The water quality improvement project for the outer moat is estimated to cost 34 billion yen, with completion expected in the mid-2030s, according to Tokyo’s estimates.

Aiming to improve the water quality of the “Sotobori” (outer moat) of the Imperial Palace, an implementation plan was announced to channel water from the Tamagawa Aqueduct and other sources. The project cost is estimated at approximately 34 billion yen, with a target completion in the mid-2030s.

The outer moat was constructed between 1604 and 1636 to surround the area, but from the Meiji period onward, the eastern side was largely filled in for railway land and other uses. The remaining canal sections on the western side are designated as a national historic site, “Edo Castle Outer Moat Ruins.”

Among these, the Tokyo Metropolitan Government identified water quality deterioration as a problem in the “Ushigome Moat,” “Shin’atsuke Moat,” and “Ichigaya Moat,” where water surfaces remain. Citing the lack of daily water inflow as the cause, the “Sotobori Purification Project” was launched in 2021 to draw water from the Tamagawa Aqueduct and the Arakawa River.

The current implementation plan outlines four projects, including installing pumps to send water from the Arakawa River, which has elevation differences, and constructing a 2-kilometer water channel from the end of the Tamagawa Aqueduct to the outer moat. The estimated construction cost accounts for rising prices, with an additional annual maintenance cost of 240 million yen.

All construction is currently in the design phase, with start dates undecided, but completion is expected within about 10 years. After completion, 500 liters of water per second will flow in, pushing old water out at the northern connection point, allowing the water in the outer moat to be replaced in about five days. The actual water flow will occur only during periods when the water temperature exceeds 20 degrees Celsius, when water quality tends to deteriorate.