Seepage Face Simulation Using PLASM

Abstract

An unconfined aquifer variation of the Prickett and Lonnquist Aquifer Simulation Model (PLASM) has been modified to simulate the dynamics of seepage face development and compute flow rates therefrom. The modifications, based on hydraulic theory and operating within the framework of the original model, enable the model to simulate zones of seepage contiguous and noncontiguous to streams, lakes, or other points of ground-water discharge. A simplistic overland flow routing procedure to move seepage flow downslope from points of discharge and permitting it to reenter where the water table is below the land surface was also incorporated. To determine the error associated with this discrete solution, the modified numerical model was compared to a series of analytically equivalent Dupuit-Forchheimer-based solutions. The numerical solutions from PLASM compare to the closed-form solutions to within the uncertainty of the model's finite-difference discretization. The comparisons indicate that errors in seepage face prediction result strictly from PLASM's inclusion of the D-F approximations, not from the formulation of the numerical seepage face simulation routine. Finally, the modified PLASM was applied to a hypothetical watershed under dynamic conditions to demonstrate its capabilities. The unconfined aquifer version of PLASM works best in situations where both thickness-to-length ratio of the aquifer and land surface and water-table slopes are relatively small, geometry typical of many real-world watersheds. Under such conditions, the modifications developed are shown to logically and consistently simulate the extent, dynamics, and flow rates of seepage zones with minimum error.