structures seldom create serious damage for short durations of extreme 

 wave 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. It may be un- 

 economical to build a structure that would withstand the hurricane condi- 

 tions without damage, hence H may be a more reasonable design wave 

 height. On the North Pacific coast of the United States, the weather 

 pattern is more unifomi; severe storms are likely each year. The use of 

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

 annual damage and frequent maintenance because of the higher frequency 

 and duration of waves greater than Hg in the spectrum. Here, a higher 

 design wave of about H-^q may be advisable. Selection of the design height 

 between Hg and H^q is based on the following factors: 



(a) degree of structure damage allowable and associated 

 maintenance costs, 



(b) availability of armor materials, and 



(c) comparative alternate size or type of armor unit and 

 their costs, 



7.121 Breaking Waves . Selection of a design wave height also depends on 

 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 <^ 1.3 H 

 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 Section 2.6, 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 process. 

 Additional research is required to fully evaluate the influence of 

 structures . 



7-3 



