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MECHANICS OF STUDY The purpose of this chapter is to discuss the design philosophy used in this report, to define risk analysis, to clarify some design concepts as they apply to risk analysis, to outline the procedure used, and to suggest one possible report outline. Design Philosophy Hydraulic design with risk analysis incorporates economic risks directly into the design process. Economic risks are expected losses that can be divided into three categories: (1) direct damage to the roadway and bridge, (2) traffic related losses, and (3) losses due to additional flood damage in the upstream flood plain. Traditionally, the hydraulic design of bridges has been based on hydrologic and hydraulic considerations with economics incorporated in a very indirect manner. The design philosophy recognizes that no single discharge can characterize the random stresses that nature will impose on a bridge site; thus, it is necessary to evaluate expected losses from the full flOOd-frequency curve by applying the appropriate exceedance probability for each discharge on the curve. The philosophy presumes that all the losses can be quantified and that an optimal design can be found by minimizing the sum of expected losses and capital costs expressed on an equivalent economic basis. The design techniques associated with this philosophy are conceptually simple. Techniques involve: (1) trying several bridge designs, (2) selecting several discrete discharges to adequately represent the flood frequency curve, (3) repetitively making hydraulic and economic computations with each discharge for a given alternate design, and (4) ultimately, selecting the alternate that has the least total cost to society considering expected losses as well as capital costs. Several notions about design require elaboration to further illustrate the implications of this design philosophy. These notions include the so called design discharge, the base flood, freeboard, and economic analysis which are discussed in detail below. The results of the risk analysis are one component of the decision making process. The decision making process will also include consideration of budgetary constraints, the need for emergency supply and evacuation routes, and the need for emergency vehicle access. The provisions of the Flood Insurance Act may also apply to the design process. The risk analysis, however, provides a means of assessing the cost of such constraints by comparing the cost of the encroachment with the constraints to the cost of the alternate that has the least total cost to society. 4