CHAPTER 7 



STRUCTURAL DESIGN: PHYSICAL FACTORS 



I. WAVE CHARACTERISTICS 



1. Desi gn Criteria . 



Coastal structures must be designed to satisfy a number of sometimes 

 conflicting criteria, including structural stability, functional performance, 

 environmental impact, life-cycle cost, and other constraints which add 

 challenge to the designer's task. Structural stability criteria are most 

 often stated in terms of the extreme conditions which a coastal structure must 

 survive without sustaining significant damage. The conditions usually include 

 wave conditions of some infrequent recurrence interval, say 50 or 100 years, 

 but may also include a seismic event (an earthquake or tsunami), a change in 

 adjacent water depths, or the impact of a large vessel. The extent to which 

 these "survival" criteria may be satisfied must sometimes be compromised for 

 the sake of reducing construction costs. Analysis may prove that the con- 

 sequences of occasional damage are more affordable than the first cost of a 

 structure invulnerable to the effects of extremely rare events. A range of 

 survival criteria should be investigated to determine the optimum final 

 choice. 



Functional performance criteria are stated in terms of the desired effect 

 of the structure on the nearby environment, or in terms of its intended 

 function. For example, the performance criteria for a breakwater intended to 

 protect a harbor in its lee should be stated in terms of the most extreme wave 

 conditions acceptable in the harbor area; the features of the breakwater 

 affecting wave transmission can then be designed to satisfy this criterion. 

 The performance criteria for a groin intended to cause accretion of sand at a 

 certain location will be dissimilar to those for a breakwater. Performance 

 criteria may also require compromise for the sake of first cost, since 

 repairing the consequences of performance limitations could be more afford- 

 able. The high construction cost of most coastal structures requires that 

 risk analysis and life-cycle costing be an integral part of each design 

 effort. 



2 . Representat ion of Wave Conditions . 



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

 structures are subjected (except for seismic sea waves). Wave characteristics 

 are usually determined for deep water and then analytically propagated 

 shoreward to the structure. Deepwater significant wave height H^ and 

 significant wave period T may be determined if wind speed, wind duration, 

 and fetch length data are available (see Ch. 3, Sec. V). This information, 

 with water level data, is used to perform refraction and shoaling 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 structure. Struc- 

 tures 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 



7-1 



