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Old 31-10-2009, 16:33   #1
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Challenge: Explain the Physics of Wind Over Tide

I just had one of my Navigation students ask me the physics of the phenomena of wind and tide in opposite directions causing short, steep waves. I cannot explain the physics but know it to be true. Anyone know the physics involved?
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Old 31-10-2009, 17:34   #2
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Not sure how there are physics to do with this? Diurnal wind (day/night cycle) happens on a thermodynamic principle, and the tides are from the gravity of the moon and to a lesser extent the sun.

Open ocean currents tend to drive the wind so they're usually together. It's pretty easy to have localized disturbances in wind, but current stays pretty true.

They work in unison often in the deep waters, but localized and at the coasts they are pretty independent of each other.

Am I missing something?
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Old 31-10-2009, 17:36   #3
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Maybe you're talking about wind driven waves acting against the prevailing current? That generates some ugly water that I would love to have nothing to do with if I could choose that option.
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Old 31-10-2009, 17:49   #4
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To clarify, I am thinking specifically of the straigh of Georgia, B.C. When, in summer you get prolonged N.W. winds of 25 knots. When the tide is flooding, it is flowing N.W. at about 2 knots. During the flood, you get short period, steep, ugly waves. When the tide changes, flowing in the same direction as the wind, long smooth rollers.
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Old 31-10-2009, 17:50   #5
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Anyone know the physics involved?
OK, a real example. Winds blowing from the west down the Potomac gusting 35 to 40 knots. Tide going up the Potomac at about a knot or so. Waves are 6 to 8 feet. As soon as we crossed the southern point (Smith Point) waves drop 2 deet instantly.

Wave height is based on wind speed plus fetch. The fetch was nearly 50 nm. The resitance of the current makes the waves stand up and the frequency drop. It is a double edge sword. The waves are very steep too. The steep waves on a short frequency are quite a handful.

You can find good examples in places where the tides are quite high and / the winds strong. A gale on a long fetch against a 5 knot current might be your worst nightmare. Add shallow water 40 miles off shore and you have Cape Hatteras vs the Gulf Stream.
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Old 31-10-2009, 19:06   #6
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Hi Rex,

I would think most research on this would have been done on the Gulf Stream and its storms.
Thats in a very heavily populated area for ships that can b quite dangerous because of the wind against current effect.
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Old 31-10-2009, 19:28   #7
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It's fairly easy to understand. In an easy anchorage like in Daytona Beach south of Memorial Bridge. Very sheltered. Light breeze, under 10 knots. Sea surface very smooth. A few hours later, one foot waves. Same conditions except the tide has changed and now is against the wind instead of with it. In another six hours or so, if the wind doesn't change, it'll be quiet and smooth again.
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Old 31-10-2009, 19:44   #8
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Agreed, we have all seen it but what are the forces involved? I can explain it or describe it to students but this one wants to know the physics involved and that I cannot explain.
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Old 31-10-2009, 20:04   #9
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Overfalls

This happens quite dramatically at the Golden Gate entrance to the San Francisco Bay when tidal currents of 5+knots oppose 30 knot winds. The waves that are created seem to just stand up and stay in place. They are quite steep and very close together. It was once explained to me as being caused by the friction of the wind on the water. I was told they were called overfalls.
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Old 31-10-2009, 20:38   #10
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Don't know physics but considering the closing speed difference of the tide with the wind and the tide against the wind along with going against the grain (kinda like combing your dogs fur backwards), it's not surprising.

I'll be watching for the scientific explanation.
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Old 31-10-2009, 20:59   #11
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I am not a physicist but what Liam Wald just described happens all over the world I suspect.
Here on LI, NY when the Great South Bay ebbs into even a rather lazy 3 - 4' Atlantic ocean swell the swell gets bigger, and bigger where there are breaking waves in what is now a tidal race. I saw 10' + swells leaving Fire Island Inlet on an ebb tide of about 4 knots without much wind at all. If there had been a wind opposite the swells they would have been slowed down with their tops being blown off and have become much closer together, piled up and dangerous.
Current opposing prevailing swell causes waves to stand up and be more menacing. I can't explain the hydrodynamics or physics but I know it to be true. It is measurable but I can't explain it by more then intuition and experience. I can't explain standing waves in the East River or Plum Gut but that seems to be a common occurrence along with whirlpools in the right conditions.
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Old 31-10-2009, 23:58   #12
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I think CalebD is spot on.

Remember that basic wave motion is transfer of energy through the linked movement of water molecules moving up and down, not in the apparent direction of the wave train.

A wave train removed from its wind source (e.g. an ocean swell caused by a distant weather pattern) should theoretically have the same basic form regardless of underlying current, but the apparent velocity of the wave train would be adjusted by the velocity of the current, as the water molecules moving up and down in the wave train would also be moving horizontally with the current.. Hence you can end up with standing waves when these two velocities cancel out.

If you now add direct wind where the waves are, this provides a head-on force to the windward face of the waves - just like they are hitting a (soft) brick wall, and the windward face gets consequently squeezed towards the leeward face. Hence the short, sharp waves. This could also affect the period, as the wind compresses the waves onto one another.

That's my first-year fluid dynamics theory anyway. I'm sure there are far more qualified people who can correct me!
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Old 01-11-2009, 03:06   #13
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Never thought about it but one might cast the problem in terms of the doppler effect! Caused by either a wave source or reciever moving relative to each other. Essentially the frequency of the wave train appears to either increase or decrease depending on the relative motion. Higher frequency yields steeper wave for the same amplitude (wave height.) In this case wind coupling to the moving fluid gives the same type of effect.
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Old 01-11-2009, 03:30   #14
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Originally Posted by LakeSuperior View Post
the doppler effect!
But if it was dopler effec there would be flat patches too? Often when you see it its quite regular short waves, steep faced.
I prefer the friction thoughts. But its all out of my league
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Old 01-11-2009, 04:56   #15
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I will try. Imagine a wave moving right to left. Although in a sense the water doesn't move there is a circular motion involved running counterclockwise. That is the water in front of the wave moves back. The current adds to the speed of that and that extra water builds up at the crest. Faster speed backwards gives greater volume which has to go somewhere. The other part is that as waves enter the shallows they slow and again increase in height and break, kind of the back overtaking the front. This is maybe the opposite the front being increased.
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