Status: Published as NCHRP Research Report 1077

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 corridors 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 practice 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 Environment: Reference Document.