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a. Available computer programs such as ERI-STARS and the Corps of Engine:rs HEC 6, b. Straight line eKtrapolation of present trends, c. Engineering jud~ment, d. The worst-case scenarios; i.e., in the case of a confluence with another stream just downstream of the bridge, assume the design flood would occur with a low downstream water-surface elevation through a qualitative assessment of the joint probability of flood magnitudes and river conditions on the main stream and its tributary. 2. If the stream is aggrading and this condition can be expected to affect the crossing, taking into account contraction scour, consider relocation of the bridge or raising the lower cord of the bridge, 3. If the stream is d3grading, use the change in elevation in the calculations of total scour, C. STEP 3. EVALUATE THE SCOUR ANALYSIS METHOD The method is based on the assu~ption that the sccu= co~pcnents develop independently, Thus, the potential local scour is added to the contraction scour without considering the effects of contraction scour on the channel and bridge hydraulics. If contraction scour is significant, an alternate method presented in Appendix A may be used, o Estimate the natural channel's hydraulics for a fixed-bed condition based on existing conditions, o Assess the expected profile and plan form changes, o Adjust the fixed-bed hydraulics to reflect any expected long-term profile or plan f0rm changes, o Estimate contraction scour using the empirical contraction formula and the adjusted fixed-bed hydraulics, o Estimate local scour using the adjusted channel and bridge hydraulics, and o Add the local Ecour to the contraction scour to obtain the total scour. Chapter 3, Design procedure, step 5. 34