Hydraulic Design of Highway Culverts - DOT On-Line Publications

Figure VI-25--Precast Double Barrel Box Culvert
4. Risk Analysis. Risk analysis is a means of assessing the economic behavior of different
design alternatives. The construction of a culvert represents a flood plain encroachment with
the associated flood risks and initial construction costs. Each design strategy can be evaluated
for an annual capital cost and an annual economic risk (cost), the sum of which is called the
total expected cost (TEC). Optimization of the economic and engineering analyses will produce
the least total expected cost (LTEC) design alternative (46).
The influence of risk (cost) in the decision-making process represents the major distinction
between traditional and LTEC design. In traditional design, the level of risk is an integral part of
the establishment of design standards such as a specified design frequency flood or limitations
on backwater. The influence of risk in the design of a specific culvert based on these design
standards will vary with site conditions. In LTEC design, there is no arbitrary design frequency.
The design process determines the response of each alternate design to discrete points on the
entire flood frequency curve. The flood frequency at which road overtopping occurs is more
meaningful than design flood frequency.
A necessary part of the risk analysis process is the establishment of acceptable design
alternatives. Engineering, legislative, and policy constraints may limit the range of alternatives.
Examples of such constraints include:
• Prescribed minimum design flood criteria as in the case of interstate highways.
• Limitations imposed by roadway geometrics such as maximum or minimum
grade lines, site distance, and vertical curvature.
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