When a gravity-driven solid-fluid mixture, such as those in geophysical flows, hits a wall-like rigid obstacle, a metastable jammed zone called hydrodynamic dead zone (HDZ) may emerge. The unjammed-jammed transition of HDZ, controlled by the intricate interactions among the obstacle, the fluid and the solid of the flow, remains an open issue to be quantified for thorough understanding its underlying physics and mechanics. This study employs a coupled Computational Fluid Dynamics and Discrete Element Method (CFD-DEM) to examine the characteristics of HDZ formed when a geophysical flow comprised of gap-graded particles and a viscous liquid impacts an obstacle. To identify key features in the zonation of HDZ, a modified granular temperature is proposed considering the influences of inherent polydispersity and both translational and rotational motions of the particles in the impacting mixture. A source-sink model is further established to offer an interpretation of the nonlinear energy dissipation process during the unjammed-jammed transition of HDZ, where the modified granular temperature serves as a function of either time or distance. The structural anisotropy is found to serve as a good indicator for illuminating the flow-structure interaction transitions. Three regimes, namely, impact-up, roll-up and heap-up regimes, have been identified according to the statistical energy conversion and dissipation in the flowing layer upon the HDZ. The influence of particle rotation is found to be more significant in the dynamical exchange of HDZ when the impacting flow contains a wider polydispersity. (c) 2021 Elsevier B.V. All rights reserved.