Numerical Modeling of Vertical Displacement and Horizontal Strain Due to Ground Water Withdrawal

A numerical modeling effort was used to quantify the relationship between groundwater decline, induced consolidation of the basin sediments, and development of horizontal tensile strain along the ground surface.  Ground subsidence and earth fissuring were modeled by employing a two-dimensional coupled seepage and stress-strain finite element analysis. A fully coupled analysis reasonably modeled the response of the applied stress to the alluvial structure due to a change in pore water pressure with resulting deformation. This coupling was achieved with the use of two finite element based computer programs, SEEP/W and SIGMA/W, developed by Geoslope (1998).

Input data for the modeling effort consisted of an array of interrelated geometric and numerical components, including alluvial lithology, hydrological data and assigned mechanical properties of the alluvial materials. The depths to bedrock in the modeled profiles were initially based on interpretations of gravity surveys with calibration by ReMi seismic profiles. The characterization of basin alluvial units used in the model was based on the interpretation of synthetic aperture radar interferometry (InSAR) data and geophysical investigation. The numerical model was calibrated using InSAR data from 1992-1999, and past survey data.

The numerical model is validated by modeling a known earth fissure complex. The model was able to simulate the observed subsidence from past survey and inSAR data within the area. The numerical model also estimated the modeled horizontal strain for several time periods. The modeled strain indicates that strain had exceeded the approximate strain threshold for fissure formation of 0.02 percent by 1981 in the area of identified earth fissures and the tensile strain at the existing fissure complex had reached about 0.04 percent by 2001. The model also predicted future subsidence and locations with the potential for earth fissure formation.