Ishioka No. 2 Power Station Spillway Channel

The channel that gives the river its water back

The Ishioka No. 2 Power Station Spillway Channel is a 107-meter open channel of stone and concrete 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).

It starts at an overflow weir set along the south wall of the settling basin and runs down the flank of the hill until it meets the Okitagawa. Water that the plant cannot use, or does not want, leaves the route here and rejoins the river a short distance below.

Every conduit needs somewhere for water to go when the turbine slows or stops. Close a gate on a channel with no relief and the water backs up until it finds a way over the wall, taking the embankment with it. The Ishioka No. 2 Power Station Spillway Channel is the answer to that problem, and it also serves as the outlet through which sediment from the settling basin and its side channel is flushed away.

Stone laid on the diagonal

The walls of the Ishioka No. 2 Power Station Spillway Channel are built in tanizumi, a coursing in which squared stones are set at a diagonal so that each block rests point down between its neighbours. The joints run at an angle rather than horizontally. Water pushing on one stone is resisted by four around it, and the pattern tolerates the small settlements that a hillside always produces.

Concrete reinforces part of the front face. The mix of the two materials is not untidy: stone where a mason's judgement suited the ground, concrete where the wall needed a monolithic strength that coursed blocks could not give. Reading which is which, and guessing why, is most of the pleasure of looking at this structure.

The gradient is steep. Standing at the top, the drop to the river is obvious in a way it is not from the road, and the sound of moving water is the first thing that tells you the channel is live.

Why the channel is on the register

Registration number 08-0215 in the 51st round covers the Ishioka No. 2 Power Station Spillway Channel, under the criterion of contributing to the historic landscape of the country. It joined the register with five other structures of the same plant, so the sequence from intake to generating hall reads as one piece of engineering rather than as six unrelated items.

Spillways are rarely thought of as heritage. They are the part of a waterworks that exists for the day something goes wrong, and they are usually rebuilt without comment. Keeping this one as a Registered Tangible Cultural Property preserves a stretch of hand-laid masonry from 1913 that has been doing an unglamorous job continuously ever since.

Seeing the Ishioka No. 2 Power Station Spillway Channel

The Ishioka No. 2 Power Station Spillway Channel belongs to Tokyo Dengen Co., Ltd. and lies within the grounds of a working, unstaffed plant. There is no entry, no path along the channel and no interpretive signage. Looking at it from public roads is the extent of what is possible, and the lower part near the river is generally the easier stretch to make out.

Reaching the area needs a car. Minaminakago Station on the JR Joban Line lies roughly 5 kilometers off and the drive takes about 15 minutes; there is no bus into the Ishioka district. Photography from public ground is not restricted. Do not walk into the channel: the bed is smooth, wet stone on a slope, and the flow can arrive without warning when a gate upstream is opened.

Kitaibaraki City's autumn cultural property opening has in some years admitted a small number of visitors to the upstream Ishioka No. 1 Power Station by application. The Ishioka No. 2 Power Station Spillway Channel has not been on that list, and the programme is revised annually, so the city's Lifelong Learning Division is the source to check.