Ishioka No. 2 Power Station Settling Basin

A tank whose job is to slow water down

The Ishioka No. 2 Power Station Settling Basin is a reinforced concrete desilting basin in the Ishioka district of Nakagocho, Kitaibaraki City, Ibaraki Prefecture, completed in 1913 (Taisho 2) and placed on the national register as a Registered Tangible Cultural Property on 3 August 2006 (Heisei 18).

It sits about 750 meters below the intake weir, at the point where the conduit widens abruptly. The plan is close to a rectangle, 30 meters long and 9 meters wide, with a footprint of 337 square meters. The concrete is rendered in mortar, a plain finish that has weathered to a grey close to that of the surrounding rock.

The principle is simple enough to grasp from the bank. Water moving fast enough to reach a turbine also carries sand. Widen the channel and the current slows; slow it enough and the sand drops to the floor instead of travelling on. Everything downstream of the Ishioka No. 2 Power Station Settling Basin, including the steel penstock and the runner in the generating hall, lasts longer because of what happens in this one shallow rectangle.

Why a plain concrete tank went onto the register

The Ishioka No. 2 Power Station Settling Basin was registered as number 08-0214 in the 51st round, under the criterion of contributing to the historic landscape of the country. Five other structures of the same plant were registered on the same day, so the complete route the water takes is protected as a set rather than as a single showpiece.

Nothing about the basin was built to be looked at. That is part of the reason it matters. Reinforced concrete in 1913 was still being tested in Japan, and utilitarian structures like this one show the material used for what it was good at, without stone facing or applied ornament to soften it. A Registered Tangible Cultural Property can stay in daily use, and this one has never been taken out of the water route.

The side channel, and the thinking behind it

A channel runs along the hill side of the Ishioka No. 2 Power Station Settling Basin, separated from the main tank. Its purpose is to carry surplus and sediment away, and a flushing gate is fitted to it. The point that separates careful design from adequate design is that this gate can also purge sediment out of the side channel itself. The engineers assumed that the arrangement built to remove sand would in time collect sand of its own, and they gave it a way out.

Anyone who has seen a silted irrigation ditch will recognise the problem. Sediment does not respect the drawing. Building a second line of defence into a structure that was already a defence is the kind of decision that only shows up when you look at where the gates are.

Seen from the road the basin reads as a long low wall of concrete with water lying quietly behind it, which is exactly how a working desilting basin should look.

Seeing the Ishioka No. 2 Power Station Settling Basin

The Ishioka No. 2 Power Station Settling Basin is owned by Tokyo Dengen Co., Ltd. and is part of an unstaffed, operating hydroelectric plant. The grounds are private, the interior is not open, and there are no facilities of any kind for visitors. What can be seen is the outside of the structure from public ground.

Minaminakago Station on the JR Joban Line is the closest railway station, about 5 kilometers away and around 15 minutes by car. No bus runs into the Ishioka district, so a taxi or a rented car is the only realistic way in. Photography from public roads is unrestricted. Climbing the embankment or entering the fenced area is not, and standing water in a concrete basin is deeper and colder than it looks.

Kitaibaraki City holds a cultural property opening each autumn at which the upstream Ishioka No. 1 Power Station has been shown to a small number of applicants. The Ishioka No. 2 Power Station Settling Basin has not been part of that programme, and the details change from year to year, so the city's Lifelong Learning Division is the place to confirm what is open.