744 LAFOND [CHAP. 22 



least 10. Individual streaks or bands varied in width from an estimated 75 to 

 600 ft. Their orientation was always parallel to the coast, which is the tendency 

 of the prevailing drift. 



As a ship cruised through the bands, it was strongly set to the right or to the 

 left, depending on whether it was in the rough or smooth band. Similar pheno- 

 mena have been observed off southern California. By determining the thermal 

 structure, it was found that the internal wave crest occurred directly under the 

 roughest zone. From the surface currents, and the thermal structure and its 

 progressive motion, it was concluded that a shallow internal wave was causing 

 the phenomenon. 



According to Lamb (1945), the vertical displacement at the interface of a 

 two-layer density system, p and p', is given by 



7] — a cos {kx — at) 



and the horizontal velocity of flow, u', in the upper layer is 



u' = — {a I h')c cos {kx — at), 



where h' = average thickness of the upper layer, a = amplitude of wave at inter- 

 face, and c = wave velocity at interface. 



If there is no appreciable flow in the y direction (normal to propagation) and 

 no appreciable transport of surface water in the direction of propagation, the 

 same volume of water per unit width must pass over the trough. The speed of 

 flow in a horizontal direction must be inversely proportional to the thickness 

 of the upper layer Z'. 



Z' = h' — a cos (kx — at). 



This indicates that u' is maximum, and in the opposite direction of propa- 

 gation to c, when h' approaches a and the phase angle (kx — at) is zero. 



In shallow internal waves, the motion over the crest is strong. The water 

 formerly passing over the trough is now funneled through this constriction. If 

 the crests are very near the surface, the speed of flow is increased. The funneling 

 of water over the crest, and the reduced speed just beyond, are believed to 

 cause the turbulence and ripples at the surface. 



When an internal-wave thermal boundary is near a sea floor, a similar action 

 ensues. If this occurs, the maximum turbulence will be under the trough but 

 the maximum speed will be in the opposite direction of wave propagation. In 

 shallow water, the internal wave direction is shoreward, and thus the maximum 

 speed near the bottom will be off-shore. The funneling of water through the 

 constriction created by the trough and bottom, always in an off"-shore direction, 

 is undoubtedly a contributor to the off-shore movement of sediment. Internal 

 waves near the bottom in deep water can also move sediment and form ripple 

 marks. 



