Proposal for compact city plans focused on flood damage mitigations and investigation of their mitigation effects using a coupling analysis of precipitation and runoff

In the context of global warming, in recent years, extremely heavy rains and subsequent flood damages have occurred more frequently. The objectives of this study are to propose city planning focused on flood damage mitigations and to investigate the mitigation effects of introducing such planning on the flood damages under projected future (the 2050s) climate conditions. Urban compactness and utilization of existing building stocks are taken into consideration in the planning. Those strategies are attracting much attention in Japan, where the population decline and severe financial condition are major social issues. The target area is Aichi Prefecture, which is the main prefecture in the third largest metropolitan area of Japan.

Two compact city plans are created in this study. In both city plans, urban compactness is achieved through population movement among urban areas (classification of population-withdrawal areas and population-induced areas). The withdrawal areas correspond to areas that will be exposed to severe flood damages in the events of future heavy rains. The two types of compact city plans, i.e., high-concentrated and low-concentrated plans, are made depending on the number of withdrawal areas. Moreover, for both city plans, existing building stocks in the population-induced areas are actively utilized to accommodate the population from the withdrawal areas.

The future (the 2050s) heavy rains and subsequent flood damages are projected using a coupling analysis of precipitation and runoff. A dynamical downscaling simulation technique is adopted to project the future climate including precipitation. The above-mentioned compact city plans are reflected by the settings of land use data and urban parameters as input conditions for the coupling analysis. Finally, the flood damage mitigation effects of the proposed compact city plans are quantitatively evaluated by comparing the extent and severity of flood damage with the results in case of using the current land use.