Off-Topic: Wave dissipation

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	Name:	MartiniMan -
	Subject:	Wave dissipation
	Date:	5/8/2008 9:58:27 AM

Answer to your question about dissipation of wave amplitude. The amplitude of a wave is directly proportional to the energy that created it. As the wave migrates from the point of creation (known as propagation) it diminishes in amplitude as a result of several factors. The biggest factor is dispersion. The same energy is dispersed over a larger area as it propagates. Dispersion also caused when the wave impacts the shore (along with a certain amount of absorption of the energy). Rip rap is extremely effective because it disperses the available energy in multiple directions and when two waves of equal amplitude but opposite phase hit each other the energy is cancelled. If it hits a concrete sidewall some of the energy is absorbed but most is reflected and the amplitude is not reduced as much. Friction also occurs with water although it is usually a small factor when compared to the overall energy of a wave.

The fact is if you measured the amplitude of a wave propagating from the back of a boat it will diminish as it moves but the reduction would not be noticeable over short distances. So in a slough there would be minimal loss of amplitude from dispersion from the spreading of the wave and friction and most of the energy makes it to shore. A boat making a big wave on one side of an open area of the lake will not have as much impact because of the reduction in amplitude over a greater distance.

Finally, a M.Sc. in Geophysics paying off! By the way, lest you wonder about a tsunami traveling across the Ocean these same factors are in play. What is different is that the amount of energy created by the movement of a 10,000 foot column of water is amazing. Not a big deal when that energy is spread out over the 10,000 feet (only enough to make a surface wave of 6 inches) but when all that energy is piled up in shallow water it becomes a 10, 20, 30 meter pile of water moving at high speed.