Hydraulic Analysis of Overland Flow on Vegetated Slope
| Autor: | Ching-Ya Tang, 唐靖雅 |
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| Rok vydání: | 2012 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 100 The existence of grasses on the ground increases the surface resistance, and thus decreases the flow velocity and sediment transport rate. Based on the theoretical analysis, vegetated flow on the slope is considered with emergent and submerged vegetation. Herbaceous species, such as grasses, or woody species, such as trees, are planted on the slope separately or simultaneously. In the study, the soil and vegetation layers are assumed to be homogeneous and isotropic porous media. Since the soil ground is considered as pervious and saturated, both surface and subsurface water flow are solved simultaneously. Compared with the no-slip condition at the surface ground used in the past researches, this study is more feasible and close to the natural phenomenon. The sheet flow or overland flow was simulated by the theory of laminar flow passing over a vegetated slope. The Navier-Stokes equations were employed to govern the flow in water layer, and the Biot’s theory of poro-elasticity was applied to delineating the flow in vegetation and soil layers. These equations were firstly simplified, modified and nondimensionalized under different vegetation situations, and then solved analytically. As a result, the velocity distributions with closed form were derived except the case of vegetated slope with woody species. The shear stress distribution and the energy distribution were also analytically derived and discussed for understanding the flow mechanism. The vegetated slope was divided into two cases--only herbaceous species planted on the slope and both herbaceous species and woody species planted on the slope. Furthermore, the flow mechanism of these two cases with emerged or submerged vegetation was investigated. In the emerged situation, the velocity profiles show that the flow runs faster for less dense vegetation, and the shear stress distribution reveals that the maximum value always occurs at the ground surface. In the vegetation layer, the rate of energy dissipation influenced by woody species is very obvious. When the water depth is higher than grass height, the velocity profiles show that an inflection point appears obviously in the vegetation layer for the denser vegetation with constant water depth. When the shear stress distribution was discussed for the submerged case without woods, a crucial parameter was found. When the dimensionless parameter δ_2 equals to 9.5 approximately with the ratio of the water depth to the grass height equal to unity, the shear stresses at the interface of water/grass layers and at the ground surface are equal and reach to a maximum. The shear stress profiles can be classified into three different types according to the parameter δ_2. From the distribution of energy rate, it is also shown that the major energy dissipation rate exists near the interface of porous medium layers. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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