Wednesday, February 15, 2017

Oroville Dam: Nation's Tallest at 770 feet

Construction of the tallest, 770 foot, dam in the country:



I don't watch videos of officials talking about the current crises of the spillways because the multitude of  platitudes such as "safety is our first priority," and the lack of information is just too frustrating. But I do read the articles about what was said. To add insult to injury, the officials decided to call it an "auxiliary," instead of "emergency," spillway. An engineer got in trouble at a press conference because he continued to use the word "emergency." But it is an emergency spillway because it was designed to allow damage if it was used that would have to be repaired. That is the definition of an emergency spillway. For example, the road embankment was an obvious design causality. But a deeper hole forming was not part of the designed allowable damage. This hole caused them to panic and call for an evacuation because they thought the emergency spillway might break in an hour. It turns out that, after the stopped the flow of water over the emergency spillway, they saw the hole was not as bad as they had feared. But they would not let the people come back.

I heard the main spillway broke because a sinkhole had formed under it. It was last inspected and repaired in 2013, so this is not an example of neglected infrastructure. The inspectors evidently don't look for sinkholes under the spillway. (I assume the Water Department will now figure out how to look for sinkholes with ultrasound or ground penetrating radar or something and then reinspect the main spillway of all the dams they own.)

The question in my mind that I can't get answered is "where is the bedrock under the soil?" They did say that the main spillway will break up from 2/3 of the way down to 1/2 of the way down where it will hit bedrock and quit eroding towards the control gates. And both sides of the dam itself should be keyed into bedrock. But the construction video talks about an earthen dam. Evidently the emergency spillway weir is built on dirt, not rock! The road being washed out was probably by design as some of the acceptable damage if the spillway is used. But I'm sure a big erosion hole was not part of the acceptable damage. If the weir is truly built on dirt instead of bedrock, then that hole can continue to erode under the weir until it reaches the water on the other side. Then you have a blowout under the weir that will probably break the weir. That is why they were talking about a 30-foot wall of water going down the Feather River. (I just saw a comment indicating "Oroville Dam Imminent Failure (LIVE STREAM) MASSIVE 200 ft WALL OF WATER!" [LiveNews] But every time point I sampled in that video was talking about the evacuation. And the audio has nothing to do with what the video is showing. But at 54:39 I did spot some video that I was informative --- the repair of the hole before the next rains come on Wed (2-15-2017). I had seen a video Monday that showed helicopters filling a hole with big rocks. But they did not show or explain where the hole was.)

The main spillway was designed to handle 125,000 cubic feet per second (or 125 kcfps). During previous "rain events," it did handle that amount. If a hole had not broke open in the main spillway, I would have still never have heard about this dam because the main spillway would have handled this rain event. But because of that hole, the officials got scared and decided to limit the flow to 55 kcfps and allow the lake to fill up at 6-feet per hour and go over the emergency spillway. 12 kcfps normally goes through the powerhouse, but they had to shut that down because the broken concrete that flowed into the Thermalito Diversion Pool was backing up the water higher than the powerhouse could handle. I heard a figure over 300 kcfps as the design capacity of the emergency spillway. But after they saw the damage being caused by just 3% of its design rate, they decided to run the main spillway at 100 kcfps and after a few hours that caused the water to quit going over the emergency spillway. By the time of that decision, the main spillway had evidently stabilized because the flow's spillway and soil erosion had hit bedrock. The Feather River is supposed to be able to handle 250 kcfps without damage to the town. They are letting the main spillway run at 100 kcfps to lower the level of the reservoir by 30 feet. That is, they are sacrificing the water storage function of the dam, the main reason it was built by the Water Department, to try to avoid another emergency overflow when rains come Wed.

In 2005 three environment-oriented organizations filed a report with a Federal agency that the emergency spillway is unsafe unless they build a concrete channel for the water. But that would cost the 19 million people in Southern California who use the water $100 million to remedy, so it was not done. During the first day, estimates to repair the damage done to the spillways were $100-200 million. And that figure will only go up. Basically, in 2005 people were advocating that the second spillway, in fact, be an auxiliary spillway instead of an emergency spillway. The director of the California Water Resources Department, when asked about the 2005 report said he was unaware of that report! So did the committee that decided to use the emergency spillway have just political appointees? Were there no hydrologists and engineers on the "experts committee" who had studied the dams construction and history see the report? Had no one researched were the bedrock is?

The "experts" also did the classic evacuation error that they learned to avoid with hurricanes --- they told everybody at the same time to get out of town. In this case they used the efficiency of a Twitter to create an even bigger traffic jam. And they have gone from "don't worry, everything is safe" before the evacuation order to "we have no idea when it will be safe and you can go back to your homes" after the order. In fact, they increased the order from 125,000 people to 200,000 people. [My wife told me Tuesday evening that she saw a blurb on the web that they were finally allowed to go back to their homes.]

I read years ago an article about how the levees protecting Sacramento are old and need to be upgraded. I wonder if any upgrades were made. If a 30-foot wall, let alone a 200-foot wall, of water was unleashed, a reporter indicated their levee system would be "tested."

Update:

The Farmer's Live posted a drone flyover that gives a better view than the helicopter views I have seen.

A February 26, 2017 summary of the spillway issues including some speculations as to why it failed in the first place.

An article with 22 photos including repair work  on the emergency spillway.

The spillway design capacity was 160,000 cfs. [Video] The expense of dredging the channel and patching the spillways has already cost $100m. They are still trying to figure out what the permanent fix would be, let alone its cost. The CDWR choose not to spend $0.5m to fix (grout) the spillway and $4m to line the channel of the emergency spillway with concrete when these issues were brought to their attention. The usage of the emergency spillway for the first time quickly caused a one-hour-notice emergency evacuation of the town at just 3,000 cfs flowing over it. Its design capacity was supposed to be 250,000 cfs.

This video indicates that it broke where they did repairs in 2014. And I'm also confused about why they are ignoring the breach over the parking lot.





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