The simple answer is yes, because anything is possible. But is it probable? Not very. The dam is 100 feet thick at the base and tapers to 12 feet at the crest.
Dams fail due to several distinct causes. The most common is overtopping of the dam capacity, usually due to unexpectedly heavy rainfall in the watershed. This may be exacerbated by accidental blockage of the spillway or inadequate spillway design.
The second most common cause is defects in the foundation of the dam. These may be due to substandard construction methods or poor maintenance. The third most common cause is failure due to piping and seepage from internal erosion and cracks in the dam structure.
Problems with conduits and valves caused by the entry of embankment material into conduits are another source of dam failure. All of these reasons combined constitute an explanation for 94 percent of all dam failures. That leaves approximately 6 percent for other causes of dam failure, of which earthquakes could be one.
The Solano Irrigation District, which manage dam operations, states that the only way Monticello Dam could fail and put Winters underwater would be a complete immediate failure along the lines of the entire structure crumbling at once. If such a failure should ever occur, it was estimated that Winters would be under 40 feet of water in about 15 minutes, with floodwaters reaching all the way to Davis.
However, it is unlikely that it would ever go over the top of the Monticello Dam because the dam was designed to let water flow around it and out onto the Highway 128 rather than go over the top. The lake level would have to rise to 455 feet, which would be 15 feet over the Glory Hole rim. The Bureau of Reclamation would like to believe that this scenario is practically impossible.
The Bureau of Reclamation has a systematic four-step program that continuously monitors the status of its dams. There is a review and inspection every four years, which involves looking at seismic, hydrologic and static parameters.
BOR headquarters in Denver and the local area office take turns being in charge of these inspections, which are exhaustive. It is visual, looking for anomalies like seepage in the internal galleries that house some of the sensing equipment relied on for data. But they also evaluate how much loading the horizontal joints are bearing and other structural parameters, evaluate seismic data.
If anything is out of specification, they will do more studies to analyze those potential weaknesses. If unwanted changes are found, they make plans for corrective actions and do it then. BOR routinely take cores of concrete from various points in their dams and test them for compressibility — how much load can they stand before giving way. In their experience, overloading of dams from seismic events is less frequent than static loads from the water being held in check. BOR engineers say that Monticello is one of the better dams they’ve ever built.
Locally, daily visual inspections of the entire dam site take place looking for any potential signs of change that could have consequences. Once every few years, the Bureau of Reclamation has people crawling all over the dam, checking every aspect of its stability. After any event in an area that could affect a dam, the dam is immediately inspected. Nothing unusual was seen at Monticello Dam after the Napa 6.0 earthquake in 2016.
What is the probability of an earthquake at Monticello Dam? There is historic evidence that a serious earthquake can occur in this area, as it did in 1892 with a magnitude 6.4 quake that leveled downtown Winters. However, it took 100 years for J.R. Unruh and Eldridge Moores, geologists at UC Davis, to establish the reason for the quake.
“It was difficult to establish until we had appropriate instrumentation because this is a concealed fault, a blind thrust as the Coast Ranges move up and into the Central Valley,” Moores said. That specific fault line extends north of Winters along the eastern edge of the Coast Range. Fault maps also show a fault line running more or less linearly adjacent to the western edge of Lake Berryessa — the Hunting Creek-Berryessa Fault Line. The Great Valley fault line running north of Vacaville is close to the site of the Monticello Dam.
“The direct effect of the Napa earthquake on the Monticello Dam would have been very small,” Rundle said, “therefore not a concern.” However, there are several faults in the area of the dam. “Earthquakes on a fault are known to affect other faults,” Rundle added. “This is called ‘fault interaction.’ It is due to the transfer of stresses or forces on one fault to another close by. A concern might be that these nearby faults in Northern California might together start to be active.
“What would happen if the Great Valley fault, which basically runs under the dam, were to fail in an event such as the magnitude 6.4 Vacaville-Winters earthquake, which was located somewhere in that area? Very probably, this will not happen. But I can’t and won’t say that it absolutely won’t happen.
Monticello Dam Failure Simulation
Steven Ward, a research geophysicist at the Institute of Geophysics and Planetary Physics at UC Santa Cruz has created a computer simulation of the first 16 hours of flooding that might be expected from the failure of Monticello Dam from a possible earthquake. This worst-case scenario envisions a nearly instantaneous breakdown of the structure and a reservoir filled to capacity. This is unlikely but informative.
However, the simulation is the worst-case scenario. Ward writes, “Rather than flush-and-gone, a dam break here is akin to opening a valve to a hose that will spray at a nearly constant rate for hours and hours. Second, just downstream is California’s Central Valley, a very flat and nearly un-channeled place. Don’t expect the flood to follow a well-defined river track as you might elsewhere. “The simulation suggests that about 1,000 square kilometers will be affected. Most areas would see water less than one or two meters deep, but the outburst would last a day or more.”
The Glory Hole spillway is at 440 feet above mean sea level (msl). The height of the dam is 450 feet msl. Highway 128 is designed to be the initial spillway if water rises that high. The water level has never been above 446.7 feet msl and has only been at or above Glory Hole (440' msl) 26 times in its 64 year history.