Discussion of 'Laboratory Investigation of Mean Drag in a Random Array of Rigid, Emergent Cylinders' by Yukie Tanino and Heidi M. Nepf

摘要

The discussers acknowledge the authors' important effort to devise a conceptual framework for the quantification of the drag coefficient of rigid stems in dense arrays of emergent vegetation. Having performed laboratory work in conditions similar to those described in the paper, the discussers found that their results can provide support for some of the hypotheses and simplifications necessary to compute flow resistance for these conditions. This discussion examines the hypotheses and simplifications that underlie Eq. 9, drawing particular attention to the role of form-induced stresses (or dispersive fluxes, Raupach et al. 1986), which are yet to be fully understood. The discussion is focused on (1) the relative magnitude of the Reynolds and form-induced shear stresses; (2) the relative magnitudes of the terms in the equation of conservation of mean momentum in the longitudinal direction; and (3) the relation between normal longitudinal form-induced stresses and the drag force.