Please use this identifier to cite or link to this item: http://hdl.handle.net/11455/89411
標題: Analysis of surface water flow over multi-layered soils
多層土壤之地表水流分析
作者: 林彥廸
Yen-Ti Lin
關鍵字: 多層土壤
多孔彈性介質理論
漫地流
均勻降雨
地下水面
multi-layered soils
poroelasticity theory
overland flow
uniform rainfall
phreatic surface
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摘要: 坡地在人為開發或因降雨、地震等自然因素發生崩塌時,會出現裸露階段,而在降雨時,若地表無植生覆蓋時將會出現大量地表逕流。因此,本研究考量均勻降雨時之多層具有滲透性之孔隙土壤的地表及地表下水流流動,以期能更接近真實之流況。 文中以水力學的觀點出發,將流場分成均質水層和具透水性之多層平行分布的土壤,並將最底層之土層再分成無限厚及有限厚兩大類型分別進行討論。水層流況以Navier-Stokes方程式來描述,而土層中之流體則視為孔隙介質流,其流動則引入宋(1993)所建立之多孔介質層流模式予以描述,並利用解析方式來分析。此外,在加入均勻降雨條件後,其求得之半解析解則係搭配4階Runge-Kutta法求得,再進一步探討地表水流與多層土壤孔隙水流間的關係,以及不同參數對地表上水深、流速等現象之影響程度。 研究結果發現,在無降雨時,地表上之流速明顯受坡度影響,且因考量土壤為透水性,故在水土交界面之速度不為零;而地表下流速,當坡度相同時,其流速主要是受到土壤孔隙率之影響,因砂土透水性較佳,故流速較快,且可在不同土壤之交界面看出速度具連續性。此外,在地表上之流體剪應力往地表方向呈線性遞增,在水、土交界面有最大值,進入土層後急遽下降趨近於零,係由於在土層中流速幾乎為定值,所以無剪應力,但在各土層交界面,由於受到流速改變之影響,在該處會產生剪應力。考慮均勻降雨情況時,水深剖面線會受到坡度、降雨強度及土壤滲透性之影響。在坡度大時,水深明顯較坡度小時低,但整體水面線之形狀大致相同;而在固定坡度情況下,降雨強度則為改變初始水深位置之主要影響因素;在土壤滲透性方面,因砂土之透水性較佳,所以部份水深剖面線會有延遲積水之情形。 而地表下之水面線初始位置,會受到土層數目、底層土壤為有限厚或無限厚、降雨強度、坡度及土壤種類等因素影響,而產生不同程度之下降,其中以多層土層之底層為無限厚砂土時,其影響初始水深程度(H0)最為明顯。同時,在坡度變陡且降雨強度變小時,未飽和含水區會開始出現,意即本研究可推估土壤中之地下水面剖線。
The landslide occurs and thr ground appears bare stage because of heavy rainfall, earthquakes, or human exploiting. When the ground surface without vegetated cover is under a rainfall event, an amount of overland flow will happen eventually. Therefore, to be close to the reality, this study presents a mathematic model of integrated surface and subsurface flows over multi-layered soils with/without rainfall. The flow field divided into homogenous water layer and permeable multi-layered soils which are parallel and the lowest bottom soil layer may be of infinite thickness or finite thickness. The flow of water layer is governed by the Navier-Stokes equation whereas the soil layer is considered as porous media, and delineated by Song's (1993) laminar model modified from Biot's theory of poroelasticity. The velocity distribution and shear stress distribution are solved analytically. Furthermore, after adding uniform rainfall event, the semi-closed form solution is obtained by the Four-order Runge-Kutta method. Besides, the relationship between surface flow and subsurface flow in the multi-layered soils is disscussed, and the factors influencing the velocity and the flow profiles are investigated. The results show that flow velocity is chiefly influenced by the slope for the condition of norainfall event. The flow velocity is nonzero at the water layer/soil layer interface because the soil is considered permeable. The subsurface flow velocity is mainly affected by the soil porosity for a constant slope and it is continuous at different soil interfaces. Besides, the shear stress distribution appears to be linear above the ground, and the maximum occurs at the interface of the water/soil layers. The shear stress, however, approaches zero inside the soil layers since the velocity is nearly constant for a certain soil except the interfaces of soil layers. While a uniform rainfall event is considered, the effect of slope, rainfall intensity and permeability on the flow profiles aresignificant. Although the flow profiles on the mild slope are lower than that on the steep slope, the shape remains similar. The rainfall intensity is the main factor of changing the initial position of the profiles for a constant slope. As to the soil permeability, the permeability of sand is better than others, so the ponding phenomenon happens. On the other hand, the initial position of flow profiles of subsurface flow depends on the soil layers, the thickness of the bottom soil layer, rainfall intensity, and slope. While the bottom soil layer is sand with infinite thickness, the effect on the initial position of the profiles is most obvious.In the meanwhile, the unsaturated zone shows up for steeper slope and smaller rainfall intensity cases. In other words, the profiles of phreatic surface are presented in this study.
URI: http://hdl.handle.net/11455/89411
其他識別: U0005-1706201416033900
文章公開時間: 2017-06-20
Appears in Collections:水土保持學系

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