the average height.

Ø One wave in over 300,000 exceeds four times the average height.

But what is not so well known is the fact that it’s not only the deep ocean that can produce “freak” waves in generally moderate conditions. If you’re running in from the sea, and you pass over a shallow bank, you can meet some quite alarmingly large waves.  It doesn’t seem right that larger waves should form in shallower water, but it comes about because a wave “feeling the bottom” is slowed down by friction, and therefore the distance between crests is reduced. The energy in the waves remains the same,  but it’s being compressed into a smaller area, and that energy has to go somewhere, so it extends upwards. In other words, the waves get bigger and steeper. It’s like pushing a carpet along the floor from one end.

Over shallow banks, and, of course, at the beach, the wave activity can be more dangerous than it is farther out in deep water. The same thing happens when waves run into an opposing current, as we know full well around here in Puget Sound, with its various tide rips. It’s interesting to note that a wave will be stopped completely by an opposing current traveling at one quarter of the speed of the wave. That wave literally hits the wall and rears up as a frightening cliff of water.

The Scripps Institute of Oceanography has shown that waves entering an area of opposing currents can quite easily have their heights raised by 50 to 100 percent in currents as low as 2 to 3 knots, creating breaking waves even in the absence of local wind.

The lesson to be drawn from this is that skippers of small yachts cruising for pleasure in coastal waters, and even in comparatively calm waters such as those of the Salish Sea, should keep a good watch for unusual patches and shadows on the water. Stay well clear of them, and keep away from charted overfalls and tide rips. They can be deadly for small boats.

Today’s Thought
Out of sight of land the sailor feels safe.
It is the beach that worries him.                                                  
— Charles G. Davis

Tailpiece
“Won’t your mother be angry if she sees you in that skimpy swimsuit?”
"Yeah, I guess so. It's hers."

1 comment:

Kevin McNeill said...

John,

I was always told as a young seaman, a state I no longer enjoy, that a sailor's place was at sea, land was but a navigational hazard.

Kevin McNeill