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<br />I <br />constant slopes as a base, the velocity in a 3-foot pipe <br />will be 1.03 larger and the velocity in an 8-foot pipe will <br />be 0.97 smaller. Less velocity change would be obtained <br />for corrugated metal pipes. <br /> <br />Some reduction in outlet velocity can be obtained by increasing <br />the number of barrels carrying the total discharge. Reducing <br />the flow rate per barrel reduces velocity at normal depth, <br />if the flowline slopes are the same. Substituting two <br />smaller pipes with the same depth to diameter ,ratio for <br />a large one raduces Q per barrel to one-half the original <br />rate and the outlet velocity to approximately 87 percent <br />of that in the single-barrel design. However, this 13 <br />percent reduction must be considered in light of the increased <br />cost of the culverts. In addition, the percentage reduction <br />decreases as the number of barrels is increased. For example, <br />using four pipes instead of three results in only an additional <br />5 percent reduction in outlet velocity. Furthermore, where <br />high velocities are produced, a design using more barrels <br />may still result in velocities requiring protection, with <br />a large increase in the area to be protected. <br /> <br />Outlet velocities can also be modified by substituting a <br />rough barrel for a smooth barrel. For a GO-inch concrete <br />pipe, n = 0.012, on a I-percent slope (So= 0.01), discharging <br />at 100 cfs, Vn = 13.8 fps and Sc = 0.00325~ using a c.m. <br />pipe (n = 0.024) results in a critical slope of O.Q15. <br />Since Sc for the c.m. pipe is greater than the actual slope, <br />the flow is subcritical and the outlet velocity will be <br />critical velocity or 8.5 fps. Manning's e~uation V = <br />(1.49 R2/3S1/2)/n shows that V varies as S /2/n. For the <br />critical slope situation (R is a constant), doubling the <br />roughness results in a four-fold increase in critical slope. <br />When using this method of velocity reduction, it should <br />be remembered that changing the flow from supercritical <br />to subcritical may result in a marked change in the headwater. <br /> <br />Substituting a "broken-slope" flow line for a steep, continuous <br />slope is not recommended for controlling outlet velocity. <br />Such a design is based on the assumption that the reduced <br />slope of the lower barrel will control depth and velocity, <br />as indicated by the Manning formula. Where the total fall <br />from inlet to outlet remains the same, a broken-slope flow <br />line reduces the outlet velocity only slightly. The initial <br />steeper slope will bring about a lesser depth and greater <br />velocity at the break in grade, followed by a small increase <br />in depth in the lesser slope section. In supercritical <br /> <br />111-4 <br />