## 2 Background  
Coupled analysis of the discrete element method (DEM) and fluid analysis is a topic of recent interest. There are advanced studies that combine SPH to reproduce debris flows [5] or the lattice Boltzmann method (LBM) and large-eddy simulation (LES) to reproduce precise fluid flows [6; 7]. In contrast, this study uses a classical model, which is different from the recent trend. This section explains why the classical method was adopted.  
The formation of water channels and the preferential flow structure in porous media are crucial issues in engineering fields, such as groundwater flow, unsaturated subsurface infiltration, and measures for soil and groundwater contamination. It is essential to simulate the heterogeneity of the pore structure for the research of such microscopic structures, and analysis via numerical simulation is an effective means of prediction.  
DEM can directly model the motion of particles interacting with each other and simulate porous media with heterogeneous pore structures. Many effective cases have been reported for reproducing particle ground structures, such as the DEM-FEM coupled analysis methods [8; 9; 10; 11; 12; 13; 14]. For the analysis of fluid dynamics, meshless smoothed particle hydrodynamics (SPH) is used. For example, [PERSON] (2022) represents the impact and entrainment behaviors of debris flow through coupled analysis of SPH-DEM-FEM [5].  
However, the SPH method is known to have a challenge, in that the accuracy and convergence deteriorate once particle distributions become disordered and the distance between particles increases [15; 16; 17]. In particular, in porous media with heterogeneous and discontinuous pore shapes, inconsistencies in particle distribution can destabilize the formation process of the flow channels. On the other hand, the finite element method (FEM) is a well-known tool traditionally applied for fluid analysis. FEM can precisely solve local pressure gradients and velocity distributions using a formulation based on a stiffness matrix. It is suitable for quantitatively evaluating the precise flow field inside porous media. In addition, by constructing a fine mesh that can also handle complex pore shapes, FEM can track the formation process of preferential water channels more stably and with higher resolution. The reasons for combining with FEM are that it may have poor computational efficiency but has fewer control parameters and a proven track record in practical applications.  
A similar study was also attempted by [PERSON] (2013), but the analysis was limited to two-dimensional fibrous porous media and could not evaluate the flow path structure in three dimensions [18]. [PERSON] (2022) simulated a porous medium using a DEM model in three-dimensional space, but the scale of the analysis domain was only 17 mm, which is insufficient to evaluate the formation process of water channels on a real scale [19].  
In this study, we use DEM to construct a simulated ground containing non-spherical particles generated by the freefall and then perform fluid analysis using FEM on the resulting large-scale three-dimensional porous structure. This makes it possible to reproduce water channels in porous media that reflect the complex structure and particle distribution of real landfill sites and to clarify the influencing factors. This approach goes one step further than conventional idealized models and will provide new findings that contribute to the analysis of porous media containing a variety of particle shapes and structures.