volumetric flow rate was computed between adjacent points from the mean ve- 

 locity and MWL measured at these points. A maximum, depth-averaged longshore 

 current of 0.67 meter per second was measured about midway in the surf zone. 



This optimization based on minimizing Q^, proved successful. In addi- 

 tion, use of a flow distribution system in the upstream waveguide wall 

 that extended beyond the breaker line increased the region for uniform 

 current profile upstream to section 3, as shown in Figure 10. For con- 

 trast, results for essentially a type (a) basin are shown in Figure 11 as 

 found by Visser (1980). Major differences in profile shape are evident 

 at the midtank position (section 2) . For the wave field-basin combination 

 employed, the author concluded that a rational procedure to establish a 

 unifonn longshore current profile had been achieved. Thus it is now poss- 

 ible to use other wave fields and basin geometries and the methods devel- 

 oped to obtain uniform current profiles in the laboratory without lengthy 

 trial-and-error efforts. Additional tests are presently underway. These 

 studies also proved that considerable deviation in a uniform profile can 

 occur in nonoptimized basin geometries with improper recirculation flows. 



Other effects due to scale are also present in laboratory models. 

 These result from improper representations of viscous effects (Reynolds 

 similarity) and surface tension (Weber number) influence on the air 

 entralnment during wave breaking in the surf zone. Viscous effects are 

 for both internal, turbulence decay, and at the boundaries from bed shear 

 (friction) and wind surface shear stresses. For these reasons, Dette and 

 Fuhrboter (1974) indicated surf zone research should be conducted only in 

 the field. However, all the laboratory experiments described were general 

 studies in which no prototype scale existed. These efforts tested the 

 theories available at the time and at the scale of the laboratory inves- 

 tigation conducted, and for this purpose they are entirely valid. The 

 ability to create a steady-state environment in which longshore current 

 measurements at a point in the surf are not dependent upon averaging time 

 is a major advantage of physical model studies. Surprisingly, the use of 

 such models to study longshore currents is not mentioned in a recent state- 

 of-the-art report by Hudson, et al. (1979). 



Site specific model tests of longshore currents have been made (e.g., 

 Gourlay, 19657; Curren and Chatham, 1980) but will not be reviewed in this 

 report. Laboratory studies of rip currents, which are strongly associated 

 with forcing functions and mechanisms are discussed in the next section. 



GOURLAY, M.R. , "Wave Generated Currents - Some Observations Made in Fixed 

 Bed Hydraulic Models," ProaeedingSj Second Australian Conference Hydraulics 

 and Fluid Mechanics, Auckland, December 1965 (not in bibliography). 



39 



