A pebble/cobble passing a head drop tends to carry a large impact momentum when it hits a groundsill; the heavy impact may result in the breakage of concrete. This study explores the mechanics and the principles of this issue by computational fluid dynamics with particle tracking. The results reveal that the kinematics of a single particle passing a head drop is primarily affected by the size of particles. A small particle tends to travel along the flow lines and brings relatively low impact energy. A large particle is more affected by gravity; saltation is more likely and will carry relatively high impact energy. The major role of the discharge magnitude is just to enable the transportation of larger particles to the downstream. With the simulation, it is possible to evaluate the impact position, load and kinetic energy. The locations prone to damage and failure potential can then be identified.