Green River Star -

By David Martin
Editor 

Fontenelle incident revisited

 


Fifty years ago, a disaster was averted that could have significantly impacted the City of Green River.

The Fontenelle Dam, completed in 1964, had sprung a leak that threatened the town and the ranches and homes located down river from the reservoir. Without swift action and some luck, reservoir water could have swept up everything in its path as it burst through the earthen dam.

The near catastrophe was revisited at Golden Hour Senior Center Friday when a dam safety officer for the National Parks Service discussed the lessons that could be learned from the event.

Mark E. Baker discussed the dam as part of documentary presentation utilizing photos, film and reports focusing on the dam.

“I’ve never seen one so close to failure,” Baker said about the incident.

Baker said the Fontenelle Dam crisis was the largest dam incident he is aware of that did not result in a dam’s failure.

Baker cited a report written by David Crandall, regional director of the Bureau of Reclamation, completed after the incident. This event would be Crandall’s baptism into the position according to oral history of the Provo, Utah regional office kept by the BOR, as he took over as regional director during the incident.

A 10-day emergency started Sept. 3, 1965, when seepage was noticed on the left spillway wall halfway up the dam. This seepage would erode a cone-shaped hole in the wall approximately 60 feet long. This resulted in the evacuation of people recreating immediately downstream from the dam, as well as advisory warnings to ranchers and the town urging residents to prepare for evacuation if the dam failed. By the end of the night, the seepage had stabilized to roughly 6,000 gallons of water per minute and the hole grew to about 200 feet long, 80 feet wide, and 60 feet deep.

The next day, the water coming out of the hole was a dark brown color, indicating internal erosion was occurring within the dam. Baker said the leak had grown to roughly 20,000 gallons of water per minute, prompting officials to order a contractor already at the site to begin piling dirt and rocks into the active leak as they began to drain water from the reservoir’s outlet works, which would lower the entire reservoir about four feet each day. This drainage was temporarily hindered by heavy rains occurring throughout the region as precipitation would add to what was draining from the reservoir.

The rock piling would only provide a temporary solution however. The next day, after about 1,700 cubic yards of rock were dumped into the hole, building pressure resulted in water bursting through the rocks in a number of high-powered surges that opened the leak further. Baker said one person witnessing the initial surge of water called people in Green River, claiming the dam had burst. Work on the dam continued the following day, the erosion inside the dam created a large sinkhole at the top of the dam.

“Vibrations were felt by people standing on the dam,” Baker said.

The sinkhole was filled with debris bulldozed from the upstream face and filled with rock. The hole was then coated with a road base to allow for travel. The sinkhole would be the last of the major events occurring at the dam. By Sept. 7, five days after the leak started, water levels in the reservoir had dropped to a point where pressure on the leak was easing up and water was no longer above the dam’s spillway. Diminished leak flows during the next week resulted in the emergency being called off Sept. 12.

Baker said the near-failure was due to the large amount of water the reservoir was storing. The winter runoff from a very moist year resulted in a total of 113 billion gallons of water being stored throughout the Fontenelle reservoir. This water and the pressure it generated began to force its way into large cracks in the sandstone, creating relief cracks along the reservoir and near the dam itself. Water began working its way through the dirt-based dam, eventually causing the leak. As the leak progressed, voids were created inside the dam as the soil would erode and become muddy, forcing the voids upward through the dam until the sinkhole opened up days later. Baker believes this was a result of the soil used in the dam’s construction not being compacted as the dam was built.

Baker said internal reports compiled by geologists recorded 29 relief cracks near the dam. The dam was the site of major reconstruction and was significantly modified in the 1980s, costing $58 million.

Lessons learned too late

Unfortunately, lessons from the Fontenelle Dam’s near failure weren’t heeded in time to prevent tragic events from taking place. In Idaho, the Teton Dam failed on June 5, 1976. The dam matched many of the same conditions that resulted in the Fontenelle Dam incident. The dam’s failure resulted in the deaths of 11 people and approximately 13,000 cattle, with damages estimated to have totaled $2 billion. Baker said the similarities between the two dams were striking.

Baker said Crandall originally intended to write an article in “Reclamation Era” magazine detailing the dam’s problems and what could be learned from the original incident. However, Crandall, when submitting a draft to his superiors, was encouraged not to publish the article. The article and other first-hand accounts of the incident were stored in the BOR’s Upper Colorado Region Library for 42 years until Baker discovered them in 2010.

Now, the BOR reviews every dam every eight years and has between three and five repair projects occurring at any given time. However, Baker said engineers can still learn from what happened at the Fontenelle Dam, saying people are doomed to repeat past mistakes if they’re forgotten.

 

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