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<br />INTRODUCTION <br /> <br />There has been increasing interest and activity in flood-plain manage- <br />ment, flood-insurance studies, and in the design of bridges and highways <br />across flood plains. Hydraulic computations of flow for such studies require <br />roughness coefficients, which represent the resistance to flood flows in <br />channels and flood plains. <br /> <br />Although much research has been done to determine roughness coefficients <br />for open-channel flow (Carter and others, 1963), less research has been done <br />on determining roughness coefficients for densely vegetated flood plains, <br />coefficients that are typically very different from those for channels. <br /> <br />There is a tendency to regard the selection of roughness coefficients as <br />either an arbitrary or an intuitive process. Specific guidelines are needed <br />to select roughness coefficients for densely vegetated flood plains so that <br />consistent values will be selected. <br /> <br />The U.S. Geological Survey in cooperation with the Federal Highway <br />Administration conducted a research study of roughness coefficients for <br />densely vegetated flood plains. The purpose of the study was to evaluate <br />methods of determining roughness values and to document roughness character- <br />istics for densely vegetated flood plains. A design guide (Arcement and <br />Schneider, 1983) was developed using the information collected for this <br />research report. <br /> <br />A variety of formulas exists for computing the flow resistance for typi- <br />cal open-channel flow. The Manning's, the Chezy, and the Darcy-Weisback <br />formulas are the ones most commonly used today. <br /> <br />Despite the limitations of the Manning's for.mula, as pointed out by <br />Rouse (1965) and Carter and others (1963), it is the one used most frequently <br />by engineers today. The Manning's formula, frequently used as a part of an <br />indirect computation of streamflow, is <br /> <br />in which <br /> <br />Q ~ AR2/3Sel/2 <br /> <br />n <br />discharge, in cubic feet per second; <br />cross-section area of channel, in square feet; <br />hydraulic radius, in feet: <br />slope of energy grade line, in feet per feet: and <br />Manning's roughness coefficient. <br /> <br />(1) <br /> <br />Q <br />A <br />R <br />Se <br />n <br /> <br />Equation 1 can be rewritten so that: <br /> <br />where: <br /> <br />in which <br /> <br />I <br />!. <br /> <br />Q KSel/2 <br />~ AR213 <br /> <br />K <br /> <br />K <br />A <br />R <br />n <br /> <br />n <br />conveyance of the channel, in cubic feet per second; <br />cross-sectional area of channel, in square feet; <br />hydraulic radius, in feet; and <br />Manning's roughness coefficient. <br /> <br />(2) <br />