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<br />Vo = average velocity directly upstream of the bridge (m/s) <br /> <br />b. Select an adjustment coefficient to account for the location of the pier or <br />abutment within the cross section. Some judgment is needed for selecting <br />the coefficient, Cf, but generally a coefficient at1.0 to 1.1 can be used. <br /> <br />Cf = 0.9, for a location near the bank of the river. <br />Cf = 1.0, for most applications <br />Cf = 1.1, for a structure in the main current of flow at a sharp bend. <br />Cf = 1.2, for a structure in the main current of the flow around an <br />extreme bend, possible cross flow generated by adjacent bridge <br />abutments or piers. <br /> <br />NOTE: HEC-18 (Richardson and Davis 1995) recommends values of C1 as <br />large as 1.7 (see Design Guildeline 8). <br /> <br />Alternatively, a hydraulic computer model could be used to determine the <br />local velocities directly upstream of bridge piers or abutments. A 1- <br />dimensional hydraulic model (Le., HEC-RAS, WSPRO) could be used to <br />compute velocity distributions within in a cross section on a relatively straight <br />reach. A 2-dimensional hydraulic model (Le., RMA.2V, FESWMS) could be <br />used to estimate local velocities in meandering reaches or reaches with <br />complex flow patterns. <br /> <br />c. Select an adjustment coefficient for shape of the pier or abutmenl. As with <br />the CSU equation for pier scour, if angle of attack, (1, is greater than 50, set <br />all shape coefficients to 1.0. <br /> <br />For piers: <br />Cs = 1.0, for a circular pier. <br />Cs = 1.1, for a square nose pier. <br />Cs = 0.9, for a sharp nose pier streamlined into the approach flow. <br /> <br />For abutments: <br />Cs = 1.1, for a vertical wall abutment. <br />Cs = 0.85, for a vertical wall abutment with wingwalls. <br />Cs = 0.65, for a spill through abutment. <br /> <br />d. Determine if the top surface of the pad can be placed level with the channel <br />bed and select the appropriate coefficient. <br /> <br />Ch = 1.0, Level . Top of pad is flush with the channel bed. <br />Ch = 1.1, Surface - Two layers of pad extend above channel bed. <br /> <br />NOTE: This is not a correction for mounding. Mounding is strongly <br />discouraged because it generates adverse side effects. The effects of <br />mounding were not addressed in the CSU study. Pad heights were kept at <br />0.2 times the approach flow depth or less. <br /> <br />6.4 <br />