DESIGN CONSIDERATIONS 



The factors upon which the design of a protective structure are based 

 may vary considerably. However, usually a "design storm" is selected which 

 fulfills the desired conditions of severity against which the study area 

 is to be protected and the frequency of which is compatible with the ex- 

 pected structure life. Once the design storm and its direction have been 

 determined, the storm surge at the structure site is determined. This 

 surge elevation is added to a selected astronomical tide and any water level 

 increases which may be expected from other sources. The elevation so de- 

 termined is the "design still water level (SWL)". 



In order to determine the height of the proposed structure, the wave 

 run-up for the waves of the "design storm" must be determined. This is 

 usually done by forecasting the "significant wave" of the storm and de- 

 termining its run-up on the proposed structure using a method such as that 

 given in reference 1. In cases where the structure is being designed for 

 a shore which has varying offshore profiles, an offshore profile typical 

 of the area should be used. When the run-up so determined has been added 

 to the design SWL, a structure height is derived. The structure height 

 derived by this process will not completely prevent overtopping because 

 storm wave heights vary statistically and about 13 percent of the waves 

 will be higher than the significant wave. Thus, run-ups higher than that 

 of the significant wave can be expected (4) and the structure height can be 

 increased to prevent overtopping from as large a percentage of all the waves 

 as is necessary for structure stability or flooding considerations. 



When a structure height has been determined in the preceding manner 

 for a typical profile it should be checked for all of the individual profiles 

 in the study area to insure the safety of the entire length of the structure. 

 In addition, the run-up of smaller waves, which break closer inshore, should 

 also be checked because these smaller waves sometimes cause run-up higher 

 than the larger waves breaking in deeper water farther offshore. Using 

 existing methods, checking many wave heights for perhaps a dozen profiles 

 becomes time consuming and, since only spot checks can be made, there is 

 no assurance that the critical wave height, or that height in the height 

 spectrum which produces the largest run-up, has been checked. 



SUGGESTED METHOD FOR CHECKING STRUCTURE HEIGHT DESIGN 



A proposed dune on a typical profile and an assumed design SWL of 

 +9 feet MLW are shown in Figure 2. In addition, it is assumed that the 

 structure height has been determined to be 3.6 feet higher than the design 

 SWL. This design can be checked for any wave steepness for all profiles by 

 determining a "critical profile" for a maximum run-up of 3.6 feet. This 

 "critical profile" is actually determined by the breaking depths of the 

 waves which produce a run-up of 3.6 feet on various slopes. When this 

 profile has been detenu ined, it can be drawn on transparent paper to the 

 same horizontal and vertical scales to which the study area profiles are 



