3D groundwater flow in heterogeneous subsurfaces underneath dikes

Abstract

Piping is a three-phase process that affects the stability of dikes and could lead to flooding of low laying areas. The three phases are: (1) groundwater flow from the river towards the polder, (2) breaching of the confining top layer and (3) erosion of the aquifer directly beneath the top layer. The current method to calculate piping is not usable for heterogeneous subsurfaces because of its 2D approach. This research investigated the behavior of groundwater flow through a heterogeneous subsurface underneath dikes with 3D-models. Software package iMOD is used to schematize the subsurface and calculate the hydraulic head with its MODFLOW calculation engine. The research area is located close to the river Waal in the center of the Rhine-Meuse delta, which is supposed to have a large risk for piping: a fossil channel belt close to the surface deposited on top of and incising in a thick permeable sand body from the Pleistocene era. The schematizations of the subsurface are based on this research area. The risk for breaching of the top layer is calculated with the results from iMOD and are compared the 2D predictions. The results show that 3D groundwater models attribute in the understanding of the process of piping. 3D behavior of erosion of the aquifer (3rd phase) is expected to occur in heterogeneous subsurfaces, which cannot be proved with 2D models. Further research has to investigate the behavior of the hydraulic head after the top layer is breached.