PART III: ESTIMATING DAMAGE RATES 



55. A key step in identification of an optimum among alternative 

 rubble-mound breakwater plans is to estimate the expected damages and life 

 cycle costs of related maintenance and repairs. The concept of designing a 

 rubble-mound breakwater for zero damage is unrealistic because a finite risk 

 always exists for the stability criteria to be exceeded in the life of the 

 structure. The stochastic nature of the physical phenomena affecting coastal 

 engineering structures requires that a probabilistic approach be applied, if 

 these maintenance cost estimates are to be more than guesses. The incident 

 wave climate can be characterized by estimating probability distribution func- 

 tions by a number of relatively well accepted methods (Battjes 1984). The 

 crucial problem for rubble-mound breakwater designs is in relating a given 

 level of damage and associated repair costs to specific incident wave condi- 

 tions. The rate at which this damage accumulates must also be predicted in 

 order to tentatively schedule maintenance and related cash flows. The fol- 

 lowing section will review some techniques proposed for making these predic- 

 tions. Their relative merits will be discussed and areas of ongoing or needed 

 future research identified. 



Damage Assessment 



56. Damages to rubble-mound breakwaters have been quantified in many 

 ways by researchers and field engineers. The current issue surrounding break- 

 age of concrete armor units has led to a number of recent publications pointed 

 at systematic assessment of damages of all kinds. One useful characterization 

 of prototype damages in terms of displaced primary armor units was proposed by 

 Groeneveld, Mol, and Den Boer (1984) and is presented in Table 2. 



Table 2 

 Classification of Breakwater Damage 



Type of Failure Displacement, % Description 



Minor 0-3 A few individual units of top layer dis- 



placed, but no gaps in top layer larger 

 than 4 units; bottom layer intact 



(Continued) 



35 



