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By way of contrast, in the LTEC design process the level of risk is a product of the analysis and is a function of individual design and site characteristics. Risk is explicitly defined and the quantified levels of risk for all the design alternatives are key factors in the decisionmaking process. e The contrast between the traditional and LTEC design processes can be further illustrated by examining the function of several important design elements in the two processes. 1.2 Function of Design Elements . The impl ications and manner of involvement of several design elements change \.hen LTEC decisionmaking process is employed The role of the design discharge is, for example, one of the IIlOSt difficult mental blocks to overcome in makin~ the transition from traditional to LTEC design. 1.2.1 Design and Overtopping Discharges ~ In the traditional design process, the design discharge is a single valued input which is generally based on arhitary standards. The design discharge is used to determi ne structure size and the resulting backwater elevation. Freeboard and structural depth are then added to the backwater elevation, and, in some situations, the resulting elevation may determine the highway profile. The risk associated with this design configuration is then assessed using the base flood Under risk analysis, a range of flood events is utilized in the analysis and the end products are the LTEC design and the overtoppi ng discharge. Thus, the discharge associated with the selected design alternative is a product of the analysis not an input parameter. The overtopping discharge is by definition that discharge described by the probability of exceedance and water surface elevation at which flow occurs over the highway, over the watershed divide, or through structure(s) provided for emergency relief. Thus, the overtoppi ng di scharge IIlUSt be accommodated by the LTEC design. e The base flood is silnply another flood included in the range of diSCharges selected for the analysis. It probably will not be the flood associated with the LTEC design, nor the largest flood considered in the analysis. 1.2,.2 Freeboard , In traditional design, providing embankment freeboard may be accomplished by either adding additional embankment elevation after the design headwater is determined or by selecting a design which results in a headwater below the desi r8d '~ilhankment freeboard. In risk analysis, this procedure has no Illeaning, since it \^IOuld simply add another trial design for consideration. . Providing freeboard to protect bridge structures from debris- and scour- related failures is, hovlever, required by policy, where practicable. This may be accGlllplished after the LTEC design is selected by specifying the desired "low steel" elevation for the bridge structure. Freeboard can be defined in this context as the positive difference between the elevation of "low steel" and the overtopping flood elevation. e 2