Guide for Selecting Manning's Roughness Coefficients for Natural Channels and Flood Plains By GEORGE J. ARCEMENT, JR., and VERNIE R. SCHNEIDER Prepared in cooperation with the U.S. Department of Transportation, Federal Highway Administration A guide presenting step-by-step procedures for selecting Manning's roughness coefficient, n, for natural channels and flood plains. Photographs of flood-plain segments can be used for comparison with similar flood plains to aid in assigning n values. TABLE OF CONTENTS Abstract Introduction Methods Channel n Values     Base n Values (nb) for Channels     Adjustment Factors for Channel n Values         Irregularity (n1)         Variation in Channel Cross Section (n2)         Obstructions (n3)         Vegetation (n4)         Meandering (m) Flood-Plain n Values     Modified Channel Method         Adjustment Factors for Flood-Plain n Values             Surface Irregularities (n1)             Obstructions (n3)             Vegetation (n4)     Vegetation-Density Method         Techniques for Determining Vegetation Density             Indirect Technique             Direct Technique Photographs of Flood Plains Procedures for Assigning n Values     Steps for Assigning n Values         Reach Subdivision (Steps 1 and 2)         Channel Roughness (Steps 3-13)         Flood-Plain Roughness (Steps 14-23)     Examples of Procedures for Determining n Values Summary References Cited FIGURES 1. Diagram of a schematic and cross sections of a hypothetical reach of a channel and flood plain showing subdivisions used in assigning n values 2. Graph showing relation of stream power and median grain size to flow regime 3. Diagram showing forms of bed roughness in sand-bed channels 4. Graph showing effective-drag coefficient for verified n values versus the hydraulic radius of wide, wooded flood plains 5. Chart of example measurement of vegetation density showing tree diameter and location in representative sample area TABLES 1. Manning's n values. Hanging Moss Creek at Interstate Route 55 in Jackson, Miss., 1964-72 ABSTRACT Although much research has been done on Manning's roughness coefficient, n, for stream channels, very little has been done concerning the roughness values for densely vegetated flood plains. The n value is determined from the values of the factors that affect the roughness of channels and flood plains. In densely vegetated flood plains, the major roughness is caused by trees, vines, and brush. The n value for this type of flood plain can be determined by measuring the vegetation density of the flood plain. Photographs of flood-plain segments where n values have been verified can be used as a comparison standard to aid in assigning n values to similar flood plains. LizardTech's Djvu plug-in is needed to view these reports. CLICK HERE TO VIEW THE ENTIRE REPORT For questions or comments, contact K. Van Wilson.