CHAPTER 7 



STRUCTURAL DESIGN - PHYSICAL FACTORS 



7.1 WAVE CHARACTERISTICS 



Wind-generated waves produce the most critical forces to which coastal 

 structures are subjected (except for seismic sea waves). A structure 

 exposed to wave action should be designed to withstand the highest wave 

 expected at the structure, if such a design is economically justified. 

 Economic evaluations depend on frequency of occurrence of extreme events 

 such as, height and duration of extreme waves, damage potential of high 

 waves, and permissible risk. Wave characteristics are normally determined 

 for deep water, and then propagated shoreward to the structure. Deepwater 

 significant wave height Ho and significant wave period Tg may be deter- 

 mined if wind speed, wind duration, emd fetch length are known. (See 

 Sections 3.5 and 3.6) This information, with water-level data, is used 

 with refraction analyses to determine wave conditions at the site. 



Wave conditions at a structure site at any time depend critically on 

 the water level. Consequently, a design Stillwater level (SWL) or range 

 of water levels must be established in determining wave forces on a struc- 

 ture. 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. The wave action a structure is subjected to m.ay 

 also vary along its length at a given time. This is true for structures 

 oriented perpendicular to the shoreline such as groins and jetties. The 

 critical section of these structures may be shoreward of the seaward end 

 of the structure depending on structure crest elevation, tidal range, and 

 bottom profile. 



Detailed discussion of the effects of astronomical tides and wind- 

 generated surges in establishing water levels is presented in Chapter 3, 

 WAVE AND WATER LEVEL PREDICTIONS. In Chapter 7, it is assumed that the 

 methods of Chapter 3 have been applied to determine design water levels. 



The wave height usually obtained from statistical analysis of 

 synoptic weather charts is the significant height, Hg. Assuming a 

 Rayleigh wave-height distribution, Hg may be further defined in approxi- 

 mate relation to other height parameters of the statistical wave-height 

 distribution: 



Hj/g or Hg = average of highest 1/3 of all waves, 

 Ms; 1.27 Hg = average of highest 10 percent of all waves (7-1) 

 H ~ 1.67 Hs = average of highest 1 percent of all waves (7-2) 



7-i 



