Mean Flow Depth 

 0.25-0.40 m 



Median Sediment Size, mm 



Figure 24. Velocity-grain size relationships for subaqueous bed forms (from Ashley (1990)) 



from planar to wave-like. The water waves may grow until they are unstable 

 and break. As the sediment antidunes grow, they may migrate upstream or 

 downstream, or may remain stationary (the name "antidune" is based on early 

 observations of upstream migration). 



e. Velocity - grain size relationships. Figure 24, from Ashley (1990) illus- 

 trates the zones where ripples, dunes, planar beds, and antidunes are 

 found. This figure is very similar to Figure 11.4 in Grafs (1984) 

 hydraulics text, although Graf uses different axis units. 



Use of subsurface structure to estimate flow regime. Several useful 

 indices of foreset laminae, which may assist in making qualitative estimates of 

 the strength of currents in modern and ancient sediments, are given by Jopling 

 (1966). These include: (1) maximum angle of dip of foreset laminae (at low 

 velocities the angle may exceed the static angle of repose, whereas at high 

 velocities the angle is less than the static angle); (2) character of contact 

 between foreset and bottomset (the contact changes from angular to tangential 

 to sigmoidal with increasing velocity); (3) laminae frequency measured at 

 right angles to bedding (there are more laminae per unit area with increasing 

 velocity); (4) sharpness or textural contrast between adjacent laminae (at 



54 



Chapter 3 Field Data Collection and Observation 



