PREFACE　前書き

Why are we writing a book about fluid dynamic mechanisms of hydraulic gate instabilities? Are there not enough of such books already available? The answer to the second question is a rather uncertain maybe. Maybe there are enough books about the classical gate vibrations such as the vertical vibration of vertical left gates. What is missing is a comprehensive overview of the developments in gate instability mechanisms in the past approximately 35 years. Interestingly, the date on the reprint of the book Flow-Induced Vibration: An Engineering Guide by Naudascher and Rockwell is 1994 and the following year saw the failure of the 87-ton Folsom Dam Tainter gate during routine operation.

The failure at the Folsom Dam was classified in the forensics report as a static failure due to excessive trunnion pin friction, despite eyewitness testimony by the operator who expressly notes “vibration and rumbling.” There is an assertion in the literature (Lewin, 1998, 2001) that among the gates he was considering “only the gate at the Washi [sic] Dam in Japan failed due to hydrodynamic causes. . .” and “The collapse of the gate of the Washi [sic] Dam is the only recorded case of complete failure of a gate due to vibration. . .”  Interestingly and despite eyewitness testimony of vibrations, the official report on the 1967 Wachi failure does not attribute the failure to hydrodynamically-induced vibration. No mechanism for vibration of such a large gate was known at the time. The mechanism that Lewin cites for this failure, the eccentricity instability mechanism, was proposed almost ten years after the failure. The gate design companies in Japan endorsed eccentricity instability as the failure mechanism since it absolved them of any liability. There was no eccentricity in their design specifications for the gate. The eccentricity instability mechanism, however, was shown to be inoperative for this gate due to insufficient eccentricity between the trunnion pin center and the skinplate center. In fairness to Prof. Lewin, we need to note that much of what is now known about the self-excited coupled-mode dynamic instability mechanism for Tainter gates was not available in the open literature in 1998, and perhaps not widely available in 2001. The authors suspect that Prof. Lewin would write his statement differently today given the availability of details of the coupled-mode dynamic instability mechanism.

Gate designers need to be better informed about dynamic instability mechanisms. Once a statement such as that by Lewin is printed, it becomes increasingly more challenging to convince designers to look rationally at the new developments that contradict such a printed statement. Our concern is that it will take another, perhaps more disastrous, failure of a Tainter gate before the hydraulic gate community develops a collective curiosity about the more complex issues behind a dynamic instability mechanism that may require a triggering event such as a corroded trunnion pin or large earthquake- or landslide-induced surface wave to initiate its occurrence.

Initially, the focus of this book was to be on just the mechanisms uncovered in the past 35 years by the senior author, Noriaki Ishii in conjunction with the other two authors, Keiko Anami and Charles Knisely, along with many masters’ and doctoral students at the Osaka Electro-Communication University in Osaka, Japan. Noriaki Ishii was a member of the forensics team that examined the Folsom Dam gate failure. Despite many evenings spent on the dam crest trying to resolve the conflict between the eyewitness testimony and the mechanisms of vibration known at the time, he had to admit short-term defeat and accept the team assessment of the cause of the failure. At the time of the forensic analysis, no mechanism of self-excited vibration for a Tainter gate, open 0.76 m, was known. All known mechanisms at the time were applicable to small gate openings, submerged discharge, or trunnion eccentricity, none of which pertained to the Folsom case. In analyzing the Wachi Dam gate failure, Noriaki Ishii came to understand that no single degree-of-freedom model, other than one with eccentricity, could correctly describe the observed vibrations at the Wachi Dam. Experience in mode-coupling for long-span gates several decades after the Wachi failure guided the search for the mechanism that might have driven the observed vibration of the Folsom Dam gate, and retrospectively that of the Wachi Dam gate. In the 10 years after the Folsom gate failure, through his experience and insight gained in trying to understand the Wachi gate failure and other types of gate vibration, Noriaki Ishii lead the effort to identify a vibration mechanism that does pertain to Tainter gates with large gate openings. That mechanism is the self-excited coupled-mode dynamic instability.

We were encouraged by reviewers and colleagues to broaden the topics considered in the book, and so we arrived at the current configuration: 

• an introductory chapter
• two chapters on broad overviews of fundamentals intended for entry-level engineers
• two chapters on more advanced concepts of dispersive wave, fluid excitation due to flow rate variation, added mass and damping in fluids
• one chapter on established instability mechanisms (nappe oscillation and vibration of flap gates)
• two chapters on other gate instability mechanisms (coupled-mode vibration of long-span gates and eccentricity instability)
• five chapters on details of the mechanism of the self-excited coupled-mode dynamic instability including test results from five full-scale gates that were tested for stability
• one chapter examining similarities and difference between coupled-mode Tainter gate instability and coupled-mode bridge flutter
• one concluding chapter, summarizing the efforts to find the mechanism of vibration for n self-excited coupled-mode vibrations of Tainter gates, that well might have been involved in the Folsom Dam gate, possibly causing or contributing to the failure of the gate.

The topics in this book are the accumulation of knowledge from many years of collaborative efforts. The content of the last six chapters is essential to any aspiring gate design engineer. Just as bridge engineers learned to accommodate dynamic wind excitation in the design phase after the Tacoma Bridge failure, so too must gate design engineers learn to accommodate both static and dynamic stability in the wake of the Folsom Dam gate failure.

Currently, large hydro projects continue to come on line around the world. We fear that without considering the dynamic instability mechanisms in this text, gate designers will bear witness to another gate failure due to some form of triggering excitation. Our hope would be that by explaining the theory in as much detail as possible, gate design engineers would adopt a dynamic design criteria that could avert such a future disaster.