Urban floods, classified as a technological disaster triggered by natural hazards due to climate change, have recently been caused by rapid urbanization and torrential rainfall. Underground facilities located in flood-prone areas are considerably vulnerable to flood disaster. Therefore, urban floods may cause not only serious property damage but also massive loss of lives. 'The Analytic Hierarchy Process' was adopted to examine the vulnerability of inland and underground inundation by analyzing the priority of flood-related influence factors. The results show that underground rainwater detention caverns are one of the most effective counter-measures for the prevention or mitigation of urban floods. In order to evaluate the mitigation effect of inundation, numerical simulation is performed using SWMM (Storm Water Management Model), which considers discharge with rainwater storage facilities in a new-town area. The simulation provides applicability for the flood mitigation system. An improved reinforcing technique in the pillar is developed to construct multi-placed rainwater detention caverns, with a comparative analysis of the strengthening effect through an experimental approach. Applying prestress switches the pillar stress back to an elastic state, securing the stability of the pillar. It is shown that the proposed pillar system has a practical advantage in that it provides the strength of in-situ ground, as a time and cost-saving structure, without relying on pre-cast concrete structure.