SAND RIPPLE GROWTH IN AN OSCILLATORY -FLOW WATER TUNNEL 



by 

 Karl E.B. Lofquist 



I . BACKGROUND 



1 . Introduction . 



Oscillatory surface waves commonly produce ripples on a sandy 

 seabed. These ripples, on the one hand, increase the roughness of the 

 bed and so increase the bottom stress and the rate of energy loss from 

 the waves which produced them. On the other hand, they enhance the 

 agitation of the sand and so are an essential part of the mechanism of 

 sand transport. When the oscillatory water motion over the bed is able 

 to move sand grains along the bottom or to lift them above it, if only 

 during a part of the cycle, a superimposed steady current, even though 

 weak, is able to move sand consistently in one direction. Ripples 

 enhance the movement and lifting of sand grains, particularly near their 

 crests where water velocities are amplified and where the sharp curva- 

 ture of the profile serves to separate the flow and the moving grains 

 from the bottom. 



The involvement of these effects in coastal engineering has en- 

 hanced interest in oscillatory-flow ripples, which have claimed much 

 study. Nevertheless, these ripples remain poorly understood. This is 

 in large part because observations remain inadequate to define the roles 

 of the many factors involved. Also, the import of observations is some- 

 times obscured by diversities in technique and, less often, by apparent 

 discrepancies. This study describes a series of laboratory experiments. 

 Its purpose is to add to available observations with special attention 

 to ripple initiation and growth. 



Observations and discussions of ripples usually concern two- 

 dimensional bed forms and one or another of several stages in ripple 

 growth from an initially level bed. These stages are: 



(a) Initiation of grain motion; 



(b) possible regimes of rolling grains or "rolling-grain" 

 ripples; 



(c) emergence of "vortex" ripples, spontaneously or from 

 rolling-grain ripples; 



(d) subsequent growth of these vortex ripples; 



II 



