An experimental investigation into the effects of underground stairs types on hydrodynamics characteristics and risk.

With the ongoing expansion of urban development into underground spaces and the increasing occurrence of extreme rainfall events, the risk of flooding in these areas has heightened. Stairs, as critical connections within underground spaces, markedly influence water inflow and evacuation during flooding events. Therefore, the impact of different stair types on the hydrodynamic characteristics of water flow in underground spaces is worth studying. This paper constructed physical models of stairs at various underground locations to investigate two types of stairs and their hydrodynamic characteristics and the risk. The findings revealed that at low flow velocities, both stair types lowered water flow in the upper section; at medium flow velocities, the main impact was observed in the lower section; and at high flow velocities, the reduction effects were weaker. For the stair types allowing lateral outflow, increasing the lateral slit height enhanced outflow volume, thereby decreasing water flow on the stairs. However, excessively high slit gaps did not substantially increase lateral outflow and may introduce safety hazards, such as the risk of children falling through. Additionally, a comprehensive analysis of flow velocity and water depth reveals that, in addition to the higher risk in the lower section, particular attention must be given to the turning point between the stair and the rest platform. The intricate vortex flow pattern at this location, characterized by elevated flow velocities and water depths, results in a risk level surpassing that of the jet flow.