Using the continuum approach to model unsaturated flow in fractured rock

The appropriateness of using the continuum approach for simulations of unsa turated flow through fractured rock is examined in a numerical study of wat er seepage into an underground opening. A continuum model is calibrated aga inst data generated with a high-resolution model that created discrete flow and seepage behavior. Probabilistic predictions of injection data and seep age rates are then compared to the values from the discrete model. The stud y shows that reasonable results can be obtained with a calibrated continuum model even for fractured systems in which the underlying flow processes ar e discrete. Calibration is a crucial step in this approach because it yield s effective, model-related, and process-specific parameters. The impact of discretization on the estimated parameters is discussed. Prediction uncerta inty is evaluated by means of Monte Carlo simulations, including both the i mpact of parameter uncertainties and the effect of local heterogeneity. In this paper, the advantages and limitations of the continuum approach are di scussed from a practical perspective, which includes a critical examination of project objectives, data needs, robustness of assumptions, and predicti on uncertainty.