Roll Waves at Myrtle Beach
January 30, 2007
These waves were photographed in the Spring of 2006, just south of Myrtle Beach, South Carolina, as the stream on which they formed moved down a channel, gently sloping away from the beach. A uniform stream of water may or may not be stable; that is to say, small disturbances which form on the surface may decay or grow. The latter will occur (under mathematically "ideal" circumstances) if the speed of the stream exceeds twice that of the waves on the surface (relative to the stream). Then these "long" waves (relative to the stream depth) will increase in size. This condition is equivalent to the slope of the stream being four times the non-dimensional drag coefficient of the channel, and is rarely satisfied in rivers. However, on roads, spillways and other man-made channels, this condition commonly occurs, due to the relative smoothness of the surface. On the photo above, the stream bed was composed of tiny grains of beach sand, and the nearly regular spacing of crests arises from the competing effects of gravity and bottom frictional drag. Such patterns are readily observed on sloping roads after a heavy downpour.