Standing waves

[PeterB]

I continue to be mystified by the hydrology of standing waves in tidal races. There’s something about paddling down a wave while actually moving backwards which short circuits my analytical abilities.

SO:

Why do standing waves fall/break against the current (what I would call the back side of the wave) , rather than with the current (what I would call the front side of the wave) ?

If this is too dense or complicated to explain in this forum , can anyone offer a good reference(preferably with illustrations) that explains to the layperson the hydrology of tidal races?

The following is an example of an explanation that I would consider not helpful:

“Oscillations of the water surface are common features of flows around critical conditions (Fr _ 1) and of both natural and structure-induced undular hydraulic jumps. The dimensionless wavelength and wave amplitude were correlated to the Froude number at the sill. The first wavelength appears linearly related to the Froude parameter, whereas the amplitude of the first wave shows a less defined behavior, possibly indicating that a maximum value occurs for sill Froude numbers around 1.6–1.7. The corresponding wave steepness (i.e., wave amplitude/length) results between 0.15–0.16, and this range also characterizes the point at which the dimensionless trough depth below the sill achieves a local maximum, probably related to hydrodynamic conditions just before wave breaking."

[Rick crangle]

While in Scotland on Loch Eishort this example dawned on me as we made way back from a day of wandering. I immediately found it to be the visual key to understanding standing waves. If you are paddling along and lay your blade flat on the water beside your moving boat, then, press down on the trailing edge, you create a void in the surface. The void is established by directing the water under the blade at the leading edge, and then, forcing it down to a new level at the trailing edge. At a point in the creation of this hole in the water level, there is an attempt to fill this void by water directly behind the hole toppling in. However, since the boat you are in is moving forward the hole behind the blade is also moving and can not be filled. The toppling water has formed a standing wave moving along with the blade creating the hole in the water. Now, think of the boat motion as current, and the hole in the water as the initiation of a void due to surface disruption, say a sudden drop in the subsurface terrain. As long as the void remains fairly constant in the flow, the wave falling into it will remain, as it tries to fill. Now to complicate things, say that there is a swell coming from the direction of the falling wave, against the current flow. As the swell moves through the site of the established standing wave, those two distinct forms will match frequency and amplify, and move forward. The standing wave will reestablish itself and the new more intense wave will continue at a distinctly more powered state. This is the condition we use to surf forward on and against a current. A standing wave can be dropped into and surfed with the flow of current below your boat and the boat riding on the cascade of water into the void. Does this make sense? Try it.

[Nick Schade]

Standing waves are not caused by current alone. In a long straight channel even a very fast current will not produce waves. Typically standing waves are produced when there is some sort of obstruction in the current. The water has to accelerate to get around the obstruction and then decellerate after the obstruction. The waves are the result of the deceleration. The waves are the way the water sheds kinetic energy as the water decelerates

One way to think of the waves is as the wake caused by the obstruction. If you follow behind a speed boat you will notice a series of waves traveling in the same direction and at the same speed as the boat. The boat is an obstruction moving across stationary water. Now make the water move, submerge the boat stationary below the surface and you will have standing waves. Standing waves are produced by the same mechanism that makes a boat wake.

Actually most of the waves we see offshore when current passes over a reef or through a channel are not standing waves, but regular wind waves that get bunched up as they hit the faster moving water near the obstruction. These waves will travel up towards the obstruction and aren't "standing" like you will see in white water.

[JohnHuth]

I'll give it a try...

Standing waves arise when you have a change in the velocity from the surface of the water to the bottom. At the bottom, the drag of rocks and such is strong enough so that the water right at the bottom is nearly at rest. On the surface, the current is the largest.

When the change in water velocity from the bottom to the surface is large enough, it creates standing waves. You can think of these a bit like eddies that are circulating vertically, as opposed to horizontally.

In waves, you can think of little parcels of water undergoing circular motion. Under the right conditions, the circular "orbit" gets broken and the waves break. This is what happens when swells feel more drag on the bottom as they reach a beach. The same thing happens with standing waves, except the origin of the waves is in the change in water velocity from the bottom to the surface.

You can also see this phenomenon in clouds. There are some cloud formations called "roller" where it looks like the clouds are forming a bunch of cylinders. This is the same kind of thing happeneing, where the air velocity varies with altitude and creates standing waves (or eddies) in the air. Inside the rollers, the air pressure is lower and water vapor condenses. This is usually a sign of an approaching warm front.

The "Froude number" is a dimensionless number (that is - no units like meter/second) that's ratio of the inertia (momentum) of a little parcel of water to it's weight. Large Froude numbers (I think...) would indicate more of a tendency to break because the water's moving faster and sort of overtakes itself. I wouldn't run around talking about Froude numbers to new kayakers, however.

[PeterB]

Thanks for three informative replies, which I cannot absorb all at once, but have printed out and will take a few days to chew over.

It would seem that tidal races and standing waves would tend to be more pronounced on the ebbing tide, as they are more likely to be affected by incoming/opposing swell, and an ebbing tide is usually stronger than a flooding tide.

E.G. At Popham, I have paddled twice in the standing waves next to Pond Island, just out from the mouth of the Kennebec River, both times on the ebbing tide . Would that same spot be not as active on the flooding tide?

[EEL]

> It would seem that tidal races and standing waves would

>tend to be more pronounced on the ebbing tide,

And as a related question, would they also be more pronounced on the from half to the lower third of the ebb tide as well?

FWIW, when we went out to the Sequin Island light there was a stretch of rougher water that must have been caused by the ebb out of the river hitting swells and wind. We were well over a NM off the mouth so that is some strong ebb current at Popham.

Ed Lawson

[JohnHuth]

There's a phenomenon that happens when you're in a region where current varies substantiall from place-to-place. This can cause waves to be magnified in some areas.

Typically, a wave moving against current will get more magnification, so, yes, incoming swells on an outgoing tide would tend to have more of this.

[rcohn]

>

>> It would seem that tidal races and standing waves would

>>tend to be more pronounced on the ebbing tide,

>

>And as a related question, would they also be more

>pronounced on the from half to the lower third of the ebb

>tide as well?

I believe that would be caused by a combination of the velocity of the current and the depth of the water. Current velocities are highest during the middle of the flood or ebb tides. Standing waves are a function of the water flowing over the local topography, which has greater influence as the local depth decreases. Shallow water waves (swells) are also affected by the decreasing depth (velocity = sqrt(gravity depth)) as the wave slows down, the height increases until the breaking point is reached.

Ralph Cohn

Black rolling SOF

White (Mark Starr) SOF

Elaho DS, blue & white with yellow trim

[Suz]

Peter,

Have you absorbed it all yet?

You asked if on a flood the place near Pond at Popham would be as active. The answer is no it won't be.

Remember the standing waves are formed by the current which on an Ebb is the tidal movement plus the river current. The flood is tidal movement against the river and therefore it won't ever be as fast on a flood as the ebb in that location.

For a local example of how standing waves form go to the mouth of the Merrimac River. From Salisbury Beach reservation, you can watch right from land, don't even have to be in your boat to see it. Observing the mouth at different stages of the ebb really allows you to predict how it will be during different conditions.

I've found that it gets the most interesting when the current is moving the fastest. It moves the fastest at max ebb just after a full moon. Best conditions are found when their is a minus tide at low tide ~ biggest tidal range.

You can have fast current but if there is no incoming swell or wind against the current, nothing will happen.

If you go there and paddle, beware that if you launch before max ebb, things will change and it could become a challenge to get back through the mouth if you go out.

If you can handle the waves, it's actually much easier to come back up the mouth when there is swell as you surf the swell against the current to make your way with very little effort.

Suz

[Bob C]

Hi Peter,

I'll draw diagrams on napkins (standard engineering practice) next time we go the Agawam to show how the standing waves are nature's way of preventing a discontinuity in fluids flowing in opposite directions (i.e., tidal current vs. river current). ;-)

[PeterB]

Thanks,

Have I absorbed it all yet? No. For a while, I was curled into a fetal position, overwhelmed with hydrological overload, but I am absorbing, slowly. Have these posts been helpful? Yes.

Bob, looking forward to drawing on napkins with you, perhaps at the Agawam Diner after an Ipswich pool session.

Do I understand that opposing swell, wind, waves etc. is an essential ingredient in the creation of standing waves? So, that race next to Pond Island at Popham , at max current on a full moon, could conceivably be inactive, with no standing waves if there were no incoming swell or opposing wind waves . If so, in that case, I would imagine just a swiftly moving current, like a river.

[Pintail]

John, I have never heard the term "roller cloud" before: are you referring to lenticular clouds? If so, their formation has nothing much to do with an advancing air mass, but rather they form orographically when saturated air is forced up over a mountain (and down again) or another obstacle (example of the latter: around the front end of an active, moving thunderstorm). It seems to me that they do tend to form very locally and in very stable conditions. (Perhaps you are referring to some other type of cloud, though?) They are beautiful to see and come in all shapes and sizes, but they always have a regular, smooth shape wherein you can see the local air movement.

[bob budd]

I think he's referring to rolling clouds.

http://weathersavvy.com/Q-Shelf_Roll_Clouds1.html

[rcohn]

>Do I understand that opposing swell, wind, waves etc. is an

>essential ingredient in the creation of standing waves?

>So, that race next to Pond Island at Popham , at max

>current on a full moon, could conceivably be inactive, with

>no standing waves if there were no incoming swell or

>opposing wind waves . If so, in that case, I would imagine

>just a swiftly moving current, like a river.

No, water flowing over obstacles is sufficient to create standing waves. Surfing standing waves on rivers is an aspect of whitewater kayaking where no wind, swells, or traveling waves are involved. I don't know anything about the race next to Pond Island or how conditions there would change there based on local wind and sea conditions.

Ralph Cohn

Black rolling SOF

White (Mark Starr) SOF

Elaho DS, blue & white with yellow trim

[JohnHuth]

>John, I have never heard the term "roller cloud" before: are

>you referring to lenticular clouds? If so, their formation

>has nothing much to do with an advancing air mass, but

>rather they form orographically when saturated air is forced

>up over a mountain (and down again) or another obstacle

>(example of the latter: around the front end of an active,

>moving thunderstorm). It seems to me that they do tend to

>form very locally and in very stable conditions. (Perhaps

>you are referring to some other type of cloud, though?)

>They are beautiful to see and come in all shapes and sizes,

>but they always have a regular, smooth shape wherein you can

>see the local air movement.

They're altocumulous clouds, not lenticular. Altocumulus is a name that has to do with both height (alto) and the puffiness (cumulus). Not all altocumulus clouds are "rollers" - "rollers" are a specific kind of altocumulus that forms when there is a large variation in wind velocity with height, causing horizontally rotating "eddies" in the air. The use of the term "roller" is not widely used, although the cousins "makerel scales" for cirrocumulus and "mares tales" for cirrus are more common. For more taxonomy of clouds (including lenticular) - I think that some slides I showed from a weather class I gave in January this year are still around (?)

Typically altocumulus clouds are the sign of a warm front encroaching on a cold front.