Simulation of groundwater exchange between an unconfined aquifer and a discrete fracture network with laminar and turbulent flows

摘要

In this study, a new approach is developed to simulate groundwater flow through both an overlying unconfined aquifer and an underlying discrete fracture network. The groundwater exchange between the aquifer and the fracture network, including magnitude and direction, is explicitly simulated based on the mass conservation, which depends on a variety of parameters related to flow and network characteristics. In the overlying unconfined aquifer, we approximate the flow using the Dupuit assumptions. In the underlying fracture network, we use the cubic law and Forchheimer's law to iteratively simulate laminar or turbulent flow in individual fractures depending on the Reynolds number. Explicitly separating laminar and turbulent flows in the fractures results in a system of nonlinear equations, which is iteratively solved. While the flow from the overlying unconfined aquifer is a small portion of the overall flow in the underlying fracture network, incorrect use of the laminar or turbulent flow equation in the fractures can lead to significant errors in simulating the flow exchange. As apertures increase, both the portion of the total flowrate in the aquifer and the portion of the flow in the fractures coming from the aquifer decrease. As the overall hydraulic gradient increases, both the portion of the total flow in the aquifer and the portion of the fracture flow that comes from the aquifer increase. The portion of the total flow in the aquifer increases with the aquifer thickness, while the portion of fracture flow coming from the overlying aquifer decreases.