Ishioka No. 2 Power Station Head Tank

The last open water before the pipe

The Ishioka No. 2 Power Station Head Tank is an elliptical reinforced concrete tank in the Ishioka district of Nakagocho, Kitaibaraki City, Ibaraki Prefecture, built in 1913 (Taisho 2) and listed as a Registered Tangible Cultural Property on 3 August 2006 (Heisei 18).

It joins the east end of the settling basin, a drum standing 11 meters on its long axis and 6 meters on its short one, occupying 64 square meters. A concrete slab covers the top. From the outside it gives away very little: a closed cylinder, faintly oval, with a small platform projecting from the south side.

Everything up to this point is an open channel, with the water surface exposed to the sky. Everything after it is a sealed steel pipe under pressure. The Ishioka No. 2 Power Station Head Tank is where the changeover happens, and the shape follows from that job. A tank has to hold a reserve deep enough that air is never drawn into the pipe, and steady enough that a sudden change in load does not empty it.

Two gates, and why they were unusual

At the boundary between the settling basin and the Ishioka No. 2 Power Station Head Tank stand two sluice gates of the Stoney type. A Stoney gate runs on a train of rollers held in a separate frame, so the rollers carry the load between gate and guide instead of the two surfaces grinding against each other. The gate can be raised against full water pressure with far less effort, and it seals better when shut.

That was advanced equipment for a small plant in 1913. Fitting two of them here, rather than one larger gate, means the flow could be cut in halves and the tank isolated without stopping the whole route. The small platform on the south side exists so that a drain valve could be worked by hand, which tells you the tank was expected to be emptied and inspected regularly.

Why the tank is on the register

The Ishioka No. 2 Power Station Head Tank carries registration number 08-0216 from the 51st round, entered under the criterion of contributing to the historic landscape of the country. It was one of six structures from the same plant registered together, an approach that keeps the water route legible as a sequence rather than as scattered relics.

Its interest lies in the combination of a young building material with imported gate technology, put together by engineers working on a modest river in northern Ibaraki. As a Registered Tangible Cultural Property it stays in service, and the concrete shows its age honestly: patched, stained by decades of wet and dry, and still holding water.

Seeing the Ishioka No. 2 Power Station Head Tank

The Ishioka No. 2 Power Station Head Tank stands on the private grounds of an unstaffed working plant owned by Tokyo Dengen Co., Ltd. There is no access to the interior, no visitor parking and no signage on site. It can be looked at from public ground, and because it sits at the head of the steel pipe, the tank and the line of the penstock running down the slope can usually be taken in together.

Getting there means driving: roughly 15 minutes from Minaminakago Station on the JR Joban Line, which is about 5 kilometers away on the line between Mito and Iwaki. There is no bus. Photographs from the road are fine; climbing towards the tank or onto the penstock line is dangerous as well as trespassing.

The autumn cultural property opening organised by Kitaibaraki City has in past years admitted a small number of applicants to the upstream Ishioka No. 1 Power Station. The Ishioka No. 2 Power Station Head Tank has not featured, so check the current year's notice with the city's Lifelong Learning Division before planning a trip around it.