Besides the economic factors that can be quantified in monetary terms in the design of hydrosystems, there are other intangible factors that are noncommensurable and cannot be quantified. Some of the intangible factors might work against the principle of economic efficiency. Examples of intangible factors that are considered in the design and planning of hydrosystems may be potential loss of human lives, stability of water course, impacts on local society and environment, health hazards after floods, litigation potential, maintenance frequency of the systems, and others. The conventional optimal risk-based design yields the most economically efficient system, which may not be acceptable or feasible when other intangible factors are considered.
As more intangible factors are considered in risk-based design, it becomes a multiobjective or multicriteria decision-making (MCDM) problem in which economic efficiency is one of many factors to be considered simultaneously. Use of a multiple-criteria approach enhances more realistic decision making, and the design frequency so determined will be more acceptable in practice and defensible during litigation or negotiation with others. Tung et al. (1993) adopted the MCDM framework to incorporate intangible factors in risk-based design of highway drainage structures through which a more defensible extended risk — based design frequency can be determined from integrated consideration of tangible and intangible factors.
In a risk-based design, in addition to quantitative measure of failure probability and risk cost, consideration of intangible factors and societally acceptable risk issues should be included if possible. In the United States, the societally acceptable frequency of flood damage was formally set to once on average in 100 years (the so-called 100-year flood) in the Flood Disaster and Protection Act of 1973; however, the 100-year flood had been used in engineering design for many years before 1973. In this act, the U. S. Congress specified the 100-year flood as the limit of the flood plain for insurance purposes, and this has become widely accepted as the standard of hazard (Linsley and Franzini, 1979, p. 634). This acceptable hazard frequency was to be applied uniformly throughout the United States, without regard to the vulnerability of the surrounding land. The selection was not based on a benefit-cost analysis or an evaluation of probable loss of life. Linsley (1986) indicated that the logic for this fixed level of flood hazard (implicit vulnerability) was that everyone should have the same level of protection. Linsley further pointed out that many hydrologists readily accept the implicit vulnerability assumption because a relatively uncommon flood is used for the hazard level, and thus
The probability that anyone will ever point a finger and say “you were wrong” is equally remote. If the flood is exceeded, it is obvious that the new flood is larger than the 10-year or 100-year flood, as the case may be. If the estimate is not exceeded, there is no reason to think about it.
Mitigation of natural hazards requires a more rigorous consideration of the risk resulting from the hazard and society’s willingness to accept that risk.
In other cases of disaster, societally acceptable hazard levels also have been selected without formal evaluation of benefits and costs. For example, in the United States, dam-failure hazards are mitigated by designing dams where failure may result in the loss of life to pass the probable maximum flood. Also, in The Netherlands, coastal-protection works normally are designed by application of a semideterministic worst-case approach wherein the maximum storm-surge level (10,000-year storm surge) is assumed to coincide with the minimum interior water level.
In the design of the Eastern Schedlt Storm-Surge Barrier, the Delta Committee in The Netherlands applied a simple risk-cost (in terms of lives) evaluation to set the design safety level. The Delta Committee set the total design load on the storm-surge barrier at the load with an exceedance probability 2.5 x 10-4 per year (i. e., the 4000-year water level) determined by integration of the joint probability distribution among storm-surge levels, basin levels, and the wave-energy spectrum. A single-failure criterion then was developed for the functioning of all major components of the storm-surge barrier (concrete piers, steel gates, foundation, sill, etc.) under the selected design load. The failure criterion was tentatively established at 10-7 per year on the basis of the following reasoning. Fatality statistics for The Netherlands indicate that the average probability of death resulting from an accident is 10-4 per year. Previous experience has shown that the failure of a sea-defence system may result in 103 casualties. Thus a normal safety level can be guaranteed only if the probability of failure of the system is less than or equal to 10-7 per year. Comparison of the worst-case approach with the probabilistic-load approach resulted in a 40 percent reduction in the design load when the actual, societally acceptable protection failure hazard was considered (Vrijling, 1993). This illustrates that when a comprehensive risk assessment is performed, societally acceptable safety can be maintained (and in some cases improved) while at the same time effectively using scarce financial resources. Some work on societally acceptable risk and intangible factors can be found elsewhere (Jonkman et al., 2003; Vrijling et al., 1995).
8.4 Applications of Risk-Based Hydrosystem Design
In this section, two examples are described to illustrate the applications of risk-based design of hydrosystems. One is pipe culverts for highway drainage, and the other is flood-damage-reduction projects implemented by the
U. S. Army Corps of Engineers. The first example involves optimal risk-based design considering only hydrologic inherent uncertainty, whereas the second example considers uncertainties from hydraulic and economic aspects.
