What Manning’s N? A Need For Clear Definitions In Computational Modeling

Manning’s n is arguably the most important parameter in open channel hydraulics for estimating flow resistance. Flow resistance can be caused by a multitude of factors, e.g., sediment grains, bedforms, vegetation, various types of obstructions (large woody material, engineering log jams, bridge piers, etc.), and channel alignment, all of which are typically lumped into a Manning’s n value. Since the debut of Manning’s equation in 1891 (Manning, 1891), hydraulic engineers have enjoyed the convenience of using one single parameter to capture the complexity of flow resistance. However, it also draws constant criticism due to the same reason. The major criticisms are the large uncertainty and the empiricism nature of how the equation was developed. Such criticisms are amplified in modern-day computational hydraulics modeling where Manning’s n is often the only tuning parameter in various one-dimensional (1D) and two-dimensional (2D) hydraulic models. Therefore, it is not only a friction factor but also a surrogate for what models cannot resolve which creates many misconceptions about what Manning’s n is under different modeling approaches. The lack of clear definitions of Manning’s n in different modeling approaches is the root cause of this confusion. This paper strives to bring some clarity to this matter, especially in the context of 2D computational hydraulic modeling. In the physical world, Manning’s n has the textbook definition of a flow resistance parameter, which can be termed the real-world total Manning’s n. In the numerical modeling world, the total Manning’s n is divided into two parts: resolved and unresolved Manning’s n values. In a 2D model, part of the flow resistance can be resolved by its terrain data and mesh. The remaining part is what should be parameterized by the unresolved Manning’s n. As a result, the Manning’s n specified in 2D models should only be the unresolved Manning’s n. It is clear that Manning’s n in 2D models is not only a physical parameter, but also a numerical one because it depends on the resolution of the terrain data and mesh, numerical schemes used, and whether subgrid treatment is used. A guideline for best practices of hydraulic model development starting with appropriate terrain and mesh resolution, followed by appropriate methods for capturing various hydraulic losses associated with flow resistance will be provided.