sediment tracer patterns. This value was estimated at the harbor based on observa- 

 tions made in the prototype during storm wave conditions. Intermediate swl's of 

 +1.2 and 2.4 m (+4.0 and +8.5 ft) were selected for limited experiments during 

 preliminary testing. 



Factors influencing selection 



of experimental wave characteristics 



In planning the experimental program for a model investigation of harbor wave- 

 action problems, it is necessary to select heights, periods, and directions for the 

 experimental waves that will allow a realistic study of the proposed improvement 

 plans and an accurate evaluation of the elements of the various proposals. Surface- 

 wind waves are generated primarily by the interactions between tangential stresses 

 of wind flowing over water, resonance between the water surface and atmospheric 

 turbulence, and interactions between individual wave components. The height and 

 period of the maximum significant wave that can be generated by a given storm 

 depend on the wind speed, the length of time that wind of a given speed continues to 

 blow, and the distance over water (fetch) which the wind blows. Selection of 

 experimental wave conditions entails evaluation of such factors as: 



a. Fetch and decay distances (the latter being the distance over which waves 

 travel after leaving the generating area) for various directions from which 

 waves can approach the problem area. 



b. Frequency of occurrence and duration of storm winds from the different 

 directions. 



c. Alignment, size, and relative geographic position of the navigation struc- 

 tures. 



d. Alignments, lengths, and locations of the various reflecting surfaces in the 

 area. 



e. Refraction of waves caused by differentials in depth in the area seaward of 

 the site, which may create either a concentration or a diffusion of wave 

 energy. 



When waves move into water of gradually decreasing depth, transformations 

 take place in all wave characteristics except wave period (to the first order of 

 approximation). The most important transformations with respect to selection of 

 experimental wave characteristics are the changes in wave height and direction of 

 travel due to the phenomenon referred to as wave refraction. For this study, the 

 Alaska District utilized numerical wave transformation models to transform 

 deepwater wave characteristics into shallow-water values. The transformation 

 models included refractive, diffractive, and shoaling effects of the offshore 

 bathymetry. Shallow- water wave characteristics were obtained at the - 12.2-m 

 (-40-ft) contour, which corresponded to the approximate wave generator location in 

 the physical model. 



Chapter 3 Experimental Conditions and Procedures 1 3 



