Sensitivity of calculated hydrological flows through multilayered hard rock to computational solution procedures

Permeability and moisture content curves for partially saturated, fractured, welded tuffs, such as those found near the site of a prospective nuclear waste repository at Yucca Mountain, Nevada, are highly nonlinear. The authors illustrate, by means of a one-dimensional problem of infiltration into multilayered fractured tuff, the numerical instabilities that can arise when analyzing the flow in such porous materials. The authors compare the results obtained using two numerical schemes for treating the nonlinearities: the first uses a first-order Piccard iteration scheme to attain convergence at each time step; the second uses a second-order Newton-Raphson iteration procedure to attain convergence. The Piccard scheme converges when flow through the rock matrix dominates but does not converge when flow through fractures dominates. On the other hand, the Newton-Raphson scheme converges in both situations