2. Nearshore Circulations . 



The data base for experimental measurements of mean water surface 

 variation and two-dimensional currents, i.e. nearshore circulations, is 

 extremely limited. Only a few field and laboratory studies have been made, 

 and the results are not readily accessible for use. While significant 

 strides have been made in development of numerical models to simulate near- 

 shore hydrodynamics, data to calibrate, verify, and test these models are 

 practically nonexistent. 



The Japanese recognized this problem in the early 19 70's and devel- 

 oped their BAGS (ballon-borne camera systems) with drogues. It was modi- 

 fied into the STEREO-BACS system by Sasaki, Horikawa, and Hotta (1976)^° 

 to permit wave height estimates to be included. Two cameras are suspended 

 by ballons that permit stereographic projections. Problems with this method 

 were discussed in Chapter 2. The results are not generally available for 

 use by other researchers. 



In the United States, the problem is currently being addressed by 

 General Government agencies such as the National Sea Grant Office of the 

 National Oceanic and Atmospheric Administration (NOAA) in their NSTS 

 project and the CERC in their Atlantic Remote Sensing Land Ocean Experi- 

 ment project. Data from the NSTS experiments are being made available 

 on magnetic tape. The data could provide the information necessary to 

 substantiate the nearshore circulation models in the near future. 



Figure 79 provides an analytic solution, based on Bowen's (1969) 

 linear theory for the same wave characteristics and bathymetry as the field 

 measurements of currents with drogues and current meters by Sonu (1972) 

 shown previously in Figure 2. Comparing the two figures showed general 

 qualitative agreement but, the symmetry in the theory was not found, no 

 quantitative comparison of results was attempted. Also, it was observed 

 that the circulations pulsated but the analytic solution was for the steady 

 state. The undulating surf zone bed was postulated as the mechanism trigger- 

 ing rip current formation. It was concluded that the theory substantiated 

 the observations in this regard. 



The simplified numerical model NCSS developed by Sasaki (1977) was 

 exercised to simulate two different cases as measured in the field by the 

 BAGS system. In one example (CASE-2) drogues were scattered outside the 

 breakers to facilitate comparison and a meandering longshore current-rip 

 current pattern was present. Figure 80(a) is the nearshore current 

 velocity field as deduced from the camera photos, and Figure 80(b) shows 

 the numerically computed velocities under corresponding wave and bottom 

 contour conditions. Sasaki (1977) stated that to the left and near the 

 pier ". . . the direction of the predicted current is different from that 

 of the observations [which] has a predominant offshore component." In 

 fact some of the strongest measured currents (> 30 centimeters per second) 

 appear precisely where the model indicates a large-scale eddy with zero 

 motion. In terms of transport stream-function contours the overall flow 



''O SASAKI, HORIKAWA, and HOTTA, op. cit. 



203 



