Future Possibilities of Intense Precipitations in Urban Areas and Adequate Plans of Urban Land Use for the Risks

Future possibilities of intense precipitations in urban and suburban areas of western parts of Japan were estimated by use of statistical methods of extreme values and Bayesian inference. In this study, observed data of precipitation of almost 30 years at 69 meteorological observation points in Japan are used for the analyses, and mesh data of amounts of precipitations were also referred to as auxiliaries. Furthermore, through overlaying the predictions on some maps relating to societal phenomena, e.g. land use regulation, distributions of public facilities and future demograhic changes, some adequate policies of land use plan in accordance with future risks of flood were proposed.

Statistical analyses of extreme phenomena revealed that precipitations in the area were fundamentally altered within 30 years from 1990 to 2018. The frequency of precipitations of over-30mm has been increased in broad areas, and the probabilistic distributions of the daily maximum number of one-hour precipitation of the last 10-year period have also been shifted to others from ones of the first 10-year period, even though the total amounts of precipitations were not changed largely. At many observation points, the shape parameters, which indicate theoretically situations of “tails” of the probabilistic distributions, show conspicuous increasements from the first period. Many of them change to positive values from negative values at time of the last period. This means that risks of extreme events have increased from what had been assumed previously at fairly amounts of points. The 100-year return levels of precipitations have also increased in general. At 23 out of 69 points they exceed 100mm per one hour, even though it was only 7 places in the first period.

Overlaying these predictions onto maps of some societal phenomena including a future prediction of population or land use changes suggests that urban land use will need to be controlled adequately especially in areas near by small rivers due to the future risks of floods as well as augment pervious areas, in order to avoid inundations. Most Japanese cities are now in the phase of depopulation except for Tokyo metropolitan area, and these proposals would possibly contribute to mitigations of risk of extreme weather events as well as improvements of urban environments.

Figure: Distribution of estimated 100-year return levels of precipitations at the observation points and Japanese DID (Densely inhabited districts).