# Inundation Risk Assessment of Underground Space Using Consequence-Probability Matrix
## 6 Summary and Conclusions  
Due to the urbanization and industrialization accompanied with climate change, the frequency and the intensity of heavy rainfall have been abruptly increasing, resulting in stormwater runoff flowing along the roads instead of being absorbed by pervious surfaces, causing underground spaces to flood. The stormwater flowing into underground spaces can lead to human casualties and cause damage to property owing to its high flow velocity, despite its low water depth, as it rapidly gains in water pressure and momentum. Accordingly, flood risk analysis is essential for enhancing the safety of underground spaces. However, previous research concerned with urban flooding has normally been conducted at the watershed scale and normally involved inundation analysis of the targeted region, sewer network analysis, and impact evaluation of flood prevention measures. The purpose of this study was to quantify flood risk by analyzing the flow momentum in the underground space connected to the ground floor. For quantitative analysis of underground space flood vulnerability, we calculated the flood risk, level of evacuation difficulty, and flood vulnerability index. For an accurate calculation of the risk level of the stormwater runoff entering an underground space through the stairs connected to the ground floor, it was necessary to calculate the inundation depth and its flow momentum associated with the flow velocity via hydrodynamic flow modeling. To do this, the flood depth and the momentum for the application site were obtained by a 2D hydraulic flow model, and the concept of flood intensity (FI) proposed by [PERSON] (1998) was employed to evaluate the severity or the consequence of flooding. In addition, to investigate the level of evacuation difficulty, the spatial layout of the rooms and exit location, together with walking speed, must be considered in the risk analysis process. In this concern, we partitioned the underground space into subzones according to the shape and layout of each room. Using the flood risk and level of evacuation difficulty thus calculated, we calculated the flood vulnerability index (FI) and assigned the FI grade to each subzone on a scale of 1 to 5. If stormwater runoff enters an underground space at an input flood depth of 1.0 m, zones far away from the inlet usually have low risk levels, and the closer to the inlet, the higher the risk level. However, when the level of evacuation difficulty of the route from the zone concerned to the exit was considered, the FI 3 was estimated for the zones at longer distances from both the inlet and exit, and FI 3 for the zones at shorter distances from the inlet and longer distance from the exit. If people are in the zones with FI 4-5, a rapid evacuation is essential for preventing human casualties. This study proposed a methodology to quantify the risk of underground flooding and can contribute to enhancing the accuracy of flood risk analysis and preventing human casualties and  
Figure 8: Inundation risk considering evacuation difficulty and hydrodynamic property. damage to property by improving preparedness and disaster response measures in zones showing a high-risk level. Furthermore, the proposed methodology incorporated with the inundation model can be applied to any underground space regardless of the locations of the stairs, the number of exits, shape of rooms, or layout of the floor. Consequently, it will contribute to mitigating flood damage in underground spaces. In a follow-up study, quantification of flood risk levels in various settings, such as different inlet configurations and large-scale building complexes with underground connections, will be explored. In addition, an automated algorithm for finding the optimal evacuation route based on the flood risk analysis is now being developed to furnish useful escape recommendation. Conceptualization, C.G.S. ; formal analysis, Y.-s.H. ; data curation, Y.-s.H., E.T.S. and T.S.E. ; supervision, C.G.S. ; writing--original draft, Y.-s.H. ; writing--review and editing, C.G.S.