## 1 Introduction
[17] proposed instability boundaries derived from experiments using human models that account for various ages and genders, analyzing differences in walking stability based on the distribution of multiple evacuees. [PERSON] et al. [18] suggested \(hU\) thresholds of 0.4 and 0.6 m\({}^{2}\)/s as instability boundaries for children and adults, respectively, based on real pedestrian experiments. In addition to flood intensity, other indicators for evaluating pedestrian stability have been proposed. [PERSON] et al. [19] introduced the specific force (\(M_{0}\)), representing the sum of hydrostatic and hydrodynamic forces, and demonstrated through real pedestrian experiments that safe evacuation is possible when \(M_{0}\) is less than 0.125. [PERSON] et al. [20] proposed a dimensionless mobility parameter as an alternative to \(hU\), analyzing the effects of the Froude number and submergence on walking stability. [PERSON] et al. [21] proposed a lumped impact parameter for evaluating human instability based on floods' total head and momentum. Recent studies have used these stability indicators to assess pedestrian stability in floodwaters on a basin scale. [PERSON] et al. [22] assessed the complex flood safety in urban areas, using the Froude number and submergence to evaluate pedestrian stability. [PERSON] and [PERSON] [23] analyzed time variations in safe evacuation zones based on specific force calculations from two-dimensional flood flow simulations in underground shopping malls. [PERSON] and [PERSON] [6] simulated evacuation scenarios by calculating walking speeds using specific force and evaluating pedestrian stability using the flood hazard index. [PERSON] et al. [24] presented pedestrian stability assessment results using critical flow velocity derived from flood flow simulations within a watershed. The stability indicators have also been applied to enhance the safety of structures. [PERSON] et al. [25] evaluated areas vulnerable to flooding based on two-dimensional flow analysis results and the incipient velocity criterion, and suggested retrofitting approaches to enhance pedestrian safety at the test site. [PERSON] et al. [26] analyzed the effects of rest platforms on safety in inundated staircases. [PERSON] et al. [27] suggested staircase designs that consider human instability by modifying the step heights and lengths. [PERSON] et al. [28] analyzed the effectiveness of -flood barriers in mitigating flood risk using computations of specific force based on flow analysis results. While these studies provide valuable insights into pedestrian stability in extensive inundated areas, they do not focus on local flow characteristics around the human body for evaluating instability. Given that stairways are key evacuation routes during floods, it is essential to analyze the impact of flood flow on people evacuating via stairs. To address this, detailed analyses of flood flow impacts around the human body on stairs have been conducted using three-dimensional simulations. [PERSON] et al. [29] analyzed flood flow impacts on stairs, revealing that jet flow occurring on stairs can influence toppling instability in the human body. Despite the numerous indicators proposed and utilized to evaluate pedestrian stability in floodwaters, it is crucial to recognize that flood risk assessment outcomes may vary depending on the choice of indicator [2; 30]. Furthermore, given that flood flow is an unsteady phenomenon, changes in stability assessment results over time should be considered. This study conducted three-dimensional flood flow simulations on a subway platform, including stairs, using OpenFOAM ver. 2312. A human body model representing a 60-year-old female, identified as a vulnerable age group, was placed near the stairway--a major evacuation facility--to analyze local flood flow variations around the body.