Comment on “Drag, turbulence, and diffusion in flow through emergent vegetation” by H. M. Nepf

Abstract

In a recent paper, Nepf [1999, p. 479] presented a model "... to describe the drag, turbulence, and diffusion for flow through emergent vegetation which for the first time captures the relevant underlying physics .... "The author also presented a set of laboratory and field experiments and claimed that they support the model. Although this paper may be considered as a valuable and stimulating contribution to the quickly developing field of ecohydraulics, it contains a number of points which require clarifications and comments.1. The Nepf model is heavily based on the turbulence energy balance, which includes the wake production term Pw. .g., Figures 7, 8, and 9). The velocity spectrum in Figure 4 clearly shows that the total turbulence energy k t is much larger than the contribution from wakes at scale d, that is, k t >> k. There is a strong low-frequency maximum at •0.1 Hz, which is, most probably, due to shear turbulence, though its origin is not completely clear as Nepf presented insufficient information on background conditions of experimental runs. Such a shape of the spectrum disagrees with the author's interpretation of (4) and (5), as well as with a statement on page 1985

Lateral dispersion in random cylinder arrays at high Reynolds number

“…Note that the measurement at d/ s n A = 0.93 falls between the extrapolation of the two expressions in (4.1), suggesting transition effects. Field measurements by Neumeier & Amos (2006), Nikora (2000) and Leonard & Luther (1995), presented in figure 15, fall within the range of √ k t /U p observed in the present study. To the authors' knowledge, these are the only field reports in which both turbulence measurements and stem density are presented for emergent plant canopies.…”

“…Vertical bars represent the uncertainty in the gradient as defined by (3.10). Field measurements of √ k t /U p in emergent plant canopies by Nikora (2000, ), Leonard & Luther (1995, +), and Neumeier & Amos (2006, unpublished values and details provided by U. Neumeier, personal comm., rectangle) are also plotted. U. Neumeier provided eight depth profiles of ( u 2 , v 2 , w 2 ) in emergent canopies, but only the profile where the estimated wind-induced horizontal and vertical wave speeds were less than 50% of the reported r.m.s.…”

Transverse and longitudinal mixing in real emergent vegetation at low velocities

“…First, energy dissipation from the stem drag force, due to the wake structure formation, was assumed to appear as turbulent kinetic energy and termed “turbulent” diffusion. The validity of this assumption is discussed in more detail in Nikora [] and in the subsequent reply by Nepf []. Second, the individual flow paths imposed by the tortuosity caused by the physical obstruction of the stems, was termed “mechanical” diffusion.…”

A mixing-length model for predicting vertical velocity distribution in flows through emergent vegetation

“…Recognition of the hydraulic processes of flows in vegetated channels is still at a preliminary stage. Vegetation influences the resistance of the watercourse considerably, creates additional drag exerted by plants, causes a violent transverse mixing due to great differences in velocities in vegetated and non-vegetated regions, and also affects the turbulence intensity and diffusion (Nepf, 1999;Nikora, 2000;Rowinski et a!., 1998). All these processes have been studied extensively, but many questions remain unanswered and constitute the basis for an important debate.…”