at the coast. Also, density effects are generally ignored when comparing 

 test results from the Great Lakes (freshwater) with ocean (saltwater) 

 beaches. Surface tension plays a role when waves break, especially for 

 large-scale differences as found on laboratory versus natural beaches. 

 Turbulence intensity and resultant turbulent shear stresses must also be 

 considered. When generated by wave breaking, the type of breaker and the 

 scale of the observation (again, laboratory versus field) produce differ- 

 ent internal turbulence intensities. Boundary shear-generated turbulence 

 (oscillatory-type) can be significantly different over rigid versus mov- 

 able beds. Movable bottoms form rippled or duned beds that locally gener- 

 ate more vorticity. Sediment concentration distributions in the vertical 

 suppress turbulent fluctuation intensities near the bed. As discussed 

 in Chapter 3, both the bottom boundary resistance coefficient and lateral 

 turbulent mixing parameter play significant roles in theories on longshore 

 currents. They are both strongly influenced by the fluid properties. 



It would be a mistake to assume that sediment characteristics (compo- 

 sition and weight, grain-size distribution, roundness, shape) are only of 

 interest for sediment transport studies. They play a role in coastal 

 currents that is yet to be fully understood or appreciated. For example, 

 the weight and size of the grains on the beach can influence the size of 

 ripples formed and the concentration distribution of suspended sediment. 

 Both properties thus change the turbulence Intensity present in the water 

 column. If more wave energy is expended to keep sediment in suspension, 

 less is available to generate currents. Also many studies have shown how 

 grain size and beach slope are related (e.g., Bascom, 1951^3),. xhe major 

 differences between reflective- and dissipative-type beach systems and 

 their slope dependence are discussed earlier. Size, distribution, and 

 shape also affect beach porosity which, in turn, influences the extent 

 of backwash in the swash zone (Kemp, 1975). Wave energy is absorbed in 

 the surf zone by porosity effects as well as boundary resistance. 



Finally, perhaps the key factor is the wave breaking phenomenon. 

 When, where and why do waves break? Because the physical understanding 

 is so weak, empirically based information is heavily relied upon. The 

 surf zone empirism that results plays a critical role in all theories of 

 longshore currents and nearshore circulations. Consequently, wave break- 

 ing processes are discussed in Chapter 3. 



IV. INSTRUMENTATION AND MEASUREMENTS 



A number of methods and problems have been discussed that relate to 

 making current measurements in the field and laboratory. This section 

 elaborates further on the instruments employed and on a number of measure- 

 ment systems devised for surf zone applications. The section concentrates 

 on the EM-type current meter and the so-called Sxy gage for estimating 

 momentum flux and wave direction. A number of other instruments are used 



13 



BASCOM, W.H. , "The Relationship Between Sand Size and Beach Face 



Slope," Trans. Am. Geophys. Union^ Vol. 32, 1951, pp. 866-874 



(not in bibliography) . 



53 



