Effects of discount rate and various costs on optimal design of caisson breakwater

In this study, the method developed by Goda and Takagi in 2000 for optimal design of a vertical breakwater caisson is extended to take into account the effects of discount rate and economic damage costs due to long-term harbor shutdown and temporal stoppage of harbor operation. The effect of discount rate is important only at smaller return periods where the damage to the caisson frequently occurs. Among the various costs, the initial construction cost and the economic damage cost due to long-term harbor shutdown caused by extraordinary sliding of caissons are found to be equally important in finding the minimum expected total lifetime cost. On the other hand, the rehabilitation cost and the economic damage cost due to temporal stoppage of harbor operation caused by excessive wave overtopping are not so important in the optimal design of the breakwater. In general, in smaller water depths the optimal return period and the corresponding optimal cross-section of the caisson are determined as those yielding the minimum expected total lifetime cost, while they are determined by the allowable expected sliding distance in greater water depths.