(Blackman and Tukey, 1958, p. 70), and the confidence limits for the analysis 

 suggest that the.y are real. The minor peak, labeled E, with period of about 

 25 to 30 seconds, also coincided (a) with the observed periodicity in the 

 groupings of high and low waves in the surf zone, as measured from the ana- 

 log record of the waves, and (b) with the pulsation period of rip current 

 i ntensity. 



The same array geometry as Figure 16, using mini-digital staffs, should 

 provide useful information in model wave basins. 



2. Mechanics of Sand Transport by Waves and Currents 



Some preliminary results have been determined from an underwater 

 photographic study of the motion of sand grains near the bottom. The 

 arrangement of instruments is shown in Figure 17 and includes: synchronous 

 timing lights mounted on a rectangular grid, digital wave staff for surface 

 wave heights, and vi brat ing-wi re transducer for wave pressure at the bottom. 



The photographic analysis of sand motion near the bottom and wave- 

 induced oscillatory currents is keyed directly to the data acquisition 

 system by means of the synchronous timing lights. The trajectory of water 

 particles near the sand bed is determined by photographing the motion of 

 neutrally buoyant particles against a reference grid as shown in Figure 12. 

 The height of the sand suspension is estimated directly from the photographs, 

 pending incorporation of a sensor for concentration of suspended material. 



Figure 18 illustrates the analysis of a typical photographic sequence. 

 The surface wave profile is plotted (top) by the computer from data provided 

 by the digital wave staff with the pressure sensor as a backup tool. That 

 portion of the plot from 180 to 191 seconds has been analyzed photographi- 

 cally, and the horizontal component of the orbital water velocity is pre- 

 sented (center) and related in time to the height of the suspended sediment 

 above the sea floor (bottom).- This particular sequence shows a maximum 

 sediment suspension during the time of maximum acceleration and deceleration, 

 rather than during the time of maximum orbital velocity. Suspension of 

 sediment during this phase of the orbit appears common for conditions where 

 the orbital velocity does not exceed about 50 cm/sec. At higher orbital 

 velocities, the sand is observed to more as a sheet flow within the layer 

 of near bottom shear dispersion, with maximum suspension during maximum 

 orbital velocity. 



33 



