The failures at the Wachi Dam on the Yura River in Japan on July 2, 1967 and at the Folsom Dam on the American River near Sacramento, on July 17, 1995 are analyzed in light of the development of the self-excited coupled-mode instability mechanism. In both failures vibration of the gate was either suspected or noted by eyewitnesses. An exploratory study of how the predictions from the theory of the self-excited coupled-mode instability mechanism fit with the limited known facts is undertaken. The theoretical predictions from self-excited coupled-mode instability theory are found to explain well these two failures and are consistent with the reported circumstances surrounding the failures.
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As listed in Table 1 in Chapter 1, two large Tainter gates, both with at least hints of self-excited vibration problems, failed within just a few days more than twenty-eight years between their failures on two different continents. Despite eye witness testimony in one of the two failures, no consensus has been reached concerning the exact cause of these failures consistent with the observations of the eyewitness testimony.
The failure at the Wachi Dam on the Yura River in Japan on July 2, 1967 initiated the line of investigations that are reported in this book. The investigative report concerning the Wachi Dam gate failure (Asahi Newspaper, 1967; Yano, 1968) explains that while gate No. 3 was being closed from a gate opening of about 30 cm, gate No. 4 was being opened. Spillway gate No. 3 spontaneously began vibrating and quickly vibrated to failure. The gate along with the overhead bridge was carried about 136 m downstream, leaving a giant hole at the gate bay At the time, however, there was no known mechanism for spontaneously exciting vibration with a gate opening of 30 cm. The failure report (Yano, 1968) suggested some unknown mechanism of vibration could potentially have been the cause of the failure.
In California some 28 years later, on July 17, 1995, during a routine opening of spillway gate No. 3 at the Folsom Dam on the American River, near Sacramento, with the gate more than 70 cm open, a sudden gate failure occurred (Japan Times, 1995; KXTV-10, 1995). The operator reported a “rumbling” and felt vibration. The forensic team investigating the failure could find no mechanism for vibration with a gate opening of 70 cm. The eccentricity instability (the focus of Chapter 8) was found to be inoperable because of the very small eccentricity between the skinplate center and the trunnion pin center. No vibration issue was included in the final forensic report.
The present chapter will examine the circumstances of these two gate failures. An exploratory study of how the predictions from the theory of the self-excited coupled-mode instability mechanism fit with the limited known facts is undertaken. The theoretical predictions from self-excited coupled-mode instability theory are found to explain well these two failures and are consistent with the reported circumstances surrounding the failures.
Although the Wachi Dam failure came first chronologically, much more data is available concerning the more recent Folsom Dam failure. We will consider the Folsom Dam failure first and then consider the Wachi Dam gate failure.