New Sediment Trap - A modified verision of the multi-sock trap ha s 

 been built and preliminary calibration begun. It differs, from the older 

 model chiefly in that the frame is demountable and the legs more mdely 

 spaced. Two more traps of this new design are being constructed. Several 

 such traps may then be placed in operation to get simultaneous observations 

 at different water depths. 



Ripple I'.lark Observations - Ripple marks were observed in some detail 

 by the divers in connection with sediment movement studies. The follov;ing 

 observations apply to fine sand bottom outside the breaker line: 



1. Ripples are the result of orbital motion of waves causing 

 a current which near the bottom is resolved into a long forward and back- 

 ward horizontal movement. 



2. Such a movement can be considered as two separate currents. 



3. These currents do not oscillate in the same sense that small 

 waves oscillate in a lake or pond, because with each reversal a new set 



of ripples is developed. 



4.. Therefore, the ripples observed are essentially "current" 

 ripples rather than "oscillation" ripples. 



Life Cycle of a Ripple Mark ; Starting from a condition of no motion, 

 i.e., no current and a ripple mark or roughness of bottom in existence, the 

 initial movement causes a transfer of sediment on the crest from the steep 

 up-current side to the down-current side as a flap. This is called the 

 initial flap motion; its result is to alter the steep side into the gently 

 sloping side of the ripple. As the current increases, sand is carried past 

 the crest and an eddy is set up in the trough of the ripple. The ripple 

 now appears to be rolling along with the current. ?/hen the current reaches 

 its peak velocity all recognition of the ripple mark as a distinct structure 

 is gone; the sand is moving in long horizontal streamers or as a blanket 

 here called a "sheet flow." ?iihen the current velocity lessens, the process 

 is reversed. First, the sand appears to be rolling, then the ripples 

 begin to form, and the final stage is the flap motion which represents the 

 last movement of the current. For an instant between trough and crest 

 velocity the ripple is stationary^ then the process is repeated in a reverse 

 direction. The return current and ripple movement complete one cycle of 

 orbital motion, i.e., one wave period* 



With rather constant wave conditions, and especially with high orbital 

 velocities, the ripple patterns on the sea floor, doTm to depths of 60 feet, 

 have a very even and symmetrical appearance, although the ripples are des- 

 troyed and reformed each time a wave passes. ¥i/hen wave action decreases 

 these same well-developed ripples, which sometimes have unbroken crest 

 lengths of 20 feet, begin to decay and become irregular. Concomitantly, 

 organisms which previously could not affect the sea floor, due to the strong 



38 



