It should be noted that the Manning’s n and Percent Impervious provided in this post are subject to further examination by the modelers and they shall be calibrated whenever it is possible. This post intends to provide some general guidelines on initial selection of manning’s n values for HEC-RAS 2D modeling. A lot of researches have been done to study Manning’s n values in 1D channel but not so many for Manning’s n values in a 2D domain. A lot of factors impact manning’s n values for 2D modeling, including but not limited to overland surface type and texture, pervious or impervious areas, 2D flow depth. In HEC-RAS 2D modeling, manning’s n values (Roughness Coefficient) for 2D flow area are normally associated with land cover types and used to account for energy friction losses of overland flow and/or channel flow (if channel flow is also modeled by 2D domain).
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