A world first: Side channel energy dissipator system used as the dam energy dissipation method on Yokoze River Dam

A multipurpose dam built as part of the Nakasuji River Development Project

The Ichubara district of Yamanacho in Sukumo, Kochi Prefecture long retained its traditional hillside village landscape with rice terraces. Right below the planned dam site lie Todoro-no-Taki Falls and Amagoi-no-Hokora Shrine, which are inhabited by the water gods according to local legend. The rich, tree-covered environment contains natural groves of chinquapin, palms, and other trees and is a confirmed habitat for the fairy pitta, the prefectural bird of Kochi and listed as a national endangered species (Class I-B).
Yokoze River Dam was constructed and commenced services in June 2020 in the Yamada district of Yamanacho in Sukumo as an eco-friendly dam with minimal environmental impact. The Yokoze River Dam, along with the Nakasuji River Dam, completed in 1998 in Kurokawa of Hiratacho in Sukumo, forms part of a comprehensive river development project for southwestern Kochi. A 72.1 m high concrete gravity dam with crest length of 188.5 m and a water volume of 170,000 m3, the dam performs the following:

(1) Flood control
At Yokoze River Dam, the design flood discharge of 210 m3/s is decelerated to 140 m3/s as a flood control measure to reduce flood damage along the Nakasuji River.

(2) Maintenance of normal function of the river water
The dam maintains and improves the normal functions of river water, such as by supplementing the existing irrigation water supply on the Yokoze River and Nakasuji River downstream of the dam.

(3) Water supply
The dam reserves water intake to supply drinking water to the city of Shimanto (up to 800 m3/day).

Emergency spillway (when the test flood surcharge reaches the water level)

A dam designed to integrate with the surrounding area, incorporating new measures

Yokoze River Dam was designed with various new measures to integrate with the surrounding area.

Energy dissipation using the world's first side channel energy dissipator system
If dam discharge is allowed to flow directly into the river downstream of it, the strong current could cause scouring in the surrounding mountains and rivers. For this reason, the flow rate is decelerated before the water is discharged downstream. In concrete gravity dams, it is common for the energy dissipator to be placed several tens of meters directly downstream of the dam. In this case, however, a side channel energy dissipator system was adopted to preserve Amagoi-no-Hokora Shrine and Todoro-no-Taki Falls directly downstream of the dam and minimize change to the surrounding natural environment. As a world’s first, this system combines the side channels located on the downstream surface of the dam body with the divide wall at the dam toe in place of the standard energy dissipator with apron and counter dam. In terms of basic geometry, the side channels are placed vertically on either side of the downstream surface of the dam at the elevation of the normal spillway opening, and the divide wall was positioned below the regular spillway opening on the footing of the dam toe. The geometry and dimensions of the energy dissipator system were examined using dam hydraulic model testing before determining the final shape.

Patent acquired for method for using VR to set field of view
During the dam body design, virtual reality (VR) was tested out in balancing symmetry for the elevator room, the water level gauge room, and the plumb line room, as well as the landscape study for the dam crest and other areas. A patent (No. 6673761) was acquired for the results of this study as a method for using VR to set the field of view.

Climbing wall on the downstream surface of the dam
As part of local revitalization efforts, in a first for Japan, a climbing wall (for bouldering) was planned and installed on the downstream surface of the dam.

Adopted side channel energy dissipator system (when the test flood surcharge reaches the water level)