action. When an individual stone or armor unit is displaced by a high wave, 

 smaller waves of the train may move it to a more stable position on the slope. 



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

 period of destructive wave action is required before a structure ceases to 

 provide protection. It is therefore necessary in selecting a design wave to 

 consider both frequency of occurrence of damaging waves and economics of 

 construction, protection, and maintenance. On the Atlantic and gulf coasts of 

 the United States, hurricanes may provide the design criteria. The frequency 

 of occurrence of the design hurricane at any site may range from once in 20 to 

 once in 100 years. On the North Pacific coast of the United States, the 

 weather pattern is more uniform; severe storms are likely each year. The use 

 of H as a design height under these conditions could result in extensive 

 annual damage due to a frequency and duration of waves greater than H in 

 the spectrum. Here, a higher design wave of H,q or He may be advisable. 



Selection of a design height between H and He is based on the following 

 factors: 



(a) Degree of structural damage tolerable and associated maintenance 

 and repair costs (risk analysis and life-cycle costing). 



(b) Availability of construction materials and equipment. 



(c) Reliability of data used to estimate wave conditions. 



a. Breaking Waves . Selection of a design wave height should consider 

 whether a structure is subject to attack by breaking waves. It has been 

 commonly assumed that a structure sited at a water depth dg (measured at 

 design water stage) will be subjected to breaking waves if dg < 1.3H where 

 H = design wave height . Study of the breaking process indicates that this 

 assumption is not always valid. The breaking point is defined as the point 

 where foam first appears on the wave crest, where the front face of the wave 

 first becomes vertical, or where the wave crest first begins to curl over the 

 face of the wave (see Ch. 2, Sec. VI, BREAKING WAVES). The breaking point is 

 an intermediate point in the breaking process between the first stages of 

 instability and the area of complete breaking. Therefore, the depth that 

 initiates breaking directly against a structure is actually some distance 

 seaward of the structure and not necessarily the depth at the structure toe. 

 The presence of a structure on a beach also modifies the breaker location and 

 height. Jackson (1968a) has evaluated the effect of rubble structures on the 

 breaking proccess. Additional research is required to fully evaluate the 

 influence of structures. 



Hedar (1965) suggested that the breaking process extends over a distance 

 equal to half the shallow-water wavelength. This wavelength is based on the 

 depth at this seaward position. On flat slopes, the resultant height of a 

 wave breaking against the structure varies only a small amount with nearshore 

 slope. A slope of 1 on 15 might increase the design breaking wave height by 

 20 to 80 percent depending on deepwater wavelength or period. Galvin (1968, 

 1969) indicated a relationship between the distance traveled by a plunging 

 breaker and the wave height at breaking Hr . The relationship between the 

 breaker travel distance x^ and the breaker height H, depends on the 

 nearshore slope and was expressed by Galvin (1969) as: 



7-4 



