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Natural Science Forum / Earth Science / Oceanography / April 2004??? about wave action on inland lake
Hi, I have a boat on Lake Lanier in GA. The other morning I took it out, and the overall wave action appeared to be different than usual. It was a flat calm day, except when a passing boat produced some waves. Most of the time the waves seem to be sort of peaked and a bit sharp, but on this morning they were more rounded and flatter. The first boat I noticed it from, I thought it might be simply due to the shape of the boat itself. But after a while I noticed it was true of all of them...which was cool with me because it meant I could go faster without getting tossed in the air too much. It still made me wonder *why* though. Could it have something to do with what creates high and low tides, and/or why sometimes when you dig a hole you seem to have more dirt than you took out and sometimes you seem to have less? If so, is there a way to predict when waves will tend to be sharper and when they will tend to be flatter, like people can predict high and low tides? | Hi, | [quoted text clipped - 13 lines] | will tend to be sharper and when they will tend to be flatter, like | people can predict high and low tides? Possible causes are change in depth of water, change in viscosity and oil slick. Alert the EPA. Androcles Dear dh_ld: > Hi, > [quoted text clipped - 13 lines] > will tend to be sharper and when they will tend to be flatter, like > people can predict high and low tides? Wave speed is a function of water depth. Wave shape is a function of dispersion (how far away the source is, temperature of the water), wave speed vs. "boat" speed, and what is happening in the air. I'd say the boats were just moving slower, which would not require as much "cresting". David A. Smith Hi, Dave, > I have a boat on Lake Lanier in GA. The other morning I took it out, > and the overall wave action appeared to be different than usual. [quoted text clipped - 11 lines] > will tend to be sharper and when they will tend to be flatter, like > people can predict high and low tides? The following may not be the explanation of what you are seeing, but could be worth bearing in mind: Monolayers of surface-active materials can significantly dampen out waves, and they're essentially not observable otherwise. It's related to the old saw about pouring oil on troubled waters. This was known in Antiquity (there is a reference in the works of Gaius Plinius Secundus, Pliny the Elder, in AD 77). "Oil" in this context is not mineral oil, which is essentially all hydrocarbons, but vegetable oil, which is largely unsaturated long-chain fatty acids. A little oil goes a long, long way, as found by Benjamin Franklin. In his letter to William Brownrigg dated Nov. 7, 1773, he recounts that a teaspoonful of oil calmed an area of about half an acre. I was put onto this by a lecture I heard some years ago about a guy responsible for a sewage-treatment pond. Suddenly, the sewage wasn't breaking down as fast as it used to, and this was creating problems. He tracked it down to a surface monolayer which was slowing down the diffusion of oxygen. He had photos showing a very clear demarcation in the wind-induced roughness of areas covered by monolayer and areas which were clear. While I haven't ever had the equipment to go into this scientifically, I've since often noticed vast differences in the choppiness of a water surface when the wind is blowing, with a very clear demarcation between them, and I think this must be due to monolayer coverage. The lecturer eventually got access to analytical equipment sensitive enough to identify the material in his monolayer. He was expecting it to be biological in nature, but it turned out to be oxidized motor oil which someone had disposed of by pouring down the toilet. Cheers, Zigoteau. |
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