Numerical Modeling and Analysis of the Tehran-Shahriar Plain Subsidence Using COMSOL Software

Abstract

The uncontrolled withdrawal of groundwater resources causes a drop of water levels and increases stress to soil particles which would culminate in the land to subside. The critical plain of Shahriar has, over the last years, been affected by the phenomenon of subsidence. Presence of vital channels, as well as economic, pilgrimage and military sites have turned it into a strategic region which increases the detrimental consequences from the subsidence. This study introduces a novel method to predict and analyze subsidence, known as poroelasticity module of COMSOL software that utilizes the concurrent resolution of equations of fluid motion in a porous medium and mechanical deformation.  The numerical model’s outputs from 2003 to 2019 in 24 points were compared and verified with levelling surveys and Sentinel 1 radar interferometric images (R²=0.97), indicating an acceptable correlation between values, good match between interferometric images and zoning subsidence maps derived from software and tendency of the values of root mean square error (RMSEA) and efficiency coefficient toward 0 and 1. The finite element modelling (FEM) output suggested that the mean rate of subsidence will have increased at a slower rate by 13.19 cm by 2031 due to subsequent compaction of upper layers of the aquifer, and by 18.38 cm in points where the thickness of clay and silt layers increases. The study finds that the region under study has a multi-layer aquifer system, which the clay thickness is thought to be a major factor that affects the subsidence variations and patterns. An increase in the thickness, together with high rates of water pumping and the number of geological units are among the major factors contributing to subsidence.