Integrated Modeling of Fully Coupled Two-Phase Surface and Subsurface Flow

An integrated model of fully coupled two-phase surface and subsurface flow was built by using GETFLOWS (GEneral purpose Terrestrial fluid-FLOW Simulator), a finite difference fluid-flow numerical simulator, to perform quantitative evaluation of exchange of overland water and groundwater, subsurface seepage and surface water runoff on the hillside slope surface. Surface water on hillside slope was modeled as open-channel flows and coupled with air-water two phase Darcy flow for underground permeation by using a generalized flow formula. In this study, a simple verification model was firstly built and validated by the extensively used Abdul and Gillham example with simulating the coupled surface and subsurface flow. Then a three-dimensional integrated model of surface and subsurface flow was built by using the DEM (Digital Elevation Model) of a natural mountain slope in Hokkaido, Japan, where runoff induced several debris flows and slope failures occurred during Typhoon 10 hit Hokkaido in 2016. Finally the results of integrated simulation was compared with the results from two-dimensional (2D) impermeable plane flow simulation using Nays2D Flood solver of the iRIC software in the same region. The results suggest that on the hillside slope upstream of embankment, the infiltration rate is equals to the rainfall intensity at the beginning of the rainfall event. After the rainfall intensity is greater than the soil infiltration capacity, runoff is generated. The runoff from upstream allows more water to infiltrate into the embankment causing the possibility of slope failure at the exit of the valley to be much greater than other locations along the highway.