STRAIGHT 

 TRANSVERSE 



TRANSVERSE SINUOUS 

 IN PHASE 



(°) 



TRANSVERSE SINUOUS 

 OUT OF PHASE 



TRANSVERSE CATENARY 

 IN PHASE 



TRAVERSE CATENARY 

 OUT OF PHASE 



CATENARY 

 SWEPT 



nAnAAAA AAAAnriA yuuuuyy 



Y^nnf^nnr nn/WW^ uuuvwww 



r^r\r\r\r\nr\ nnnAOAA uuuvuyw 



current innnnAnf /TVWW^ uuuvuuv 



Figure 22. Plan view of basic ripple types (modified from Allen 1968) 



be straight crested and can be termed two-dimensional (Figure 23a). If 

 the flow structure varies significantly across the predominant direction 

 and vortices capable of scouring the bed are present, a three- 

 dimensional bed form is produced (Figure 23b). 



c. Plane beds. A plane bed is a horizontal bed without elevations or 

 depressions larger than the maximum size of the exposed sediment. 

 The resistance to flow is small, resulting from grain roughness, which 

 is a function of grain size. Plane beds occur under two hydraulic 

 conditions: 



• The transition zone between the region of no movement and the 

 initiation of dunes (Figure 24). 



• The transition zone between ripples and antidunes, at mean flow 

 velocities between about 1 and 2 m/sec (Figure 24). 



d. Antidunes. Antidunes are bed forms that are in phase with water sur- 

 face gravity waves. They resemble regular dunes, but their height and 

 wavelength depend on the scale of the flow system and characteristics of 

 the fluid and bed material (Reineck and Singh 1980). Trains of 

 antidunes gradually build up from a plane bed as water velocity 

 increases. As the antidunes increase in size, the water surface changes 



52 



Chapter 3 Field Data Collection and Observation 



