Guide for selecting Manning's roughness coefficients for natural channels and flood plains

Introduction 1 Methods 2 Channel n Values 2 Base n Values (nb) for Channels 2 Adjustment Factors for Channel n Values 6 Irregularity (HJ) 6 Variation in Channel Cross Section («2) 6 Obstructions (n3) 6 Vegetation («4) 6 Meandering (m) 8 Rood-Plain n Values 8 Modified Channel Method 8 Adjustment Factors for Flood-Plain n Values 8 Surface Irregularities (HJ) 8 Obstructions («3) 8 Vegetation (n4) 8 Vegetation-Density Method 9 Techniques for Determining Vegetation Density 9 Indirect Technique 10 Direct Technique 10 Photographs of Flood Plains 13 Procedures for Assigning n Values 28 Steps for Assigning n Values 28 Reach Subdivision (Steps 1 and 2) 28 Channel Roughness (Steps 3-13) 28 Rood-Plain Roughness (Steps 14-23) 29 Examples of Procedures for Determining n Values 32 Summary 32 References Cited 37FIGURES 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 3 2. Graph showing relation of stream power and median grain size to flow regime 5 3. Diagram showing forms of bed roughness in sand-bed channels 5 4. Graph showing effective-drag coefficient for verified n values versus the hydraulic radius of wide, wooded flood plains 10 5. Chart of example measurement of vegetation density showing tree diameter and location in representative sample area 12