Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models (2024)

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2024 N A T I O N A L C O O P E R A T I V E H I G H W A Y R E S E A R C H P R O G R A M NCHRP RESEARCH REPORT 1077 Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models Xiaofeng Liu Ali Mahdavi Mazdeh The Pennsylvania State University University Park, PA Lyle W. Zevenbergen Lyle W. Zevenbergen, LLC Fort Collins, CO Casey M. Kramer Natural Waters, LLC Olympia, WA Subscriber Categories Highways • Bridges and Other Structures • Hydraulics and Hydrology Research sponsored by the American Association of State Highway and Transportation Ofcials in cooperation with the Federal Highway Administration

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C O O P E R A T I V E R E S E A R C H P R O G R A M S AUTHOR ACKNOWLEDGMENTS The research reported herein was performed under NCHRP Project 24-49, “Guidelines for Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models,” by the Depart- ment of Civil and Environmental Engineering at The Pennsylvania State University (PSU). PSU was the contractor for this study. Dr. Xiaofeng Liu, P.E., an associate professor of civil engineering at PSU, was the project director and prin- cipal investigator. The other authors of this report are Dr. Ali Mahdavi Mazdeh, a postdoctoral researcher at PSU; Lyle W. Zevenbergen from Lyle W. Zevenbergen, LLC; and Casey M. Kramer from Natural Waters, LLC. The work was done under the general supervision of Dr. Xiaofeng Liu. Mr. Zevenbergen and Mr. Kramer worked as subcontractors to PSU on this project. Scott Hogan from FHWA provided the Iowa River 2008 flooding case data. Scott Steele at Pennoni Associates, Inc., provided the Delaware Bridge 1-242 case data. Aquaveo provided free licenses of SMS. Reinaldo Garcia from Hydronia provided free licenses of RiverFlow2D. Dr. Yong Lai at the U.S. Bureau of Reclamation and Gary Brunner (formerly at U.S. Army Corps of Engineers, now at HDR) provided assistance and comments. Hassan Ismail, a former postdoctoral research associate in Dr. Liu’s group, participated in the early stage of this project and proposed the research plan. The research team is grateful to all of them. CRP STAFF FOR NCHRP RESEARCH REPORT 1077 Waseem Dekelbab, Deputy Director, Cooperative Research Programs Ahmad Abu-Hawash, Senior Program Officer Sheila A. Moore, Program Associate Natalie Barnes, Director of Publications Heather DiAngelis, Associate Director of Publications Kristin C. Sawyer, Editor NCHRP PROJECT 24-49 PANEL Field of Soils and Geology—Area of Foundations and Scour Solomon T. Woldeamlak, Minnesota Department of Transportation, Oakdale, MN (Chair) Luke Assink, Washington State Department of Transportation, Olympia, WA Jeffrey A. DeGraff, Vermont Agency of Transportation, Barre, VT Kathryn L. Eagan, BSC Group, Inc., Boston, MA Abiot Gemechu, Virginia Department of Transportation, Richmond, VA Abderrahmane Maamar-Tayeb, Texas Department of Transportation, Austin, TX Jose Vasconcelos, Auburn University, Auburn, AL Scott Hogan, FHWA Liaison Patricia Bush, AASHTO Liaison

NCHRP Research Report 1077: Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models presents guidelines for the selection and application of Manning’s roughness values for two-dimensional (2D) hydraulic models. The development of the guidelines involved extensive modeling and analysis. The research findings will be of interest to hydraulic engineers at state departments of transportation (DOTs) and other public agencies. Measures to improve the accuracy, reliability, and consistency in selecting roughness values for the application of 2D hydraulic models at highway crossings and in transportation cor- ridors will represent a major step forward in the evolving state of practice. Some state DOTs have readily embraced the application of 2D models for transportation facility planning, analysis, and design; other states have held back and taken a more wait-and-see approach as the tools and techniques have continued to advance. At present, whether a one-dimensional (1D) or a 2D modeling approach is taken, inconsistencies can exist from state to state, and even within states at the various district levels, on how to develop accurate and reliable hydraulic models. The fundamental issue is the selection and assignment of Manning’s roughness values for a variety of channel types and land uses. Manning’s roughness values, along with channel and floodplain geometry (i.e., topographic and hydrographic surface data), are recognized to be the two most important components for developing and calibrating hydraulic models. Identifying feasible, cost-effective, and consistent guidelines to improve the state of the prac- tice in selecting roughness values for hydraulic modeling is highly desirable. The potential payoff to owners of transportation assets, in particular roadways, bridges, and culverts, is significant if the accuracy and reliability of hydraulic models can be improved. Development of detailed guidelines on hydraulic modeling was needed to improve the state of practice on a national level and in the rapidly advancing field of 2D hydraulic models. Under NCHRP Project 24-49, “Guidelines for Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models,” Penn State University was asked to (1) develop guidelines for the selection and application of Manning’s roughness values in 2D hydraulic models for transportation-related riverine settings and (2) propose draft language to the AASHTO Drainage Manual, the Federal Highway Administration (FHWA) Hydraulic Design Series Number 7: Hydraulic Design of Safe Bridges, and the FHWA’s Every Day Counts (EDC) Two-Dimensional Hydraulic Modeling for Highways in the River Environ- ment: Reference Document. F O R E W O R D By Ahmad Abu-Hawash Staff Officer Transportation Research Board

1 Summary P A R T I Literature Review and Identifying Knowledge Gaps 11 Chapter 1 Literature Review and Knowledge Gaps 11 1.1 Introduction 11 1.2 Flow Resistance 14 1.3 Manning’s n 22 1.4 Current Guidance on Selection of Manning’s n 22 1.5 Synthesis of Literature Review and Knowledge Gaps 25 Chapter 2 Analytical Program to Achieve Research Objectives 25 2.1 Introduction 26 2.2 Analytical Program P A R T I I Execution of Analytical Program and Results 33 Chapter 3 Verification of Existing n Values for Surface Roughness and Spatial Effects of n 33 3.1 Comparison of Manning’s n in Different Models 37 3.2 Effect of Relative n Values in Composite Channels 44 Chapter 4 Flow Resistance Factors, Required Conditions for Resolving a Feature, and Effect of Model Choice 44 4.1 Channel Obstruction 79 4.2 Bedforms 101 4.3 Channel Irregularities 103 4.4 Stage and Discharge Relations and Available Guidelines 104 4.5 Vegetation 107 4.6 Summary of Simulation Cases and Examples 114 4.7 How to Obtain Total Manning’s n 117 Chapter 5 Development of 2D Modeling Best Practices 117 5.1 Overview of 2D Modeling Best Practices 117 5.2 Best Practices in 2D Hydraulics Modeling 127 Chapter 6 Development of a Decision-Making Process for Selecting Manning’s n for 2D Models 127 6.1 Overview 127 6.2 Decision-Making Process C O N T E N T S

135 Chapter 7 Verification and Demonstration of Proposed Guidelines 135 7.1 Iowa River 2008 Flooding Case 152 7.2 Susitna River Case 168 7.3 Delaware Bridge 1-242 Case 179 References 183 Appendix A Evaluation of Solvers in HEC-RAS 2D 191 Appendix B Detailed Evaluation of Hole-in-Mesh Method 199 Appendix C Manning’s n Sensitivity Analysis