基于COMSOL Multiphysics的非饱和裂隙土降雨入渗特性研究

Study on rainfall infiltration characteristics of unsaturated fractured soil based on COMSOL Multiphysics

基于COMSOL Multiphysics软件对非饱和裂隙土降雨入渗特性进行数值模拟研究。通过将裂隙和基质分别离散成有限单元,建立了能充分模拟土中裂隙流、基质流以及裂隙-基质流量交换的离散裂隙-孔隙介质模型。结合"空气单元"的概念,对裂隙土的上边界进行模拟。该方法不仅能描述降雨初期雨水沿裂隙优先入渗的现象,还能描述当降雨量大于裂隙土入渗量时雨水沿地表流走的现象。通过对地表以下2 m深度内低渗含裂隙土体进行模拟,分析了裂隙的几何特征、基质的水力特性、前期水分条件以及降雨强度对非饱和裂隙土降雨入渗过程的影响。结果表明,在非饱和裂隙土中,存在两个主要的渗流过程:一是水沿裂隙优先流动;二是水不断从裂隙吸入基质中,基质吸收水的作用抑制了裂隙中优势流的发展。与裂隙的几何特征相比,基质的水力特性对非饱和裂隙土渗流的影响较大。增大基质的饱和渗透系数可能使由裂隙流主导的渗流过程转变为由基质流主导的渗流过程,而基质的非饱和特性与裂隙土的初始含水率改变了土体的储水能力,从而加速或延缓了降雨入渗至某一深度的时间。降雨强度对土体入渗速率和入渗量均有影响,当超过裂隙土的入渗能力时,多余积水沿地表流走,断面入渗率随时间趋于稳定值。

A numerical simulation study on the rainfall infiltration characteristics of unsaturated fractured soil was performed based on COMSOL Multiphysics software. By discretizing the fracture and the matrix into finite elements, a discrete fracture-porous medium model was established to fully simulate the fracture flow, matrix flow and fracture-matrix flow exchange in the soil. The upper boundary of the fractured soil was simulated by using the concept of “air element”. This method can describe not only the phenomenon of preferential infiltration for rainwater along the fractures at the beginning of rainfall, but also the phenomenon of rainwater flowing away along the surface when the rainfall is greater than the infiltration of the fractured soil. By simulating the low-permeability fractured soil at a depth of 2 m below the ground surface, the influences of the geometric characteristics of the fracture, the hydraulic properties of the matrix, the previous moisture condition and the rainfall intensity on the rainfall infiltration process of the unsaturated fractured soil were investigated. The results show two main seepage processes in the unsaturated fractured soil: firstly, water flows preferentially along the fracture;secondly, water is continuously imbibed into the matrix from the fracture,and the matrix imbibition inhibits the development of preferential flow in the fracture. Compared with the geometric characteristics of the fracture, the hydraulic properties of the matrix have a greater influence on the seepage flow in the unsaturated fractured soil.Increasing the saturated permeability coefficient of the matrix may change the seepage flow dominated by the fracture flow into that dominated by the matrix flow. The unsaturated properties of the matrix as well as the initial water content of the fractured soil change the soil moisture storage capacity, thereby accelerating or delaying the time for rainwater to infiltrate into a certain depth. The rainfall intensity has an influence on both the infiltration rate and infiltration amount in the soil. When it exceeds the infiltration capacity of the fractured soil, the excess rainwater flows away along the surface, and the infiltration rate across the section tends to stabilize with time.