Due to the increasing urbanisation trend and more recurrent flood events worldwide that are affecting exposed habitable areas like underground spaces. This state-of-the-art review presents a comprehensive analysis of the available literature that focuses on defining instability criteria for hazard assessment during human evacuation of underground flooded stairs. The studies are outlined in three main groups: theoretical, experimental campaigns and numerical approaches. Several methods for defining specific criteria were found, most of these criteria were described as a function of water depth (D) alone or in combination with velocity (V) highlighting the importance of these two water-related parameters for flood-related hazard evaluation. Succinctly, the most relevant findings and limitations of these studies are discussed and summarised in tables. A comparison with other flood-related hazard criteria in plain and mild-slope areas is presented. Finally, potential future investigation lines are proposed. Thus, this state-of-the-art review could offer a comprehensive overview of the topic and stimulate new exploratory studies in this research field yielding valuable insights that can be easily transmitted to stakeholders or non-experts and foster urban resilience during floods.

Urban underground infrastructures are highly vulnerable to intense rainfall events, particularly access stairs, where preferential runoff paths and the most probable evacuation routes can conflict. This study presents a pluvial flood hazard assessment for underground access stairs in the Barcelona Metropolitan Area Metro network. It integrates the EU ICARIA project modeling framework and the hazard assessment criteria based on hydraulic parameters identified by the Spanish national research project FAVOUR. Both current and future climate change rainfall scenarios are considered. The results showed that out of 415 underground access points, 27 face a high risk of floods, while 35 more have potentially high-risk conditions. These figures could rise to 38 (40% increase) and 47 (74% increase) respectively by the end of the century since climate change is projected to increase rainfall intensity and frequency. By quantifying hazard levels across the network, this study allows the identification of points of the infrastructure where hazard conditions can be more critical. Furthermore, the results presented could potentially support targeted adaptation strategies such as entrance retrofitting, improved drainage design, and emergency planning to develop resilient and sustainable cities. The proposed methodology demonstrates how ICARIA’s modeling framework can effectively evaluate and anticipate flood hazards in complex urban environments at the asset level.

Given the increasing flooding incidents in metro stations globally, underground space infrastructures must guarantee users' safety. This manuscript characterises the hydraulic behaviour of a flood‐prone access stairs of the Paral·lel Metro station in Barcelona. Using a real‐scale stairs model at the Technical University of Catalonia (UPC) and the Computational Fluid Dynamics (CFD) software FLOW‐3D. Hydraulic parameters; water velocity, and depth were analysed under different discharges. Human instability criteria, Momentum of Flow ( M _F ) and Specific Force Per Unit Width ‘SFPUW’ ( M ₀ ) indicated that the most hazardous zone was the lower part of the stairs. Sectorised analyses showed that specific locations like the walls and proximities to risers presented lower safety criteria values compared to the centre of the stairs and steps' borders. Also, the numerical model closely mirrored the aerated flow experimentally observed. These findings provide useful information for future research campaigns aiming to improve urban resilience to face underground evacuation of flooded stairs.

The integration of remote sensors with nature-based solutions (NBS) offers new opportunities for suitable hydrological monitoring and risk reduction. This study implements 2D-1D numerical modelling tools to guide the allocation of both sensors and NBS interventions. Using the Cerisano catchment as a case study, the software IBER-SWMM (3.3.1–5.2 respectively) was employed to reproduce rainfall-runoff under different scenarios. Outputs were analysed to identify hydrologically sensitive zones where sensor deployment and NBS implementation would maximise monitoring efficiency and mitigation benefits. Results demonstrate how modelling supports effective decisions that are often limited by budget and site-specific constraints.

The increasingly recurrent urban floods pose significant risks to underground spaces and their users. Often essential in these scenarios, stairs present unique challenges during floods as they serve as pathways for evacuation and for the incoming water. With most of the prior studies focusing on studying a single individual, the impact of group dynamics on flooded stairs remains underexplored. This study addresses this gap investigating how these dynamics influence evacuation efficiency and safety through a 3D numerical analysis. This research introduces realistic human body dummies along the stairs to simulate various pedestrian arrangements under a discharge of 0.423 m ³ /s. Hydrodynamic forces and Momentum of Flow (MF) and Specific Force Per Unit Width (SFPUW) safety criteria are calculated to evaluate hazardous conditions. Pedestrian arrangements include a single individual at different positions, as well as tandem and side-by-side arrangements. Results revealed significant variations in the water-dragging forces based on positions and grouping dynamics. Notably, compared to isolated cases, side-by-side arrangements exhibited increased water-exerted force on both side-by-side and downstream individuals, while tandem configurations showed reductions in dragging forces on trailing pedestrians. The affectation of forces diminished when the separation between them increased. This study highlights potential bottlenecks, evaluates evacuation strategies, and provides insights to enhance safety protocols in underground infrastructure during flood events, contributing to urban resilience and flood risk mitigation strategies.