The concept of roughness in fluvial hydraulics and its formulation in 1D, 2D and 3D numerical simulation models

This paper gives an overview of the meaning of the term "roughness" in the field of fluvial hydraulics, and how it is often formulated as a "resistance to flow" term in 1D, 2D and 3D numerical models. It looks at how roughness is traditionally characterized in both experimental and numerical fields, and subsequently challenges the definitions that currently exist. In the end, the authors wonder: Is roughness well understood and defined at all? Such a question raises a number of concerns in both research and practice; for example, how does one modeller use the roughness value from an experimental piece of work, or how does a practitioner identify the roughness value of a particular river channel? The authors indicate that roughness may not be uniquely defined, that there may be distinct "experimental" and "numerical" roughness values, and that in each field nuances exist associated with the context in which these values are used.