Water Policy (Dec 2021)
Anticipated maximum scale precipitation for calculating the worst-case floods
Abstract
Against increasing number of unprecedented heavy rains and typhoons reflecting climate change, the Japanese Government decided saving life as the top priority considering a ‘worst-case’ scenario. Accordingly, the Flood Risk Management Act was amended in 2015 to use the anticipated maximum scale precipitation (AMSP) for flood inundation calculation. In order to estimate the AMSP, the Ministry of Land, Infrastructure, Transport and Tourism (MLIT) chose historical maximum areal precipitation in the form of duration–area–depth (DAD) curves rather than climate change projections' dataset d4PDF. In this paper, policy development and detailed estimation procedures for the AMSP were reviewed and discussed. It was concluded that the current climate change projections are still not accurate enough to be used as the basis for real local operations, while long accumulated ground observations and ground-based radars are available in good quality all over Japan. But at the same time, historical maximum should always be updated as past records are renewed. Also, regional partitioning should not be done at too coarse of scale for proper regionalization of DAD. Such strategy would serve as a useful reference for other nations. HIGHLIGHTS To produce the flood hazard maps under the worst-case scenario, the anticipated maximum scale precipitation (AMSP) is used.; The regionalized worst-case DAD, that is, the DAD estimated by the areal maximum precipitation observed in the history in a climatologically uniform region, is used to estimate the AMSP in any basin in the region.; The ground observations by JMA since 1957 and the Radar-AMeDAS analyzed datasets since 1988 are considered more reliable than the d4PDF dataset assembled by climatological projections using GCMs and RCMs to be used for calculating inundation for flood hazard maps for local practice all over Japan.; The areal maximum precipitation prescribed for a climatologically uniform region is subject to renewal in the future. Therefore, periodical updating of the AMSP is necessary. At the same time, reflection of climatological projections for the areal maximum and geographical partitioning of climatologically uniform regions should be considered.; Ground observations and model-based climatological projections need to go hand-in-hand.;
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