7.11 DETERMINATION OF WAVE CONDITIONS 



All wave data applicable to the project site should be evaluated 

 for possible use as design criteria. Visual observation of storm waves, 

 while difficult to confirm, may provide an indication of wave height, 

 period, direction, storm duration, and frequency of occurrence. Instru- 

 mentation has been developed for recording wave height and period at a 

 point. Instrumentation for recording wave direction is presently in the 

 development stage, thus direction data must be obtained from visual obser- 

 vations. Wave direction is usually necessary for design analysis. If 

 reliable visual shore or ship observations of wave direction are not 

 available, hindcast procedures (Sec, 3.5, SIMPLIFIED WAVE PREDICTION 

 NK)DELS) must be used. Where reliable, statistical deepwater wave data 

 are available, these can provide the necessary shallow-water wave data. 

 If wave data are not directly available at the site, the best available 

 procedure must be employed, with sound engineering judgment, to transform 

 available deepwater and extreme offshore wave data to the structure site. 

 (See Section 2.238, Wave Energy and Power, and Sections 2.3, WAVE REFRAC- 

 TION, and 2.4, WAVE DIFFRACTION.) 



7.12 SELECTION OF DESIGN WAVE 



The choice of a design wave height depends on 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 are known, the next step 

 is to determine if wave height at the site is controlled by water depth. 

 (See Section 2.6, 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 con- 

 ditions should be determined 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 be found at a location 

 other than the seaward end of the structure. The possibility of such 

 conditions should be considered in establishing design waves and water 

 levels. 



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

 breaking 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 vigid, semirn-gidj 

 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 faulure of the entire 

 structure, the design wave is normally based on H^, the average height 

 of the highest 1 percent of all waves. For semirigid structures, the 

 design wave is selected from a range of H to H . Steel sheet-pile 

 cell structures are semirigid, and can absorb wave poimding; therefore, 

 a design wave height of H,- may be used. For flexible structures, such 

 as rubble-mound or riprap structures, the design height is usually the 

 significant height Hg. Waves higher than Hg impinging on flexible 



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