Press Release Back Number(2005)

Cause determined for water discharge from Minamimata Dam gate

November 24, 2005
Chubu Electric Power Co., Inc.

On October 24, 2005, an opening of approximately 0.6 meters developed in the gate used to discharge earth and gravel (width: 4.85 meters) on Chubu Electric Power's Minamimata Dam. The dam straddles the Minamimata River in the Himekawa River drainage system (in Hakuba-mura, Kita-Azumi-gun, Nagano Prefecture). An estimated 300 cubic meters of water was discharged in this incident, which lasted for approximately three minutes, beginning at 1:12 pm. (This incident was announced in the city of Nagano on October 24, 2005.)


The cause of this incident and related issues were investigated while expert opinion was sought. The cause determined by this investigation was reported today as summarized below to the head of the Chubu Kinki Industrial Safety and Inspection Department (under the Ministry of Economy, Trade and Industry) and to the head of the Hokuriku Regional Development Bureau (under the Ministry of Land, Infrastructure and Transport).


Until specific countermeasures are implemented, operations at the Minamimata Power Plant will remain suspended.


1. Cause

    To ensure that the gate is opened to the appropriate extent, the opening and closing of this gate is controlled automatically based on water-level data provided by hydrographs embedded within the dam.

    Water-level data is converted to digital signals and transmitted to a gate-control relay. The problem was apparently caused by an electrical current level differing from that of these digital water-level signals, which was detected by the gate-control relay, leading to the opening of the gate.

    Since this irregular signal occurred following the installation of additional simple-camera signal distributors in December 2003, the installation of these additional units has been identified as the cause of the irregular current.


2. Future response

    The automatic gate control equipment will be operated through next spring to allow time for potential recurrence of this irregularity and for consideration of potential countermeasures.

    Even if the above irregularity does reoccur, since with continued cessation of power generation it will be possible to discharge water intake to directly below the gate and to close the gate swiftly through remote operation from the management office of Himekawa Dam No. 2, no increase in water discharge from the dam will result.