Steady nonuniform shallow flow within emergent vegetation

Abstract

Surface flow redistribution on flat ground from crusted bare soil to vegetated patches following intense rainfall events elevates plant available water above that provided by rainfall. The significance of this surface water redistribution to sustaining vegetation in arid and semiarid regions is undisputed. What is disputed is the quantity and spatial distribution of the redistributed water. In ecohydrological models, such nonuniform flows are described using the Saint-Venant equation (SVE) subject to a Manning roughness coefficient closure. To explore these assumptions in the most idealized setting, flume experiments were conducted using rigid cylinders representing rigid vegetation with varying density. Flow was induced along the streamwise x direction by adjusting the free water surface height H(x) between the upstream and downstream boundaries mimicking the nonuniformity encountered in nature. In natural settings, such H(x) variations arise due to contrasts in infiltration capacity and ponded depths during storms. The measured H(x) values in the flume were interpreted using the SVE augmented with progressively elaborate approximations to the roughness representation. The simplest approximation employs a friction factor derived from a drag coefficient (C d ) for isolated cylinders in a locally (but not globally) uniform flow and upscaled using the rod density that was varied across experiments. Comparison between measured and modeled H(x) suggested that such a ''naive'' approach overpredicts H(x). Blockage was then incorporated into the SVE model calculations but resulted in underestimation of H(x). Biases in modeled H(x) suggest that C d must be varying in x beyond what a local or bulk Reynolds number predicts. Inferred C d (x) from the flume experiments exhibited a near-parabolic shape most peaked in the densest canopy cases. The outcome of such C d (x) variations is then summarized in a bulk resistance formulation that may be beneficial to modeling runon-runoff processes on shallow slopes using SVE.

Effect of submerged vegetation density on flow under favorable pressure gradient

“…They suggested that drag coefficient must be varying along vegetation length in stream-wise direction. Their finding illustrated that C d (x)/< C d > increases and then decreases with increasing distant from flume inlet forming a hump-shape that has been studied elsewhere [28]. The drag coefficient for dense vegetation differs from that of single vegetation of the same form.…”

“…Most of the studies used solid circular cylinders in regular rows for quantifying the impact of vegetation on flow characteristics and drag coefficient [28][29][30][31][32][33]. However, cylinders do not adequately represent vegetation because of the differences in flexibility, roughness, shape, etc.…”

Effects of precipitation intermittency on vegetation patterns in semi-arid landscapes

“…Indeed, overland flow of water during intense rainfall events on semi-arid flat plains is the subject of ongoing research (e.g. [89,96,72]). A detailed description of the overland water flow and infiltration into the soil that occurs before water is consumed by plants relies on a clear distinction between the surface water density and the soil moisture.…”

Resistance to Flow on a Sloping Channel Covered by Dense Vegetation following a Dam Break

“…For an isolated cylinder, the local C d (labeled as C d ,iso ) can be determined from the bulk velocity and rod diameter by forming an element Reynolds number Re d = UD / ν . An approximate expression for C d ,iso that describes data for isolated cylinders and for Re d < 10 5 is given by (Cheng, ; Wang et al, ) where and …”

“…A number of formulations have been proposed to link S f to vegetation drag coefficient C d assuming a steady uniform flow. These formulations, or variants on them, have been shown to capture blockage, sheltering, angle of separation, among others (Baptist et al, ; Carollo et al, ; Chapman et al, ; Cheng, ; Cheng & Nguyen, ; Dijkstra & Uittenbogaard, ; Etminan et al, ; James et al, ; Järvelä, ; Kouwen et al, ; Kim et al, ; Konings et al, ; Tanino & Nepf, ; Wang et al, , ; Zhao et al, ). However, the dam break problem leads to transient surface waves (Kobayashi et al, ) as well as large horizontal gradients in Froude numbers (Ishikawa et al, ) not present in conventional uniform canopy flow studies.…”