Principal Civil and Environmental Consultants, Inc. Knoxville, TN
Abstract Description: Manning’s “n” has been with us for over 100 years. It is the empirical coefficient in the Manning Formula intended to account for energy losses due to friction, sinuosity, and other factors in a uniform channel. In applying the Manning Formula to a broader range of engineering problems, Manning’s “n” values have been extended for natural channels and floodplains with irregular and variable cross-sections and patterns. These applications include the routing of dam breach hydrographs in both 1-dimensional (1D) and 2-dimensional (2D) analyses.
By their derivation, the established empirical values for Manning’s n represent channel and floodplain roughness but it also accounts for the energy losses caused by turbulence and form roughness in 3-dimensional space. Much like the qualifiers of “steady” and “uniform” the qualifier for the energy loss term is just a footnote in many analyses. For 2D models, the energy loss from lateral flow paths is directly accounted for in the analysis through the relationship between the computational mesh and the underlying terrain. If the Mannings “n” values are not modified, a 2D model will over count the energy losses due to lateral flow variation, however the magnitude of the over counting will vary with the channel’s characteristics.
Dam breach events are routinely modeled using the U.S. Army Corps of Engineers Hydrologic Engineering Center, River Analysis System (HEC-RAS). The results are typically summarized graphicly as inundation maps that show the arrival time and maximum depth of floodwaters. These breach models may be developed using either or both 1D and 2D approximations for the governing equations of fluid mechanics. The effect of Manning’s “n” selection on 2D modeling was examined for breaches at multiple dams using HEC-RAS 2D. The value of Manning’s n was reduced between 10 to 50 percent for each model to determine the effect on the inundation boundary, arrival time, water surface elevation, and flood depth.
Learning Objectives:
Understand what Manning's n represents and how it is applied in 1D and 2D modeling in HEC-RAS.
Demonstrate the variation in modeled inundation parameters as Manning's n varies in a 2D HEC-RAS model.
Provide an approach for selecting Manning's n for 2D inundation modeling using HEC-RAS.
