Fully conservative coupling of HEC-RAS with MODFLOW to simulate stream–aquifer interactions in a drainage basin

This work describes the application of a methodology designed to improve the representation of water surface profiles along open drain channels within the framework of regional groundwater modelling. The proposed methodology employs an iterative procedure that combines two public domain computational codes, MODFLOW and HEC-RAS. In spite of its known versatility, MODFLOW contains several limitations to reproduce elevation profiles of the free surface along open drain channels. The Drain Module available within MODFLOW simulates groundwater flow to open drain channels as a linear function of the difference between the hydraulic head in the aquifer and the hydraulic head in the drain, where it considers a static representation of water surface profiles along drains. The proposed methodology developed herein uses HEC-RAS, a one-dimensional (1D) computer code for open surface water calculations, to iteratively estimate hydraulic profiles along drain channels in order to improve the aquifer/drain interaction process. The approach is first validated with a simple closed analytical solution where it is shown that a Piccard iteration is enough to produce a numerically convergent and mass preserving solution. The methodology is then applied to the groundwater/surface water system of the Choele Choel Island, in the Patagonian region of Argentina. Smooth and realistic hydraulic profiles along drains are obtained while backwater effects are clearly represented.