IV. PRELIMINARY DESIGN 



1. Selection of Design Waves and Water Levels. 



The selection of design conditions is related to the economics of 

 construction and annual maintenance costs to repair structure in the event of 

 extreme wave action. These costs are related to the probability of 

 occurrence of extreme waves and high water levels. There will usually be some 

 design wave height which will minimize the average annual cost (including 

 amortization of first cost). This optimum design wave height will give the 

 most economical design. 



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DESIGN WAVE HEIGHT GIVING 

 MOST ECONOMICAL DESIGN 



TOTAL AVERAGE 

 ANNUAL COST 



/ 



AMORTIZED 

 FIRST COST 



^' 



MAINTENANCE 

 & REPAIR COSTS 



DESIGN WAVE HEIGHT 



Intangible considerations such as the environmental consequences of 

 structural failure or the possibility of loss of life in the event of failure 

 must also enter into the decision of selecting design conditions. These 

 factors are related to the specific purpose of each structure. 



The following design conditions are assumed for the illustrative purposes 

 of this problem. 



a. Water Levels (MLW datum) . 



(1) Storm surge (less astonomical tide): use 3.0 m . 



(2) Astronomical tide (use water level exceeded 1 percent of 

 time) : 1.5m. 



(3) Wave setup (assumed negligible since structure is in 

 relatively deep water and not at beach). 



8-46 



