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1. A secondary drainage method for draining at a time of flooding, the method comprising a secondary drainage system that comprises a rainwater storage tank having inflow switching valves installed in water flowing-in portions to temporarily prevent water from flowing in from said water flowing-in portions, a drainage pipe for guiding, into said rainwater storage tank, the water in locally remaining zones of flooded land or of flooded property even after wide-area flooding of land due to a localized severe rain storm or river flooding has receded, and a drainage channel or drainage pump for draining the water stored in said rainwater storage tank, comprising the steps of: preventing the flow of water into said rainwater storage tank by closing said inflow switching valves during initial stages of river flooding or of localized severe rain storm that occurs in a target region, opening said inflow switching valves and guiding the water in locally remaining zones of flooded land or of flooded property into said rainwater storage tank after the wide-area flooding of land due to the river flooding or localized severe rain storm has receded, and draining the water stored in said rainwater storage tank by the drainage channel or drainage pump.
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