Monte Carlo simulation model for economic evaluation of rubble mound breakwater protection in Harbors

The US Army Corps of Engineers has a mission to conduct a wide array of programs in the arenas of water resources, including coastal protection. Coastal projects must be evaluated according to sound economic principles, and considerations of risk assessment and sea level change must be included in the analysis. Breakwaters are typically nearshore structures designed to reduce wave action in the lee of the structure, resulting in calmer waters within the protected area, with attendant benefits in terms of usability by navigation interests, shoreline protection, reduction of wave runup and onshore flooding, and protection of navigation channels from sedimentation and wave action. A common method of breakwater construction is the rubble mound breakwater, constructed in a trapezoidal cross section with gradually increasing stone sizes from the core out. Rubble mound breakwaters are subject to degradation from storms, particularly for antiquated designs with under-sized stones insufficient to protect against intense wave energy. Storm waves dislodge the stones, resulting in lowering of crest height and associated protective capability for wave reduction. This behavior happens over a long period of time, so a lifecycle model (that can analyze the damage progression over a period of years) is appropriate. Because storms are highly variable, a model that can support risk analysis is also needed. Economic impacts are determined by the nature of the wave climate in the protected area, and by the nature of the protected assets. Monte Carlo simulation (MCS) modeling that incorporates engineering and economic impacts is a worthwhile method for handling the many complexities involved in real world problems. The Corps has developed and utilized a number of MCS models to compare project alternatives in terms of their costs and benefits. This paper describes one such model, Coastal Structure simulation (CSsim) that has been developed specifically for planning level analysis of breakwaters.