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

Abstract

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 1-D, 2-D & 3-D numerical models. It looks at how roughness is traditionally characterised 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.

A mixing length model for estimating channel conveyance

“…25 and 5 m and for four values of roughness in the range 0 . 002 to 2 m. While the concept of a roughness height larger than the depth of the channel itself may appear nonsensical it should be borne in mind that this is not a direct physical parameter (Morvan et al, 2008) and, as the zero-velocity depth is equal to 0 . 033k s , a solution can still be derived.…”

“…The distance is calculated explicitly by the model between each computational point and the walls and bed. This splits the channel into zones of influence from bed and walls (De Cacqueray et al, 2009;Morvan et al, 2008), and the mixing length varies within the cross-section according to the distance to the nearest solid boundary. The hydraulics are determined therefore by the distance to the point that would be expected to be most significant in generating local shear.…”

“…Intuitively, this is sensible: the hydraulic radius describes the ratio of the amount of water in the channel (and hence its weight) to the length of wall in contact with the water. The hydraulic radius is a characteristic of the geometry and thus a simple way of incorporating the cross-section geometry in the model (Chanson, 1999;Morvan et al, 2008). However, it is not clear why this simple combination of area and perimeter should control the velocity, rather than some other, more complex relationship.…”

Information gap analysis of flood model uncertainties and regional frequency analysis

“…[18] Manning's equation is inevitably an approximation to the relationship between discharge and stage in a real channel, due to uncertainties in the estimation of Manning's n, and assumptions in the physics used to model the hydraulic processes [Morvan et al, 2008]. There will therefore be a discrepancy between the depth predicted in equation (5) and the actual flood depth, for a given discharge.…”

A semi-analytical model for form drag of river bedforms

“…In modeling river flow and predicting water levels, it is of particular importance to understand the processes that determine flow resistance, as the output of river-reach models has appeared sensitive to flow resistance of the main channel and the floodplains (e.g. Casas et al, 2006;Morvan et al, 2008). The accuracy of the output of a model system (e.g.…”