A quantitatively effectiveness of hybrid sewerage systems allowing rainwater flow into sewage facilities for disaster prevention of inland flooding

In Japan, recent low birthrate and aging population have progressively led to a crisis for infrastructure. In particular, the quantity of drainage from homes and factories is remarkably decreasing, along with the decline of economic activity resulting from the overseas move of Japanese companies. As a result, a condition of overcapacity of sewage systems has arisen. On the other hand, the risks of natural disaster, such as damage from local flooding by heavy rain, are rapidly increasing. But the capacity of rainwater drainage systems remains chronically short. We suggest that underground space is not being used effectively in urban sewage systems. In our study, which focused on the rainwater sewage systems and sewage systems of enormous social capital infrastructure, we commented on the conditions and problems of two systems. To improve the spatial conditions of the two systems, we suggested that space reallocation is required. As such, we proposed a framework of hybrid sewerage systems and showed its various benefits in terms of environmental issues and disaster prevention of inland flooding. Targeting a specific district in Japan, we estimated that the capacity of rainwater drainage systems would improve from 61.6mm/hr. to 65.0 mm/hr. by introducing hybrid usage of sewerage systems. The possibility of inland flooding damage in this district could be reduced up to 20.7%. Realizing hybrid sewerage systems would contribute greatly to appropriate reallocation policy for underground water system space, compared with the river reforming works and the rainwater drainage pipes' constructions. Considering the problem from an institutional viewpoint, the sewage business is planned and operated based on the decisions of city planners, so to construct and perform hybrid sewerage systems, it is important to re-adjust the duties of various relevant authorities. From a technical viewpoint, to allow rainwater into sewage facilities, it is important to devise rules for controlling rainfall inflow and outflow to and from sewage disposal plants. Namely, it is required that inflow loads are reduced at sewage disposal plants. We approach the institutional and technical problems in carrying out hybrid sewerage systems. We suggest that rainwater drainage systems should be treated as a part of whole city systems, as opposed to separating rainwater drainage systems and sewage systems. We construct the Intelligent Rainwater and Sewage Systems (IRSS) concept for handling environment issues and disaster prevention by fully utilizing the capacities of rainwater drainage systems and sewage systems. Finally, we should still expect and be prepared for minimal damage such as local flooding, and should work for a sustainable life environment in the long term. To improve city planning in safety, relief and comfort, we hope IRSS can be included as a part of space reallocation policies by fully exploiting unused capacities.