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<br />bridge. However, special conditions (angle of at~ack, <br />pressure flow, decrease in velocity or discharge resulting <br />from high flows oV2rtopping approaches or going through <br />relief bridges, iC2 jams, ete,) may cause a more severe <br />condition for scour with a flow smaller than the overtopping. <br />or lOO-year flood. <br /> <br />2. Determine the water-surface profiles for the disc~arges <br />judged to produce the most scour from step 1, using the FHWA <br />program WSPRO, BRI-STARS, HEC-2, or other existing methods <br />that employ the FH;vA bridge analysis, The engineer should <br />anticipate future ~onditions at the bridge, in the stream's <br />watershed, and at downstream water-surface elevation <br />controls. <br /> <br />3. Determine if there are existing or potential future factors <br />that will produce a combination of high discharge and low <br />tailwater control. Are there bedrock or other controls (old <br />diversion structures, erosion control checks, other bridges, <br />etc.) that might be lowered or removed? Are there dams or <br />locks downstream that would control the tailwater elevation <br />seasonally? Are there da~s upstream or downstream that <br />could control the elevation of the water surface at the <br />bridge? Select the lowest reasonable downstream water- <br />surface elevation and the largest discharge to estimate the <br />greatest scour potential, Assess the distribution of the <br />velocity and discharge per foot of width for the design flow <br />and other flows through the bridge opening. Consider also <br />the approach flow and the flow distri~ution downstream (the <br />contraction and expansion of the flew), This should take <br />into consideration present conditions and anticipated future <br />changes in the river. <br /> <br />4. From computer analysis and from other hydraulic studies, <br />determine the discharge velocity and depth input variables <br />needed for the scour calculations, <br /> <br />5, Collect and summarize the following information as <br />appropria~e, <br /> <br />a. Boring logs to define geologic substrata at the bridge <br />site. <br /> <br />b. Bed material size and gradation distribution in the <br />bridge reach. <br /> <br />c. Existing strean and flood plain cross-section through the <br />reach. <br /> <br />d. Stream geomorphic plan form. <br /> <br />e. Watershed characteristics. <br /> <br />32 <br />