Why Japan built a concrete cathedral just north of Tokyo that's designed to flood

Just north of Tokyo there's a concrete cathedral built for one purpose: to flood. In 1991 a typhoon put more than 31,000 homes in Saitama underwater. The region sits in a shallow basin, its small rivers barely slope, and houses already crowded the banks, so raising levees and widening rivers alone couldn't fix it. So engineers built a new river beneath the city. Floodwater spills over low barriers into shafts about 230 feet deep, races through a tunnel almost four miles long, and slows down in a giant hall of 59 pillars before four turbine pumps send it into the Edo River. Those pillars also hold the empty hall down against groundwater. In 2017, similar rain flooded just 202 homes. Sources: https://www.ktr.mlit.go.jp/edogawa/edogawa00014.html https://www.mlit.go.jp/river/pamphlet_jirei/kasen/gaiyou/panf/pdf/2019/kasengaiyou2019_5.pdf