on the front face of the wave than on the trailing face. This asymmetry 

 of the waves demonstrates that the acceleration on the front face of wind 

 waves is greater than predicted by elementary wave theory and suggests 

 that the peak force exerted by the waves on a structure may also be 

 greater than the peak force predicted by elementary wave theory, or meas- 

 ured in ordinary wave tanks. 



3. Wind- Driven Spray . 



Seawater, carried over a seawall, can add significantly to the problems 

 of controlling coastal flooding in severe storms. Seawater can be carried 

 over the seawall in the form of wind-driven spray, or in the form of wave 

 runup and overtopping. It is believed that the height to which waves can 

 carry water up a beach or over a levy is increased by strong onshore winds. 

 Fiihrboter (1974) suggested that the wind-driven spray can significantly 

 increase the windloading on structures in the surf zone. Quantitative 

 studies with a natural distribution of wave heights and periods are lacking. 



4. Surface Currents. 



Wind blowing over a water surface always generates a surface current 

 in the direction of the wind or, in the northern hemisphere, to the right 

 of the wind. When the wind is directed toward the shore, or parallel to 

 the shore to the right of the wind, this surface current has a shoreward 

 component. Continuity of mass requires a subsurface current away from the 

 shore which may contribute to beach erosion. Subsurface currents, result- 

 ing from a seaward flow at the surface, may lead to sediment transport 

 toward the beach and contribute to natural beach restoration. Suitable 

 laboratory experiments involving both waves and surface currents should 

 contribute significantly toward an understanding of this process. Lab- 

 oratory experiments may be essential. 



5. Wind-Generated Turbulence . 



Wind shear at the water surface adds vorticity to the water, increasing 

 the turbulence and the effective viscosity of the water and the effective 

 mixing coefficients for heat, salt, or any polluting substance. Little 

 quantitative data relative to this effect are available, and suitable 

 laboratory experiments in a wind-water facility could be extremely useful. 



6. Wind-Stress Relationships . 



Storm surge is the most important coastal engineering problem where 

 wave action is not the most important natural phenomenon. High winds, pro- 

 duced by severe storms, pile water against the coast and cause severe 

 flooding in low-lying coastal and estuarine areas. The principal cause 

 of the water motion which produces these floods is the shear stress between 

 wind and water. This stress is generally estimated in engineering practice 

 by expressions of the type: 



T = Pa Cj U2 , (1) 



13 



