National Academies Publishes New Look at Manning’s Formula

NCHRP Research Report 1077: Selection and Application of Manning’s Roughness Values in Two-Dimensional Hydraulic Models, from TRB’s National Cooperative Highway Research Program, presents guidelines for the selection and application of Manning’s roughness values for 2D hydraulic models. While it is geared toward highway design and large civil engineering projects, plumbing engineers will find it fascinating to read as there is always debate in our profession on the “n” value to use for the Manning equation.

According to the National Academies of Sciences, Engineering, and Medicine, 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. Whether a one-dimensional 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.

You can download it for free at the National Academies’ website.

Source: National Academies of Sciences, Engineering, and Medicine

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