6 . Dissipation and Turbulence . 



Dissipation of ocean waves is often considered negligible, but 

 where waves propagate for appreciable distances over shallow water, 

 or in any depth under significant wind action, dissipation may be 

 significant. In the absence of currents, dissipation occurs by two 

 mechanisms. The most effective mechanism is wave breaking. This 

 usually occurs relatively close to shore, but for the largest ocean 

 waves with heights of 30 meters, some breaking can be expected on the 

 Continental Shelf. And while waves are being generated by the wind, 

 some of the steeper waves are breaking, whatever the depth of water. 

 Breaking related to currents is discussed further in Section II, 12. 



The other major mechanism for dissipation is bottom friction. This 

 becomes important when waves travel a long distance in shallow water. 

 For ocean waves, the bottom boundary layer is turbulent and hence only a 

 semiempirical approach is possible, e.g., see Knight (1978) and Jonsson 

 (1980). More recently, Brevik (1981) has presented a simple two-layer 

 model, which yields good agreements with Danish measurements. 



The introduction of currents adds to the difficulty of estimating 

 bed shear and dissipation. First attempts were made by Jonsson (1966) 

 and Bijker (1966, 1967). More elaborate approaches to the problem have 

 been made: Lundgren (1972), Bakker (1974), Smith (1977), Grant and 

 Madsen (1979), Bakker and van Doom (1978, 1980), Engelund (1979), 

 Greulich (1980), Christof fersen (1980a, b), Christof fersen, Skovgaard 

 and Jonsson (1981), Freds^e (1981), Bakker and van Kesteren 

 (unpublished, 1981). The investigations use a variety of eddy viscosity 

 assumptions and mixing length hypotheses. Most models assume that the 

 problem of interest is the interaction of waves and currents in a wave- 

 dominated environment. Some consider the reverse case: a dominant 

 current with waves on it (Smith, 1977; Engelund, 1979). Some of the 

 papers give detailed advice as how to calculate the bed shear and 

 velocity variation near the bed for the current at an arbitrary angle to 

 the wave motion, e.g.. Grant and Madsen (1979), and Chris tof fersen 

 (1980b). Soulsby and Dyer's (1981) results deduced from boundary-layer 

 measurements in accelerating tidal flows may help improve the modeling 

 of wave-current boundary layers. 



For experimental results see van Hoften and Karaki (1976), Bakker 

 and van Doom (1978, 1980), George and Sleath (1979) (oscillating bed), 

 Iwagaki and Asano (1980), Brevik and Aas (1980), Brevik (1980), van 

 Doom (unpublished, 1981), Kemp and Simons (in preparation, 1982), all 

 limited to plane flume flow. Bijker (1967), and Rasmussen and FredsjSe 

 (1981), measured sediment transport in a basin, with the waves 

 perpendicular to the current, see Section III, 3. Kemp and Simons' 

 measurements are probably the most comprehensive; they include both mean 

 velocities and turbulent fluctuations, as do those of Iwagaki and Asano. 



40 



