70,000 in Puerto Rico urged to evacuate immediately as dam is in “imminent” danger of failure
This should serve a stark warning to DWR - and the public - at Oroville dam. An exact failure mode is occurring in Puerto Rico Dam that Oroville Spillway Gates already have the symptoms for.
As of tonight, a Puerto Rico dam is facing imminent collapse. Mass evacuations are underway. A large crack developed. The "failure mode" of the dam is from "jammed gates" where they cannot release water. The dam will then experience an overspill which is feared to result in a full "breach".
Oroville's Main Spillway Gates have experienced "jamming" to where full reconstruction of the side seals were warranted. Don't think this couldn't happen to Oroville as the side seal problems and alignment issues depending on the balanced strength of the Gates' anchor tendons. Just a 0.017 inch shift in either trunnion pin (strain) will cause the 0.040 inch side gap spacing to disappear. Bronze guide shoes could then assert higher forces on the slide plates. If these forces get too high, the gates jam and become stuck.
This Failure Mode at Puerto Rico (large crack) is amazingly similar to conditions at Oroville in that two large cracks are already in the massive concrete Piers (Gates 1 and 8). The cracks, one is 16+ feet, are evidenced from a design defect (design mistake) causing differential settlement of the spillway bridge abutment with respect to the two headworks structures. DWR has done nothing to fix these cracks. All they have done is continued to cover them with orange paint so they can watch to see how much they've grown from year to year.
DWR decided not to fix the anchor tendons and the cracks in the headworks. If the gates jam and fail, like what is happening in Puerto Rico, the Emergency spillway is going to be forced to be operated (no choice). There is not enough armoring on the Emergency spillway to handle flows more than 13,000 cfs for 2017/2018. This is a gamble that the Main Spillway Headworks and Gates will not fail. Well, here you have a real life example of a "jammed gates" failure mode underway.
Puerto Rico's dam (just over 120+ feet and 2 sq miles of reservoir area) is very small compared to Oroville dam (770 ft tall and much greater square miles in reservoir area). The high risk stakes are much greater at Oroville.
If the Emergency Spillway has to take a much larger flow, the secant wall will not protect the 64ft high concrete Weir section next to the Main Spillway Headworks. If this section "topples", the Headworks would see a deepening "V" breach. This could escalate to a full loss of head control, the potential destruction of the upper spillway at the headworks, and worst case, erosion/breach migration to the main earth dam.
=== Article clip (emphasis mine)
70,000 in Puerto Rico urged to evacuate immediately as dam is in “imminent” danger of failure
September 22, 2017 at 10:21 pm
SAN JUAN, PUERTO RICO – Tens of thousands of residents in northwestern Puerto Rico were ordered to evacuate Friday amid fears that a dam holding back a large inland lake was in imminent danger of failing because of damage from Hurricane Maria’s floodwaters.
Officials worried that as many as 70,000 people could be in the path of a massive amount of rushing water in the event the Guajataca Dam releases into the Guajataca River, which flows north through low-lying coastal communities and empties into the ocean.
The dam suffered a “fissure,” Puerto Rico Gov. Ricardo Rosselló said in a news conference Friday afternoon. Residents in the municipalities of Quebradillas, Isabela and part of San Sebastian could be affected if the dam collapses, he said, and it could be a catastrophic event.
“To those citizens . . . who are listening: Please evacuate,” Rosselló said as buses were dispatched to ferry residents out of harm’s way. “We want your life to be protected. . . . Please, if you’re listening, the time to evacuate is now.”
Abner Gomez, executive director of Puerto Rico’s emergency management agency, said in an interview late Friday that the dam’s gates suffered mechanical damage during the storm, making it impossible for them to open and let out normal water currents. Officials worry that could cause the dam to spill over.
Gomez said that under current conditions, with water rising after the hurricane, “there is no way to fix it” right now. Additional water flowing into the lake could create sudden dangers, so emergency evacuation was the only option, he said. If the dam spills over or fails structurally, he said, “thousands of people could die.”
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70,000 in Puerto Rico urged to evacuate immediately as dam is in “imminent” danger of failure
Update to Puerto Rico Dam Breach:
The "fissure" or "crack" has not been identified where it is located. Guajataca Dam is an "earthen dam" like Oroville dam. Reports indicated that flooding has increased from this dam failure condition but it is not clear if the increased flow is from this "crack" or "fissure".
Engineers have stated that the cause of the mechanical gate failure (i.e. cannot open) is from structural conditions related to the hurricane storming.
Photos reveal that "back cutting" erosion into the earthen dam is occurring in a damaged area of a formed spillway. Other engineering reports noted that there was a crack in the northern part of the dam. However the back cutting erosion is about mid-way in the dam (centered slightly north).
More information will likely be coming in the news.
Back Cutting erosion eating away at the base of the dam. Section of the Spillway and dissipation riprap junction destroyed.
Guajataca Dam, Puerto Rico - Built in 1929 by the US Army Corps of Engineers. ~1,000 feet long, 120 feet high, and a 2 square mile reservoir area.
