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Guide for Selecting Manning's Roughness Coefficients

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Guide for Selecting Manning's Roughness Coefficients GUIDE FOR SELECTING MANNING’S ROUGHNESS Turner-Fairbank Highway COEFFICIENTS FOR NATURAL CHANNELS Research Center 6300 Georgetown Pike AND FLOOD PLAINS McLean, Virginia 22101 Report No. 3 t u FHWA-TS-84-204 U.S. Department of Transportation Final Report April 1984 -1 HishrwaV AdministrWon Archived ‘This document is available to the U.S. public through the National Technical Information Service, Springfield, Virginia 2216 FOREWORD This Technology Sharing Report provides procedures for determining Manning's roughness coefficient for densely vegetated flood plains. The guidelines should be of interest to hydraulic and bridge engineers. Environmental specialists concerned with flood plains and wetlands may also find this report useful. The report was prepared by the United States Geological Survey, Water Resources Division, with technical guidance from the FHWA Office of Engineering and Highway Operations Research and Development. Sufficient copies of the publication are being distributed to provide a minimum of one copy to each FHWA region office, division office, and to each State highway agency. Additional copies of the report can be obtained from the National Technical Information Service, Springfield, Virginia 22161. DirectdrJ Office of / Director, Office of Engineering and Highway Implementation Operations R&D NOTICE This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. The contents of this report reflect the views of the contractor, who is responsible for the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policy of the Department of Transportation. This report does not constitute a standard, specification or regulation. The United States Government does not endorse products or manufacturers. Trade or manufacturer's names appear herein only because they are considered essential to Archivedthe object of this document. Technical Report Documentation Page 1. Report No. 2. G overnment Accession No. 3. Recipient’s Catalog No. 'FHWA-TS-84-204 I I 4. Title and Subtitle 5. Report Dote GUIDE FOR SELECTING MANNING'S ROUGHNESS COEFFICIENTS . April 1984 FOR NATURAL CHANNELS AND FLOOD PLAINS 6. Performing Organization Code 8. Performing Organization Report No. 7. Author’s) G. J. Arcement, Jr., and V. R. Schneider 9. Performing Organization Name and Address 10. Work Unit NO. (TRAIS) U. S. Geological Survey Water Resources Division 11. Contract or Grant No. P.O. Box 66492 13. TYPO of Report and Period Covered Baton Rouge, LA 70896 9 12. Sponsoring Agency Name and Address Office of Implementation Final Report Federal Highway Administration 6300 Georgetown Pike 14. Sponsoring Agency Code McLean, Virginia 22101 15. s upplementory Notes Co-COTR: John M. Kurdziel Roy Trent 16. Abstract Although much research has been done on Manning's roughness coefficients for stream channels, very little has been done on the selection of roughness values for densely vegetated flood plains. A procedure for the determination of Manning's roughness coefficient (n) for channels and flood plains analyzes the different roughness factors that affect the roughness coefficients for channels and flood plains. By determining the value of each factor and combining those values, the n value can be determined. Another procedure deals with densely vegetated flood plains where 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 are presented as a comparison standard to aid in assigning n values to similar flood plains. 17. Key Words Archived18. Distributiov Statement Roughness Coefficient, Manning Equation, No restrictions. This document is Channels, Stable, Unstable, Channel Flow, available to the public through the Flood Plains, Flow Resistance, Hydraulic National Technical Information Service, Roughness, Hydraulic Friction, Overland Springfield, Virginia 22161. Flow, Vegetation Effects 19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Prtce Unclassified Unclassified 68 Form DOT F 1700.7 (8-72) Reproduction of completed page authorized Page Introduction------------------------------------------------------- 1 Approach--- ------- _________________-------------------------------- 2 Method for assigning n values for channels------------------------- 3 Selection of base n values (nb)------------------------------- 4 Adjustment factors for channels- ----------------------~~~~~~~- 8 Irregularity (nl)---------------------------------------- 8 Variation in channel cross section (n2)------------------ 8 (&stru&ions (n3)---------------------------------------- 12 Vegetation (n4) ____-------------------------------------- 12 Meandering (m) ---------------------~~~~~~~-~~~-----~---~~ 13 Methods for assigning n values for flood plains-------------------- 13 Modified channel method--------------------------------------- 13 Adjustment factors for flood plains---------------------- 16 Surface irregularities (nl)------------------------- 16 &structions (n3)----------------------------------- 16 Vegetation (n4)---------------------------------------- 16 Vegetation density methods------------------------------------ 17 Techniques for determining vegetation density------------ 17 Indirect technique---------------------------------- 18 Direct technique------------------------------------ 18 Pho+-ogra@s of flood plains---------------------------------------- 23 procedures for assigning n values---------------------------------- 39 Description of steps for assigning n values------------------- 39 Channel roughness (steps 3-13)--------------------------- 39 Flood-plain roughness (steps 14-23)---------------------- 50 Examples of procedures for determining n values---------- 51 60 &lw+-& references------------------------------------------------ 61 Archived ii LIST OF FIGURES Page Figure 1. Graph showing relation of stream power and median grain size to flow regime----------------------------- 6 2. Forms of bed roughness in sand-bed channels------------- 7 3. Plot of effective-drag coefficient versus hydraulic radius for wide, wooded flood plains using verified n values---------------------------------------------- 19 4. Example measurement of vegetation density of repre- sentative sample area--------------------------------- 21 5. Example of determination of Manning's n by direct technique of vegetation density----------------------- 22 6-20. Photograph showing flood plains having verified n values: 6. Cypress Creek near Downsville, La., n=O.lO------- 24 7. Bayou de Ioutre near Farmerville, La., n=O.ll---- 25 8. Bayou de Iioutre near Farmerville, La., n=O.ll---- 26 9. Bayou de Loutre near Farmerville, La., n=O.ll---- 27 10 . Coldwater River near Red Banks, Miss., n=O.ll---- 28 11. Coldwater River near Red Banks, Miss., n=O.ll---- 29 12. Yockanookany River near Thomastown, Miss., n=O.l2----------------------------------------- 30 13. Yockanookany River near Thomastown, Miss., n=O.12 ----------------------------------------- 31 14. Flagon Bayou near Libuse, La., n=0.13----------- 32 15. Pea Creek near Louisville, Ala., n=0.14---------- 33 16. Pea Creek near Louisville, Ala., n=O.14---------- 34 17. Tenmile Creek near Elizabeth, La., n=0.15-------- 35 18. Sixmile Creek near Sugartown, La., n=O.18-------- 36 19. Thompson Creek near Clara, Miss., n=O.20--------- 37 20. Thompson Creek near Clara, Miss., n=0.20--------- 38 21. Flow chart of procedures for assigning n values--------- 40 22. Sample form for computing n values---------------------- 42 23. Sketch of a hypothetical cross section showing reaches, segments, and subsections used in assigning n values-- 47 LIST OF TABLES Table 1. &se values of Manning's n------------------------------ 4 2. Factors that affect roughness of the channel------------ 9 3. Factors that affect roughness of flood plains----------- 14 4. Cutline and example of procedures for determining n valuesArchived for a hypothetical channel and flood plain----- 52 iii SYMBOLSANDUNITS svmbol Definition Units A cross-section area of flow ft2 CAi the total frontal area of vegetation blocking the flow ft2 c* effective drag coefficient for vegetation ----- d84 particle diameter that equals or exceeds that of 84 percent of the particles ft gravitational constant ft/s2 height of water on flood plain ft conveyance of a section ft3/s length of channel reach being considered ft length of representative sample area ft m correction factor for meandering of channel or flood plain ----- n Manning's roughness coefficient, including boundary and vegetation effects ftl/6 "b base value of Manning's roughness coefficient for the surface material of the channel or flood plain ftl/6 Cnidi summation of number of trees in a sample area multiplied by diameter ft “0 Manning's roughness coefficient, excluding the effect of the vegetation ft1/6 "1 value of Manniq's roughness coefficient for the effect of surface irregularity ftV6 "2 value of Manning's roughness coefficient for variations in shape and size of channel and flood plain ft1/6 n3 value of Manning's roughness coefficient for obstruc- ftl/6 tions "4 Archivedvalue of Manning's roughness coefficient for vegetation ftl/6 "4' value of Manning's roughness coefficient used in determining no, representing
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