Reliability analysis methods can be applied to design hydrosystem infrastructures with or without considering risk costs. Risk costs are those cost items incurred owing to the unexpected failure of structures, and they can be broadly classified into tangible and intangible costs. Tangible costs are those measurable in terms of monetary unit, which include damage to property and structures, loss of business, cost of repair, etc. On the other hand, intangible costs are not measurable by monetary unit, such as psychological trauma, loss of lives, social unrest, damage to the environment, and others. Without considering risk costs, reliability has been explicitly accounted for in the design of storm sewer systems (Yen and Ang, 1971; Yen et al., 1976; Yen and Jun, 1984), culverts (Yen et al., 1980; Tung and Mays, 1980), and levees (Tung and Mays, 1981a; Lee and Mays, 1986). Cheng et al. (1986) demonstrated how to apply the AFOSM method to calculate the risk reduction associated with freeboard in dam design. Melching et al. (1987) suggested different flood peak-risk curves for forecasting and for design. However, it is the risk-based least cost design of hydrosystem infrastructure that promises to be potentially the most significant application of reliability analysis.
The risk-based design procedure integrates the procedures of uncertainty and reliability analyses in the design practice. The procedure considers the tradeoff among various factors such as failure probability, economics, and other performance measures in hydraulic structure design. Plate and Duckstein (1987,
1988) list a number of performance measures, called the figures of merit, in the risk-based design of hydraulic structures and water resource systems, which are further discussed by Plate (1992). When risk-based design is embedded into an optimization framework, the combined procedure is called the optimal risk-based design.
