Abstract
Flow through vegetation has a significant impact on sediment transport and ecosystem robustness in the coastal and fluvial environment. Numerical models (Nepf and Vivoni, 2000; Uittenbogaaard, 2003) have been developed to simulate this type of flow. The success of these models depends on proper characterization of the main processes and appropriate setting of pre-defined empirical coefficients. Among others, the drag coefficient CD is one of the most important coefficients, which influences the mean velocity and the turbulence characteristics (Nepf and Ghisalberti, 2008).
 Tanino and Nepf (2008) and Cheng (2011) have derived empirical relationships of CD for flow through emerged rigid vegetation. Both studies confirm that CD is related to canopy properties (plants density, diameter, etc.) as well as flow conditions. However, in both studies CD is estimated by simply equating the vegetation drag force to the water level gradient. Bed shear stress and Reynolds stress were ignored. More importantly, the CD provided by these expressions is depth averaged, which is not suitable for modelling flow and canopy that both vary in vertical (Nepf and Vivoni, 2000). In this study, the CD relation proposed by Cheng (2011) is modified. This new relation depends on the local flow conditions and canopy properties in the vertical. Further, this relation is implemented in an iterative scheme of a 1DV flow model. The modelling results are compared with experiment data of flow through emerged and submerged rigid vegetation. Our results show that when special defined Reynolds number is small, this relation performs less well compare to that when it is larger.
Highlights
The interaction between aquatic plants and hydrodynamic force and its implication on the long-term landscape development have received intention from ecology, geology and hydraulic engineering.v is the Kinematic viscosity, rv (z) is defined as the ratio of the volume occupied by water per unit area in a certain depth ( h) to the total front area of vegetation per unit area. (z) is the solid area of vegetation per unit area
Reynolds number Rv (z) varies with flow velocity u (z) and plant characters r (z) in depth, the CD is modified to be a function of z as well
The drag force is quantified by the quadratic law (Equation 1.) originated from Morison [1950], which uses a drag coefficient to characterize the average drag force provided by one stem: F (z)
Summary
The interaction between aquatic plants and hydrodynamic force and its implication on the long-term landscape development have received intention from ecology, geology and hydraulic engineering. The vegetation resists flow, damps waves and alters the turbulent intensity predominantly though exerting additional drag force on the water particles passing it. In these models, the drag force is quantified by the quadratic law (Equation 1.) originated from Morison [1950], which uses a drag coefficient to characterize the average drag force provided by one stem:. The modified relation depends on the local flow conditions and canopy properties in the vertical. This relation is implemented in an iterative scheme of a 1DV flow model. The modeling results are compared with experiment data of flow through rigid vegetation
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