A Study on Improving Hydrologic Simulation Model Algorithms for Groundwater Table Analysis in Slopeland with Drain Pipes
Date Issued
2007
Date
2007
Author(s)
Hung, Kuo-Chan
DOI
zh-TW
Abstract
The research studies the effects of drainage pipes on the groundwater table in slopelands with pipes using a hydrologic model. The variations of groundwater table and pore water pressure are the ones of major factors in the view of slope stability. Embedding drainage pipes at the side slope to facilitate groundwater drainage and to reduce the pore water pressure is a common method in engineering aspect. However, it is difficult to estimate the efficiency in lowering the groundwater table by experiences. The object of this research is establishing a numerical model describing the variation of groundwater table in slopelands with pipes, furthermore, studying the efficiency of different setting scenarios in lowering the groundwater table.
There are the main aspects in the issue of the slopelands with pipes: 1. the numerical modeling in variably saturated groundwater flow is charactered by mass unbalance and specific seepage face flow at downstream. It is difficult to obtain the accurate groundwater table and the distribution of pore water pressure unless conquers the problem of mass unbalance and seepage face estimation; 2. A 3-dimentional (3D) model is capable in simulating an irregular domain, however, the efficiency lowers. The calculating efficiency can be improved by simplified the 3D model and matching the tolerance; and 3. The pipe flow and groundwater flow are different flow mechanisms. The pipes have effect in converging water flow and make a singular-like point in soil when coupling the two mechanisms. Furthermore, when mass of water flow in the pipes, the pipes become full, and some of the water in pipes may feedback to the surrounding soil matrix through the holes on the pipes. Based on the above, the full pipe flow does not present a pressurized pipe flow. It also needs a physical-based method to estimate the interchange flow and make the boundary conditions consistency between the two flow mechanisms.
The research solves the mass unbalance and seepage face estimation using the method proposed by Celia(1990) and Neuman(1973), respectively. The research proposes a quasi-3D model to improve the efficiency in 3D simulating. The method divides the slope with a few vertical 2D simulating domain along the slope, and ensures the calculating accuracy and efficiency in simulating a 2D simulating domain with a drainage hole using the method proposed by Fippes(1986). To deal with the problem in simulating the water flow in slopelands with pipes, the research proposes the idea of “virtual pipes”. With the virtual pipes, the problems of the switch between channel flow/full pipe flow in pipe flow simulation and the boundary condition consistency in coupling the quasi-3D model and pipe flow model can be solved.
The numerical modeling in variably saturated groundwater flow is charactered by mass unbalance and specific seepage face flow at downstream. The research established a 3D and a vertical 2D model by finite element method (FEM) with Galerkin technique. The domain divides into some pyramids and some triangles in 3D and 2D domains, respectively. Also, the research solves the mass unbalance and seepage face estimation using the method proposed by Celia(1990) and Neuman(1973), respectively. The study concludes the distribution of pore water pressure and seepage face in variably saturated groundwater flow can be simulated correctly by FEM combining the above two techniques.
The research proposes a quasi-3D model to improve the efficiency in 3D simulating. In the quasi-3D model, the 3D domain divides into few vertical 2D domains. Each vertical 2D domain is simulated by the vertical 2D model, the interchanging quantities between adjacent vertical 2D domains are also calculated and a virtual slice at the most downstream position is added to solve the seepage face. The study concludes the simulating results of the established quasi-3D model are close to that of the 3D model. The quasi-3D model can improve the efficiency significantly.
With regard to the vertical 2D model, the existing drainage hole has the effect of convergent water flow, and results high hydraulic gradient in surrounding soil. The error in simulating this zone is larger. It needs fine grids configuration in this zone to lower the calculating error. The research uses a simplified gird configuration and single drainage point proposed by Fipps(1986). Based on the above, the model simulates using an equivalent hydraulic conductivity in simplified gird, which is called the resistance adjusted method. The research extents the resistance adjusted method into saturated/unsaturated soil. The study concludes the above combination can obtain similar groundwater table, distribution of pore water pressure and drainage water flow rate compared with the simulated results with fine grid configuration. The method can also improve the efficiency significantly.
The research proposes the concept of “virtual pipes” to solve the switch between the channel flow/full pipe flow and to be the basis of coupled model of the quasi-3D model and pipe flow model. With the virtual pipes, the channel flow and the full pipe flow can be solved by the same algorithm. With a proper area of virtual pipe, the water volume into the virtual pipe after the full pipe regime happened in the pipe flow model can equal to the water content in soil bulk representing by the calculating node in the quasi-3D model, furthermore, the water level in the virtual pipe can equal to the head value of the calculating node. Therefore, the state variables in the two models are consistent. The study concludes the coupled model can be validated with the previously flume experiment. The coupled model can also switch between channel flow/full pipe flow stably and describe the hydrologic response in soil.
The research simulates the effects of the different lengths, embedded heights, diameters, embedded intervals of pipe, the different saturated hydraulic conductivities of soil matrix and the different slopes on the highest groundwater table, the time of water flowing into pipes, the time of full pipes happened and the drainage flow rate. Besides, the research compares the simulating results with the cases of slopelands without pipes. The study concludes, the longer pipes, the deeper embedded pipes can lower much more groundwater table. The effect of diameter of pipes affects less under channel flow regime.
Subjects
排水管
邊坡
地下水位
Drain Pipes
Slopeland
Groundwater Table
Hydrologic Simulation Model
Type
thesis
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