Manning’s n for 2D Hydraulic Modeling of Dam Breaches

Manning’s roughness coefficient (Manning’s n) is the empirical coefficient in Manning’s Formula intended to account for energy losses due to surface roughness and form drag (bedforms, vegetation, obstructions, sinuosity, and other factors) in an open channel. The established empirical values for Manning’s n represent channel and floodplain roughness but also account for the energy losses caused by turbulence and form roughness in 3-dimensional space. Adapting Mannings n values for 2D models is an evolving practice, but in general it is expected that Mannings n values in 2D models will be less than those used in 1D models. If the Manning’s n values are not modified, a 2D model will overcount the energy losses. The magnitude of the reduction is a function of the flow resistance imposed by the 2D model’s terrain and mesh. Dam breach events are routinely modeled using the U.S. Army Corps of Engineers Hydrologic Engineering Center, River Analysis System (HEC-RAS). This paper does not provide guidance on selecting Mannings n for 2D models. However, recognizing that Mannings n should generally be reduced for 2D models, the effect of reduced Manning’s n on 2D model results was examined for breaches at multiple dams using HEC-RAS 2D.