Ishioka No. 2 Power Station Intake Weir

Where the water route of the Ishioka No. 2 Power Station begins

The Ishioka No. 2 Power Station Intake Weir is a gravity concrete weir on the Okitagawa River (大北川) in the Ishioka district of Nakagocho, Kitaibaraki City, Ibaraki Prefecture, built in 1913 (Taisho 2) and entered on the national register as a Registered Tangible Cultural Property on 3 August 2006 (Heisei 18).

By the standards of river engineering it is a small structure: 19 meters across the channel, 4.7 meters from riverbed to crest. Concrete carries the load. The surface the river actually touches is stone, laid course by course across the face, so the weir reads as masonry rather than as a poured block. A stone apron spreads out in front of the crest to take the fall of the overflow, which is the part of any low weir that gives way first.

The water arriving here has already worked once. The main building of the neighbouring Ishioka No. 1 Power Station sits roughly 100 meters upstream, and the Ishioka No. 2 Power Station Intake Weir catches the river again just below it. From the gate on the left bank the flow enters a conduit that runs about 750 meters downstream to a settling basin, and from there to a head tank and a penstock.

Why the weir was registered

The Ishioka No. 2 Power Station Intake Weir was registered under the criterion of contributing to the historic landscape of the country, in the 51st round of registrations, as number 08-0213. Six structures of the same power station went onto the register on the same day, which is unusual: registration is normally granted structure by structure, and a complete water route surviving intact from intake to generating hall is rare enough that the whole sequence was taken together.

What makes the weir worth keeping is not size but date. Reinforced concrete was still a young material in Japan in 1913, used with caution and often hidden behind more familiar surfaces. Here the engineers poured a concrete mass for strength and then dressed it in stone, a compromise between what the material could do and what a river structure was expected to look like. Registration under the Registered Tangible Cultural Property system keeps a working structure in service rather than freezing it, which suits a weir that has never stopped diverting water.

What repays a closer look

The most deliberate detail at the Ishioka No. 2 Power Station Intake Weir is the way the intake gate is set. Rather than facing the current head on, the sluice gate on the left bank is placed at a right-angled turn, so that water has to change direction before it enters. Grit and gravel carry straight on and pass over the weir; the conduit takes the cleaner water off the bend. Nothing about it is decorative, and it works without any moving part beyond the gate itself.

Beside the intake gate sits a second, smaller opening for flushing sand. Silt collects against any weir, and a structure with no way to purge it slowly fills until the intake chokes. Behind the gates the No. 1 open channel begins, the first length of the conduit, built as part of the same registered unit.

Water level changes the character of the place completely. After rain the whole crest disappears under a sheet of falling water and the stone apron below is invisible. In dry weather the coursed stonework on the face is exposed and the joints can be read from the bank.

Seeing the Ishioka No. 2 Power Station Intake Weir

The Ishioka No. 2 Power Station Intake Weir belongs to Tokyo Dengen Co., Ltd. and forms part of a working, unstaffed power station. There is no visitor centre, no parking area and no interior access, and the grounds are private. Viewing is from public roads and from the riverbank only, and the intake gate is easier to make out in winter when the vegetation on the banks has died back.

The nearest railway station is Minaminakago on the JR Joban Line, about 5 kilometers to the southeast, roughly 15 minutes by car. No bus route serves the Ishioka district, so a car or taxi is needed. Photography from public ground is not restricted; flying a drone over the plant or stepping inside the fence is not acceptable, and the riverbank near a weir is slippery and can rise quickly when the gates upstream are opened.

Kitaibaraki City runs an autumn cultural property opening each year at which the upstream Ishioka No. 1 Power Station has been shown by advance application. The Ishioka No. 2 Power Station Intake Weir has not been included in that programme, so anyone hoping to get closer should check the current year's announcement with the city's Lifelong Learning Division rather than assume access.