<p>The mechanisms of granular flow-structure interactions and impact dynamics serve as a foundation for bridge engineering design. However, the design of bridge piers to counter granular flow continues to be influenced by the particle size distribution and the Froude characteristics’ impact on pier performance. A three-dimensional numerical model is established in this study, using the Discrete Element Method (DEM), and its reliability is confirmed through flume tests. The interaction mechanisms and dynamic impact characteristics of granular flow, coupling with particle size and Froude number (<i>F</i>r), and pier’s shapes, were explored. The flow characteristics of granular flow have been revealed, as well as the interplay mechanism between granular flow and pier, the energy evolution mechanism, and escalation. The impact force distribution of granular flow on pier was clarified. A comparative analysis was conducted on the peak impact force resistance coefficient (<i>C</i>_d) for pier of assorted cross-sectional shapes. We have further developed a unified particle size-bridge pier-special design diagram, quantifying the influence of particle size and <i>F</i>r on the hydrodynamic <i>α</i>. The analysis indicates that the existing models calibrated by limited experiments may overestimate the peak impact force on round and round-end bridge piers, while underestimating it for square bridge piers. </p> Graphical Abstract <p></p>