A Swell Way to Look at Waves

Winter 2004/2005

As they move through water, ships of all sizes create waves - a seemingly harmless, sometimes beautiful action. Recently, however, researchers have become increasingly aware of environmental problems associated with ships operating in shallow coastal regions.

One such problem is that ship-generated waves can significantly contribute to shoreline erosion problems. In addition, loose or soft bottom materials transported by these waves can harm marine life in environmentally sensitive regions.

Unfortunately, the impact of waves generated by ships is difficult to assess and depends on a variety of factors, including ship size and speed (as well as ocean or river bottom topography and soil conditions.)

For example, in shallow water, waves interact with the bottom of the ocean, which can affect both the wave amplitude (height) and the propagation (motion) direction. Consequently, a ship will generate a substantially different wave pattern in shallow water than in deep water (most of the prediction and analysis of ship-generated waves has focused on the deepwater case; extending computational methods to shallow-water operations can be problematic.)

In his ESTC Award-winning paper, however, SAIC's Carl Scragg discusses his novel method for predicting waves generated by ships moving in shallow water. The method determines the spectral representation of waves, a way to analyze the wave field in terms of the energies contained in each of the different wavelengths. (Once you determine the wave spectrum, you can use it as the basis for analyzing the impacts of operating a ship at reduced speeds or increased distances from environmentally sensitive areas.)

With Scragg's method, you can calculate the waves anywhere, without using giant computers. (One of the problems with calculating shallow-water waves is that they are computationally very intensive.)

"The nice thing about spectral components is that once you understand how much energy there is in each wavelength, then you have a very straightforward way to calculate what the waves would be like at 50, 500, or 5,000 meters from the back of the ship or from the sides," said Scragg. (Environmental damage generally occurs far from vessel and shipping channels.)

In fact, Scragg said that one of the unique aspects of his method is that it works for both deep and shallow water cases. Specifically, it involves finding what the waves generated by one infinitesimal disturbance moving at a set speed and depth would look like, and then finding a collection of these disturbances that match the wave field generated by a real ship.

Scragg's paper, "Spectral representation of ship-generated waves in finite-depth water," appeared in Journal of Offshore Mechanics and Arctic Engineering.

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