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<br />\ <br />- --~~ <br />\ <br />~~~ol <br />~ <br />, <br />~ <br /> <br />'. <br />I <br /> <br />contraction will also be satisfactory for the other flow conditions. <br />All of the discussion on bridge crossings has pertained to use <br />of three cross sections to model the geometry of flow. The actual <br />number of cross sections required and their location relative to each <br />other and to the bridge depends upon the analytical technique used to <br />calculate the head loss. Nevertheless, all techniques which are <br />based on solving the energy equation must accomplish the same purpose <br />-- that is, to calculate the energy loss through a contraction. <br />Analytical techniques may require cross sections to be located between <br />sections 3 and 4, fig. 4.01, but if additional sections are included <br />they must model the flow boundaries --not just the flood plain width. <br /> <br />, <br />'. <br /> <br />Section 4.03. Hydraulic Roughness Values <br /> <br />Sources of n-Values <br /> <br />Sources for roughness values are found in publications such as <br />references 2 and 27. Roughness values can also be obtained from <br />hydraulically similar streams in the study area for which data is <br />available, or from field measurements of water surface profiles and <br />discharges. Of these sources of data, field measurements are by far <br /> <br />the most reliable. <br /> <br />Published data by the U. S. Department of Agriculture and the <br />U. S. Geological Survey provide excellent sources of information for <br /> <br />roughness values. When information is not available from these sources <br /> <br />the roughness value can be selected from the following table. <br /> <br />4.07 <br />