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The new Cascade concept of flood control is demonstrated in laboratory experiments in which upstream dams in a series of dams constructed along a river overflow from emergency spillways while the final downstream dam is required only to use its normal spillway and never do its emergency spillway. Multiple small dry dams lacking a slide gate in a normal spillway should be constructed in a series rather than as a single large dam to prevent flood disasters and to preserve the natural environment. Dry dams for flood control have recently been reviewed, planned, and built at sites in Japan. In this paper, we compare the Cascade method to conventional flood control in laboratory experiments conducted based on the condition that dams all have the same reservoir capacity. Results have shown that the Cascade method using multiple dry dams was considerably more effective than conventional flood control. Furthermore, the additional flood control effect of a dry dam equipped with closable and openable gate in its regular spillway was experimentally confirmed although there is no such kind of the gate on an ordinary dry dam. This new dry dam should be constructed in the river’s upper reaches away from the existing downstream storage dam needing still more its capacity for water utilization, thus ensuring the amount of water available by closing the regular spillway after the dry dam is filled to capacity. The flood control capacity of dams including the new dry dam is stronger than that of an ordinary storage dam thanks to the dry dam’s storage function.

We introduce a new "disaster immunity" concept in place of conventional "disaster management capacity" that reflects dynamic transitions in society and nature more accurately than the fixed conventional "disaster management capacity" concept. Because awareness deeply impacts on disaster management, the new concept captures disaster dynamics and could play an important role in disaster reduction. Since global warming involves disaster hazard intensification, it is not enough to simply strengthen existing measures. As an example, Japan responds to particular temperate zone patterns through long-term disaster management infrastructures. Society and nature in Japan have disaster management capacity matching typical temperate zone hazards. A rapid transition to subtropical climate patterns within the next several decades to a century is expected to generate large gaps between disaster hazards and disaster management capacity of human society and nature, leading to an imbalance. Under unstable conditions, society and nature have become increasingly vulnerable due to decreased "immunity." Increasing "disaster immunity" is thus an urgent and important issue.

This paper examines flood disasters caused by climate change focusing on two floods occurring on Japan's southwest island of Kyushu during record precipitation exceeding 1,000 mm. Our examination results emphasize the urgent necessity of implementing dam measures to prevent serious dam accidents at all costs and adequate action against driftwood and other waste materials to prevent them from interfering in dam operation. Changes in natural weather and other patterns, e.g., heavy rainfalls 1,900 mm recorded in the Miyazaki Prefecture during Typhoon 14 in 2005 and 1,200 mm in the Kagoshima Prefecture Sendai River basin in five days of torrential rain in 2006, have made it clear that conventional measures for coping with such occurrences are no longer adequate. Just 300 mm of precipitation during Typhoon 10 in 2003, for example, triggered catastrophic results in Hokkaido, where heavy rainfall rarely occurs and there is non immunity against 300 mm rainfall. Since global warming and its attendant influences are expected to continue and to bring condition of non immunity against an increased potential of disaster to whole country, the need for better knowledge and ideas on disaster prevention are urgently required.

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