Modeling influences of crowd density on pedestrian-water interactions during flood evacuations

Abstract

Flood evacuations frequently involve pedestrians wading through flowing water, yet the influence of crowd density on pedestrian–water interactions remains poorly understood. This study investigates hydrodynamic forces acting on pedestrians under varying crowd densities using three-dimensional computational fluid dynamics (CFD) simulations based on the Volume of Fluid (VOF) method in Simcenter Star-CCM+ v2302. Three representative configurations – a single pedestrian, a single-file line, and a staggered formation – are analyzed to quantify drag force and overturning moment. The results demonstrate that increasing pedestrian density significantly reduces individual hydrodynamic loads due to wake shielding and flow redistribution. Compared with an isolated pedestrian, the average drag and moment on individuals decrease by approximately 85 % and 90 %, respectively, in dense single-file and staggered arrangements. These findings suggest that group movement provides hydrodynamic and energetic benefits during wading, as individuals experience lower loading within the protective flow structures created by neighboring pedestrians. The results establish a quantitative basis for improving flood evacuation models by incorporating crowd-level fluid interactions, thus enhancing the accuracy and realism of pedestrian movement simulations in flooded environments.