  Large-eddy simulations are conducted to contrast momentum and passive scalar
transport over large, three-dimensional roughness elements in a turbulent
channel flow. Special attention is given to the dispersive fluxes, which are
shown to be a significant fraction of the total fluxes within the roughness
sublayers. Based on point-wise quadrant analysis, the turbulent components of
the transport of momentum and scalars are found to be similar in general,
albeit with increasing dissimilarity for roughnesses with low frontal blockage.
However, strong dissimilarity is noted between the dispersive momentum and
scalar fluxes, especially below the top of the roughness elements. In general,
turbulence is found to transport momentum more efficiently than scalars, while
the reverse applies for the dispersive contributions. The effects of varying
surface geometries, measured by the frontal density, can be pronounced on both
turbulent and dispersive fluxes. Increasing frontal density induces a general
transition in the flow from a rough boundary-layer type to a mixed-layer-like
type. This transition results in an increase in the efficiency of turbulent
momentum transport, but the reverse occurs for scalars due to reduced
contributions from large scale motions in the roughness sublayers. This study
highlights the need for distinct parameterizations of the dispersive scalar
fluxes and the importance of considering the contrasts between momentum and
scalar transport for turbulent flows over very rough surfaces.
