(Fig. 28d) . For this control, a minimum of a pair of onshore navigation 

 targets to sight on and a transit to read cutoff angles to the sled are 

 required. 



b. Sensor Configurations . The optimum design for measuring the dis- 

 tribution of coastal currents along a horizontal profile should consist 

 of at least nine bidirectional ducted current meters, in groups of three, 

 and alined orthogonally to one another (Fig. 29) at elevations near the 

 surface, at middepth, and near the bottom. Each sensor measures the cur- 

 rent speed concurrently along the three axes and from these data the mean 

 vector can be resolved and velocity computed. Alternately, paired electro- 

 magnetic sensors may be used. Where vertical orbital velocities are 

 negligible (shallow water or high-frequency waves), pairs of B-10 meters 

 mounted horizontally at right angles have been used (Fig. 30) ; however, 



in the reconstruction of the vertical velocity profile, less than three 

 points in the vertical are inadequate. The wave gage is best situated 

 adjacent to the lowermost group of meters, thus assuring that pressure 

 records will be collected even with the least number of submerged current 

 meters . 



c. Operations . Common to all these objectives is the incremental 

 movement of the platform, usually commencing in deep water and ending 

 where water depth becomes less than 0.91 meter. The reason for not 

 carrying the measurements up to the dry beach is that the placement of 

 sensors below the upper rim of the cylinders would introduce a bias into 

 the records. The actual operation requires towing the sled to 9.14 meters 

 offshore, where the first record is usually obtained, followed by its 

 consecutive shoreward displacement to a predetermined location or pre- 

 determined water depth. In most operations the sled needs to be decoupled 

 from the towing vessel, especially in heavy swell. 



Minimum personnel required for continuous surveys where sensors are 

 used in fixed position throughout the experiment, include a LARC operator, 

 a deckhand to release the sled cables, a transit operator, and a data 

 station operator. However, standby divers must be available for emergency 

 underwater work; when surface currents are measured the divers are used 

 to readjust spars on the sled each time the water depth has changed. 



d. Data Collection . Several data recording options may be exercised 

 with each experiment (Table 3). A data file convenient to analyze should 

 consist of less than 2 12 words per sensor. Usually 5 to 7 minutes of re- 

 cordings per station along the shore-normal profile are adequate for ana- 

 lyzing that part of the record which contains the wave-induced currents. 

 Since the principal aim of such a study is to observe the spatial actions 

 without a change in the local wave, current, and wind climates with longer 

 recording times, the uniformity of conditions during the measurement 

 period may be foresaken. An example of the mean longshore velocities 

 along a profile is shown in Figure 31. The time-series analysis which 

 includes Fast Fourier Transform (FFT) programing is also used to calculate 

 the energy spectra for each sensor (Fig. 32). 



52 



