NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Wind drift, was: DR thread from Nov-Dec '04
From: Trevor Kenchington
Date: 2005 Jan 20, 09:50 -0400
From: Trevor Kenchington
Date: 2005 Jan 20, 09:50 -0400
Bill, > 1. If I understand it correctly, tides can be amplified by what I > will term the "natural frequency" of a basin. Tides are totally different because the driving force is periodic (and very regular in its period). To grossly over-simplify: The Moon pulls on the North Atlantic, dragging a bit of it into the Bay of Fundy. As the Moon's pull reverses, the water in the Bay sloshes back following the natural period of that basin. It is just ready to slosh back in again as the Moon once again drags water that way, so the resonant "in sloshing" coincides with the inward flow dragged by the Moon. Hence the amplification. A little bit of energy from the Moon twice a day gets captured within the natural sloshing of the Bay and the total energy within the system builds up, creating some exciting tides. Wind acting over a lake is different because the wind won't switch on and off, nor reverse direction, with any regular period, so it can't switch on and off with the period of any specific lake. Once in a while, you might see the wind blow, chop around to the reverse direction and then change once more at just the right time to reinforce the seiche in the lake but even a couple of well-timed changes would be a very rare event. The semi-diurnal lunar tide, in contrast, has been doing its thing twice a day for millions of years with only a very slow change in its period. > 2. The Annapolis Book of Seamanship states regarding seiches, "On > tideless lakes, strong wind-driven currents called seiches may > develop. The effect of wind-driven current is figured like that of > a tidal current." Even the best textbooks contain errors. The National Ocean Service's "Tide and Current Glossary" starts its definition of "seiche" with "A stationary wave usually caused by strong winds and/or changes in barometric pressure" and then goes on to supplementary details, including the equation for the period of a seiche (2L/gd in my copy though I think there is something missing due to the PDF lacking some symbol font). That first sentence of the definition is a bit misleading, since winds and barometric pressure cannot "cause" the stationary wave. The wave is driven by gravity (hence the "g" in the equation). The role of wind or pressure is to set up the water so that gravity can set it moving. The Annapolis statement that the effect "is figured like ... a tidal current" is a bit opaque. They may be trying to say that seiches operate as standing waves, which is also the form that tides take in most ocean basins. [That is the kind of standing wave which has one or more nodes, where there is no rise of tide, although there are strong tidal streams, with antinodes elsewhere which do have large amplitudes. Not to be confused with the standing waves seen immediately below a weir, where something that looks much like a wind wave remains fixed in relation to the weir as water races past -- which is the same as the stern wave of a planing powerboat seen by an observer on the boat.] > 3. If I understand your more complete explanation, that wind/current > piles up water at the leeward end of the lake, and there are two > probable outcomes once the fluid sets up (piles up?) on the leeward > end of the body. > a. A subsurface current in the opposite > direction attempts to "level out" the basin > b. The water remains > "set up" (piled up?) at the leeward end of the body. I think those are the two alternatives, while the wind continues blowing at the same strength. However, I don't recall reading a serious discussion of the issue. In reality, there will be some intermediate cases, of course, with some small subsurface counter-current but not enough to materially reduce the set-up. Plus there will be cases where the shape of the shoreline, combined with the direction of the wind, encourages a surface counter-current at one side of the lake -- much like a rip current forming on one side of a cove when wave transport sets up the water against the beach. > 4. By your definition of seiche, when the wind decreases/stops, > gravity once again prevails and the piled up water on the leeward > side of the body rushes to the former windward end to establish > equilibrium (level off), setting up a wave. Yes. Gravity pulls the water back towards level but that sets the water in motion, giving it momentum. The momentum carries the water past level and piles it up at the opposite end of the lake. Gravity then pulls it back towards level and so the process continues. However, friction swiftly bleeds energy out of the system and the wave dies away. You can demonstrate this quite easily in a bucket. (I use a small fish tank when teaching tides and related phenomena to Power Squadron classes.) If you are a real fanatic, you can amuse yourself while waiting for a restaurant meal to be delivered by creating seiches in your wine glass -- or better still the two-dimensional version called an "amphidromic system", which is the form that tides take in most real seas. Repeated experiments into the change in period as you reduce the depth of the wine by draining the glass can really add to the entertainment. > 5. If the basin is sympathetic, this "sloshing" may be rather > dramatic for a tideless body of water. No. All basins have their own natural period (based on length and depth, as in the equation given above, albeit in corrupted form). If the wind sets up the water surface and then dies away, there will be a seiche which will follow that natural period. "Sympathy" only comes in if you have a periodic driving force, such as the tides. When the period of the basin and the period of the force coincide, you do indeed get dramatic results, as in Fundy. That is going to be very rare in lakes. But given the right relationship of lake length and depth, the sloshing can be considerable _relative_to_ people's expectations that lake shores ought to stay in place. The little 2-metre tides on this side of Nova Scotia don't seem much for an ocean coastline but a sudden change of a metre either side of the normal level of a lake would cause serious problem to many people living along its shore. > I feel you have adequately dispelled the heartland folk lore > concerning why a seiche occurs, particularly for a lake that is > relatively shallow at its southern tip and would have thermoclines. > > Given I may have made some logical leaps above, but the definition > of seiche remains a wee bit unresolved for me. It is either: > > A two line reference stating it is a wind driven current. > > A knowledgeable explanation stating it is akin to a standing wave, > AKA sloshing in a basin after the wind dies. > > I apologize for the second degree. A lawyer would say, "asked and answered." > I am attempting to "get it right." > > I hope my summary is close to accurate. Friends might claim > decreasing my sphere of ignorance is thankless. They are > incorrect. Thank you. No problem. Hope I have clarified things a bit further. Trevor Kenchington -- Trevor J. Kenchington PhD Gadus@iStar.ca Gadus Associates, Office(902) 889-9250 R.R.#1, Musquodoboit Harbour, Fax (902) 889-9251 Nova Scotia B0J 2L0, CANADA Home (902) 889-3555 Science Serving the Fisheries http://home.istar.ca/~gadus