the Great Lakes. Hindcast data are normally presented for relatively 

 deepwater conditions. Since detached breakwaters are placed in the nearshore 

 environment, the selected design wave height must be analytically propagated 

 shoreward to the structure. The deepwater significant wave height and 

 significant or peak spectral wave period can be used along with water level 

 and bathymetric data to perform refraction and shoaling analyses which 

 determine wave conditions at the site. Several numerical models are available 

 to perform these operations and are presented as part of the CMS (Cialone et 

 al. 1992). 



The choice of design wave conditions for structural stability should 

 consider whether the structure is subjected to the attack of nonbreaking, 

 breaking, or broken waves. Wave conditions at a structure site depend 

 critically on the existing water level. Consequently, a design still-water level 

 (swl) or range of water levels must be established in determining wave forces 

 on a structure. Structures may be subjected to radically different types of 

 wave action as the water level at the site varies. A given structure might be 

 subjected to nonbreaking, breaking, and broken waves during different stages 

 of a tidal cycle. Critical design conditions are the wave and water level 

 combinations which result in maximum forces or minimum structural stability. 



Selection of design wave heights for nearshore structures will often be 

 controlled by depth-limited waves. The depth-limited breaking wave height 

 for the given design water level should be calculated and compared with the 

 unbroken design storm wave height, and the lesser of the two chosen as the 

 design wave. Maximum depth-limited breaking wave heights can be estimated 

 using procedures found in Chapter 7 of the Shore Protection Manual (1984). 



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 wave height distribution. The selected design height depends 

 on whether the structure is defined as rigid, semirigid, or flexible (Shore 

 Protection Manual 1984). For rigid structures, such as cantilever steel sheet- 

 pile walls, where a large wave within the wave train can cause failure of the 

 entire structure, the design wave height is normally based on Hj (=1.67 H s , 

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

 design wave height can range from H ]0 (=1.27 H s , the average of highest 10 

 percent of all waves) to H } . Steel sheet-pile cell structures are semirigid and 

 can absorb wave pounding; therefore a design wave of H 10 may be used. For 

 flexible structures, such as rubble-mound structures, the design wave height 

 H 10 is typically used. Waves higher than the design wave height impinging 

 on flexible structures seldom create serious damage for short durations of 

 extreme wave action. 



Damage to rubble-mound structures is usually progressive, and an extended 

 period of destructive wave action (waves greater than design conditions) is 

 required before a structure ceases to provide protection. It is therefore 

 necessary in selecting a design wave to consider both the frequency of 

 occurrence of damaging waves and economics of initial construction, 



Chapter 4 Structural Design Guidance 



79 



