period, and direction at a point. Wave direction information is usually 

 necessary for design analysis, but may be estimated from directional wind data 

 if physical measurements of wave direction are not available. Visual observa- 

 tions of wave direction during exteme events are important in verifying 

 estimates made from wind data. If reliable visual shore or ship observations 

 of wave direction are not available, hindcast procedures (Ch. 3, Sec. V, 

 SIMPLIFIED METHODS FOR ESTIMATING WAVE CONDITIONS) must be used. Reliable 

 deepwater wave data can be analyzed to provide the necessary shallow-water 

 wave data. (See Ch. 2, Sec. II, 3, h. Wave Energy and Power, and Ch. 2, Sec. 

 Ill, WAVE REFRACTION, and IV, WAVE DIFFRACTION.) 



4 . Sel ecti on of Design Wave Conditions . 



The choice of design wave conditions for structural stability as well as 

 for functional performance should consider whether the structure is subjected 

 to the attack of nonbreaking, breaking, or broken waves and on the geometrical 

 and porosity characteristics of the structure (Jackson, 1968a). Once wave 

 characteristics have been estimated, the next step is to determine if wave 

 height at the site is controlled by water depth (see Ch. 2, Sec. VI, BREAKING 

 WAVES) . The type of wave action experienced by a structure may vary with 

 position along the structure and with water level and time at a given 

 structure section. For this reason, wave conditions should be estimated at 

 various points along a structure and for various water levels. Critical wave 

 conditions that result in maximum forces on structures like groins and jetties 

 may occur at a location other than the seaward end of the structure. This 

 possibility should be considered in choosing design wave and water level 

 conditions. 



Many analytical procedures currently available to estimate the maximum 

 wave forces on structures or to compute the appropriate weights of primary 

 armor units require the choice of a single design wave height and period to 

 represent the spectrum of wave conditions during an extreme event. The peak 

 spectral period is the best choice in most cases as a design wave period (see 

 Ch. 3, Sec. V, SIMPLIFIED METHODS FOR ESTIMATING WAVE CONDITIONS). The choice 

 of a design wave height should relate to the site conditions, the construction 

 methods and materials to be used, and the reliability of the physical data 

 available. 



If breaking in shallow water does not limit wave height , a nonbreaking 

 wave condition exists. For nonbreaking waves, the design height is selected 

 from a statistical height distribution. The selected design height depends on 

 whether the structure is defined as rigid, semirigid, or flexible. As a rule 

 of thumb, the design wave is selected as follows. For rigid structures, such 

 as cantilever steel sheet-pile walls, where a high wave within the wave train 

 might cause failure of the entire structure, the design wave height is 

 normally based on H, . For semirigid structures, the design wave height is 

 selected from a range of H,q to H, . Steel sheet-pile cell structures are 

 semirigid and can absorb wave pounding; therefore, a design wave height of 

 HiQ may be used. For flexible structures, such as rubble-mound or riprap 

 structures, the design wave height usually ranges from He to the significant 

 wave height Hg . Hiq is currently favored for most coastal breakwaters or 

 jetties. Waves higher than the design wave height impinging on flexible 

 structures seldom create serious damage for short durations of extreme wave 



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