Waves around the island

Cecil's Columns/Staff Reports
1

Cecil answered a question about waves cashing upon the beaches on all sides of an island. To explain why it happened, he used a strange analogy about 2 roller-skaters holding a beach towel. As they skate either side of a pole, the towel wraps around the pole (which supposedly assimilates waves around an island).

The reason I open the thread is in dispute. His example of a beach towel is inadequate because all the fibers of a towel are connected…that is, by hitting one part of the towel, you’re going to disrupt the whole towel. Waves are a completely different substance…hence, by obstructing one portion of a wave, it will have practically no effect on the rest of the wave, and hence a wave hitting an island isn’t going to cause a “wrapping around” effect.

What say ye?

2

Here’s the relevant column.

3

Cecil’s explanation seems correct. He says

So the part of the wave nearest the impact is affected by friction and it curves inwards a bit and hits the shore pretty much head on. And the part of the wave next to that is slowed and bent a bit more and still hits pretty much head on etc. Sure it’s not not like the end of the wave is pulled by a skater - but it’s only “a homely metaphor”. Each part of the wave is slowed and bent by the part before it.

4

Cecil’s explanation seems better to explain a large swell than regular, choppy, short-wavelength wind-generated surf, which would indeed crash at all times on all beaches.

5

Please forgive me in advance for this display of ignorance.

Doesn’t cohesion have something to do with the question in the OP?

In this case, is cohesion within a wave inconsequential? (Maybe negligible is a better word?)

6

Yes to both.

To the OP, yes the towel is not a perfect metaphor, but it does describe what you see (if not explain it). Cecil did a very apt job in explaining the role of friction on a surface gravity wave. Yes, a large wave train hitting an island will tend to wrap around, but not due to any interal cohesion. The force of friction (from the seafloor) will induce a change of angle for the incoming wave.

To jjimm: Yes, the effects on larger waves will be more noticable. The effect will also be felt further offshore. However, the shorter wind generated waves will still feel the effects of friction and will turn towards shore. (I have to say shorter because most of the larger swells are also somehow wind generated.)

Here’s a good picture of the phenomina: linky
I was disappointed in one thing in Cecil’s column: I couldn’t find any errors to call him on! :wink:

-geek
(who is turning in his thesis in physical oceanography tomorrow)