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Design Discharge Understanding the new role of design discharge is one of the most difficult tasks in making the transition from traditional design to risk-analysis design. Traditionally, designers start with a semi- arbitrary design discharge, select a bridge opening that will pass this discharge, and add freeboard to computed water-surface elevations to establish embankment and low bridge steel elevations. In risk analysis the design discharge for any combination of embankment and bridge opening size is the discharge whose elevation upstream of the encroachment is equal to the minimum elevation of the encroachment. (In other words, it is on the verge of overtopping the highway.) In risk analysis, there is no one discharge that dictates the design, and the design discharge is one of the results of the analysis. Since the risk analysis described in this report is oriented towards traffic requirements, design discharge that results from the risk analysis is an indicator for impending traffic interruptions. Designers maYr in fact, choose to make several preliminary traditional sets of computations based on arbitrary discharges to determine some of the trial bridge designs to be used in the risk analysis. Such computations are, however, side issues to the risk analysis because trial bridge designs could be selected by other methods. The final design discharge that results from the risk analysis mayor may not correspond to any of the discharges used in the preliminary computations. Risk analysis can be applied to many aspects of design. This report is oriented to determine embankment elevations and bridge lengths, but the concepts are applicable to other components of design. Other components of design that could be determined by risk analysis include scour depths used to establish foundation elevations, clearance requirements for low steel elevations on bridges, and protection measures. Each of these components may be associated with design discharges that are quite different from one another. For example, Laursen (1970) made a sensitivity risk analysis to illustrate that scour estimates should be based on the maximum flood that might occur because the results of losing a typical bridge are so catastrophic. In other words, the design discharge associated with scour computations may have a very low exceedance probability (say .0001) in a given year; whereas, the design discharge associated with establishing the embankment elevation and bridge length may have 'a relatively higher exceedance probability (say .04) in a given year. The Base Flood The base flood (l-percent-chance flood) is referred to in Executive Order 11988--Floodplain Management. Executive Order 11988 is intended to avoid to the extent possible adverse impacts associated with occupancy and modification"of flood plains. Flood plains, as a mimimum, include areas subject to inundation by the base flood. 5