Considering the high friction velocities, the assumption of a 

 uniform velocity distribution can be questioned. 



Coastal Engineering Significance . This paper is important in 

 demonstrating that waves and currents together can result in drastically 

 higher bottom shear stress than for currents alone. A procedure is 

 presented for experimental measurement of the friction factor and tested 

 against data from Inman and Bowen (1962). 



24. JONSSON, I.G., "Energy Flux and Wave Action in Gravity Waves 

 Propagating on a Current," Journal of Hydraulic Research^ Delft, 

 The Netherlands, Vol. 16, No. 3, 1978(a), pp. 223-234 (Errata in 

 Vol. 17, No. 1, 1979). 



Keywords. Currents, Large-Scale; Flow, Irrotational; Mass Transport; 

 Setdown; Theory; Wave Action. 



Discussion. Small-amplitude waves are considered, propagating with slow 

 variations of depth and current in both horizontal directions. In many 

 respects it is a generalization to three-dimensional flow of the results 

 obtained in JONSSON, SKOUGAARD, and WANG (1970). 



The currents are steady, the flow irrotational, and all results are 

 taken to second order in wave height. Dissipation is neglected. 



A significant new result is that wave action flux is proportional 

 to the total energy flux with the mean energy level as a datum. This 

 leads directly to the concept of the wave ray and the wave action 

 conservation principle. 



The setdown of the mean water surface is easily found as the mean 

 velocity head at the bottom, using a result for progressive waves from 

 the above reference. The result shows the importance of distinguishing 

 between the average-over-depth current velocity and the Eulerian mean 

 velocity below wave trough level. 



It is also shown that the so-called mass transport in water waves 

 is a boundary problem rather than a physical necessity. 



Coastal Engineering Significance . The most important finding is the 

 simple physical interpretation of wave action flux. This physical 

 interpretation in hydraulic terms is familiar to engineers. 



25. JONSSON, I.G., "Combinations of Waves and Curents," Stability of 

 Tidal Inlets, P. Bruun, ed., 1st ed., Developments in Geotechnical 

 Engineering, Vol. 23, Elsevier Scientific Publishing Co., 



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