# Inundation Risk Assessment of Underground Space Using Consequence-Probability Matrix
## 1 Introduction  
With increasingly fast-paced industrialization, sudden climate change has been underway across the globe. This leads to an increase in the intensity and the frequency of heavy rainfall. In addition to continuing industrialization, urbanization is also proceeding at an unprecedented rate, with increases in the number and heights of buildings that are used for various purposes, such as residential, commercial, and public buildings. As the number of buildings in urban centers are reaching saturation, underground space is being increasingly utilized for underground facilities, such as subways, shopping malls, substations, parking lots, and many others, which are densely distributed underneath the urban area. As such complex facilities increase, covering more and more ground, impervious surfaces also continue to increase. Typically, the size of the impervious layer in Seoul was 7.8% in 1962, but rose to 47.7% in 2010 [1]. With the simultaneous progress of urbanization and climate change, the frequency of flood damage has also increased. The stormwater that accumulates on the ground moves to underground spaces along the roads drawn by the force of gravity and infiltrates the ground space through various pathways. The stormwater runoff that flows into underground spaces can cause human casualties and damage to property owing to its high flow velocity, despite its low water depth, as it rapidly gains in water pressure and momentum. Underground spaces vulnerable to flooding are becoming more complex and larger. Underground linked complex facilities have blind spots where safety cannot be secured and can therefore be very vulnerable to disasters, as presented in Figure 1.  
Floods in an underground space are most likely to reach the lowest floor along the interlayer connection structure under the influence of gravity [2; 3; 4]. Moreover, various facilities and equipment installed in underground spaces, such as substations and utility pumps, can cause hybrid disasters through electric shock, power outage, fire, water service interruption, destroyed facilities, collapsed structures, and subsidence. In this context, higher priority must be made to safety as the number of underground facilities increase. Previous researches on urban inundation have normally been performed at the watershed scale and have typically involved flooding analysis of the targeted region, sewer network analysis, and impact evaluation of flood prevention measures. But there is a shortage of hydrodynamic-based modeling that analyzes, in detail, the flow submerging underground spaces [5]. Nowadays, a variety of models and approaches are used to determine inundation and flood damage [6; 7; 8; 9]. However, there is a lack of hydrodynamic-based modeling that analyzes, in detail, the flow through underground access stairways. A numerical model is a favorable tool to predict the damage and range of flooding. Although a three-dimensional (3D) hydraulic model yields the most accurate analysis results for numerical simulations of flooding conditions, it is still time-consuming, and the analysis involves complicated processes owing to the excessive computational load due to the huge number of calculation grids. Therefore, it is desirable to apply a horizontal two-dimensional (2D) hydraulic model to calculate the flow velocity and flood depth in a flow field in both longitudinal (i.e., streamwise spreading) and transverse directions [10; 11; 12]. In some of the existing underground flood damage analysis studies, only the change of flood depth was analyzed according to the time of immersion in the underground space, and the type of damage was analyzed according to the flood damage history statistics of the underground flood. In this study, to quantify the severity and the impact of flooding in an underground space, the concept of a consequence-probability matrix was used. The flood depth and the flow momentum were computed by a 2D hydraulic flow model, and the flood intensity (FI) was employed to evaluate the severity or the consequence of flooding [13].