system. These meters are not without their own problems. For example, 

 the acoustic type is very sensitive to air bubbles in the water column to 

 make it relatively intractable in the surf zone without further develop- 

 ment (van der Graff, NSTS Workshop, Scripps Institute of Oceanography, 

 La Jolla, California, personal communication, February 1981). All these types 

 have not been extensively used in field experiments of surf zone currents, 

 to date. 



Articles by Kalaitis (1975), Appell (1977), and Mero and Appell (1977) 

 provide further discussions of dynamic calibration tests. Turbulence 

 effects are discussed by Bivins and Appell (1976). Other design shapes 

 are also being developed for the EM type (Crump, 1976; Aubrey, personal 

 communication, February 1981) that may lead to more uniform frequency 

 response characteristics. 



2. Measurement Systems . 



It can be reasonably argued that the scale of the phenomena observed 

 (or perceived) at the coast depends on the observation method. Sasaki 

 (1972) divided the coastal scale into (a) the spacing or rip currents, 

 i.e. a unit cell scale, and (b) the velocity field within a cell. He 

 then made (somewhat subjective) tables of desirable features and com- 

 parisons of observation methods for both scales. Color aerial photos 

 taken at low tide were judged best for the larger scale phenomena. 

 Horikawa and Sasaki (1972) also developed two measurement systems for 

 taking successive overhead photos of floats to quantitatively describe the 

 nearshore currents and rip currents at the smaller scales. One system, 

 SHIELS, used two hovering helicopters to take successive stereo photos 

 but was very expensive. The second, simpler and far cheaper method, 

 BAGS, uses a single balloon-borne, motor-driven camera but wave height 

 determinations are impossible. This system is still used today by the 

 Japanese. Figure 17 schematically illustrates the BAGS system. The pier 

 is used to tether the balloon and for workers to readily cross the surf 

 zone while tossing the drogues. Wind drift of the balloon between pictures 

 contributes measurement errors and limits use to calm days. They normally 

 photograph rip currents usually expected near piers. This could be a 

 valuable research tool but needs further development. Nonsurfboarding 

 drogue shapes, tethering without piers, and electrical systems to precisely 

 locate the camera at all times for position corrections must be developed. 

 An excellent discussion of the use of these remote sensing systems can 

 also be found in Sasaki (1977). 



Sled systems have been used extensively in the surf zone. They can be 

 instrumented heavily with wave staffs, pressure sensors, current meters, 

 sediment concentration devices, etc. Also, they can be winched out into 

 deeper water beyond the breakers and they follow the bottom contours. One 

 of the first sleds developed is described by Lowe, Inman, and Brush 

 (1972)16, Others that followed are reported by Teleki, Musialowski, and 



^^LOWE, R,L. , INMAl-J, D.L. , and BRUSH, B.M. , "Simultaneous Data System for 

 Instrumenting the Shelf," Proceedings ^ iZth Coastal Engineeving Conference^ 

 ASGE, Vol. I., 1972, pp. 95-112 (not in bibliography). 



58 



