The evolution of hydrosystem design methods can be roughly classified into four stages: (1) historical event-based design, (2) return-period design, (3) conventional risk-based design, and (4) optimal risk-based design with consideration given to a variety of uncertainties.
Historical event-based design. In the design of hydrosystem infrastructures and the establishment of land-use management practices to prevent and/or reduce damages resulting from natural disasters, the risk (damage) assessment typically has been implicit. The earliest structures and land-use management approaches for flood protection were designed or established on the basis of their ability to withstand previous disastrous floods. For example, Chow (1962) noted that the Dun waterway table used to design railroad bridges in the early 1900s was primarily determined from channel areas corresponding to high — water marks studied during and after floods. Thus previous large floods of unknown frequency could pass through the designed bridges safely. Also, after a devastating flood on the Mississippi River in 1790, a homeowner in Saint Genieve, Missouri, rebuilt his house outside the boundary of that flood. Similar rules were applied in the design of coastal-protection works in The Netherlands at the time the Zuiderzee was closed (1927-1932) (Vrijling, 1993).
Rules based on previous experience work well in some cases. For example, the house in Missouri was not flooded until the 1993 flood on the Mississippi River, and the Zuiderzee protection works survived the 1953 storm that devastated the southwestern part of The Netherlands. However, in most cases these methods are inadequate because human experience with floods and other natural hazards do not include a broad enough range of events. As noted by Vrijling (1993), “One is always one step behind when a policy is only based on historical facts.”
Return-period design. In the early part of the twentieth century, the concept of frequency analysis began to emerge as a method to extend limited data on extreme events to probabilistically estimate the magnitude of rarely occurring events. Frequency analysis of observed events is a key aspect of meteoro — logic, hydrologic, and seismic hazard analyses. Thus, using frequency-analysis methods, it is possible to estimate events with magnitudes beyond those that have been observed. This necessitates the selection of a societally acceptable hazard frequency (see Sec. 8.3.6).
Using the return-period design approach, a hydraulic engineer first determines the design discharge from a frequency-discharge relationship by selecting an appropriate design frequency or return period. The design discharge then is used to determine the size and layout of the hydrosystem that has a satisfactory hydraulic performance. In the return-period design method, selection of the design return period is crucial to the design. Once the design return period is determined, it remains fixed throughout the whole design process. In the past, the design return period was selected subjectively on the basis of an individual’s experience or the societally acceptable hazard frequency (Sec. 8.3.6). Selection of the design return period is a complex procedure that involves considerations of economic, social, legal, and other factors. However, the procedure does not account for these factors explicitly.
Conventional risk-based design. Risk-based design is a procedure that evaluates among alternatives by considering the tradeoff between the investment cost and the expected economic losses due to failures. Specifically, the conventional risk- based design considers the inherent hydrologic uncertainty in calculation of the expected economic losses. In the risk-based design procedure, the design return period is a decision variable instead of being a preselected design parameter value, as with the return-period design procedure.
The concept of risk-based design has been recognized for many years. As early as 1936, Congress passed the Flood Control Act (U. S. Statutes 1570), in which consideration of failure consequences in the design procedure was advocated. The economic risks or the expected flood losses were not considered explicitly until the early 1960s. Pritchett’s work (1964) was one of the early attempts to apply the risk-based hydraulic design concept to highway culverts. At four actual locations, Pritchett calculated the investment costs and expected flood damage costs on an annual basis for several design alternatives, among which the most economical one was selected. The results indicated that a more economical solution could be reached by selecting smaller culvert sizes compared with the
traditional return-period method used by the California Division of Highways. The conventional approach has been applied to the design of highway drainage structures such as culverts (Young et al., 1974; Corry et al., 1980) and bridges (Schneider and Wilson, 1980). Inherent randomness of hydrologic processes is integrated with reliability analysis in seismic, structural, and geotechnical aspects in the design of new dams (Pate-Cornell and Tagaras, 1986) and evaluation of alternatives for rehabilitating existing dams (McCann et al., 1984; Bureau of Reclamation, 1986; National Research Council, 1983).
Risk-based design considering other uncertainties. In the conventional risk-based hydraulic design procedure, economic risks are calculated considering only the randomness of hydrologic events. In reality, there are various types of uncertainties, as described in Sec. 1.2, in a hydrosystem infrastructure design. Advances have been made to incorporate other aspects of uncertainty in the design of various hydrosystem infrastructures. For example, both hydrologic and hydraulic uncertainties were considered in the design of highway drainage structures (Mays, 1979; Tung and Mays, 1980, 1982; Tung and Bao, 1990), storm sewer systems (Yen and Ang, 1971; Yen and Jun, 1984; Tang and Yen, 1972; Tang et al., 1975, 1976), levee systems (Tung and Mays, 1981b), riprap design of stable channels (Tung, 1994), and river diversion (Afshar et al., 1994). Inherent hydrologic uncertainty, along with parameter and model uncertainties, was considered in design of levee systems (Wood, 1977; Bodo and Unny, 1976). Economic uncertainty, along with hydrologic and hydraulic uncertainties, has been considered in flood-damage-reduction projects (U. S. Army Corps of Engineers, 1996).
