Iwabuchi Sluice Gates in Akabane
The Old Iwabuchi Sluice Gate (aka Red Sluice Gate or Akasuimon) is a flood gate located in Akabane (Kita City), at the spot where the natural Arakawa River becomes the Sumida River. The construction of the gate in the 1920’s was accompanied by the digging of an artificial canal, starting at the same location: This canal is the Arakawa River that flows through Tokyo. The system was designed so that, in case of a flood, the gate would be closed and overflow water would instead get into the canal and quickly dumped into the sea. The Red Gate still stands but its role has been taken over by the new Iwabuchi Sluice Gate (in blue). This page has more context about the floods of the Sumida River and the gates.
The gates are in the middle of a large park that I visited a few weeks ago.