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Wow almost every statement here is wrong. #1: the dam is managed in the winter about 1 million acre feet below the top, for flood control purposes. The spillwa

Wow almost every statement here is wrong. #1: the dam is managed in the winter about 1 million acre feet below the top, for flood control purposes. The spillway has been operating all winter long. #2: The operator cannot decide to use the emergency spillway. Water just goes over it when it approaches the top of the dam. #3: no water has gone over the dam. That would destroy it.

crisdux 10y ago
Regarding #2, they can choose to decrease flow out of the main spillway and let it go over the emergency spill way - From my understanding, that is exactly what happened over the weekend. They were trying avoiding continued damage to the main spillway.
- maxerickson 10y ago
  Quoting from one of the LA Times updates linked from the link: The result is a balancing act — drain as much water as possible as quickly as possible while trying to minimize further damage to the spillway. The main spillway was returned to action, but at reduced capacity compared to what it could normally do under the current conditions. After all, the spillway needs to last for the remainder of the rainy season. The stated capacity of the main spillway is 250,000 cubic feet per second, once it became clear the emergency spillway was not holding up, they increased the main spillway to 100,000 cubic feet per second. So limiting damage to the main spillway is an ongoing concern.
  - mintyfresh 10y ago
    The capacity of the intact spillway is ~250,000 cfs - the capacity of the Feather River through Oroville and Yuba City below, currently in the lower flood stages with 100,000 cfs outflows from the main spillway, is much lower than that. In addition to attempting to minimize further erosion of the main spillway, they set an outflow level that wouldn't flood the towns downstream.
    - stuckagain 10y ago
      It's difficult to describe the hydrological situation of Yuba City and Marysville to people who have never been there. The rivers run through the towns way up above the street level, owing to mining debris washed down from the mountains in the Gold Rush era, and later reinforced by Corps of Engineers works. So if the rivers overflow their banks in Yuba City, the city will be ruinously flooded. It's really freaky to stand in downtown Marysville and look up at the river bank 15 feet above you.
      - heartbreak 10y ago
        On Street View that doesn't appear to be the case at all. It looks like there's a 15-20 foot levee between Marysville/Yuba City and the Feather River.
      - flukus 10y ago
        Do they have release valves where they can release water and potentially cause minor flooding over a wide area instead of potentially catastrophic flooding wherever a "natural" breach occurs?
    - friism 10y ago
      Both the main and the emergency spillway send water into the Feather river. When the reservoir is at capacity, the dam operator has no control over how much water goes into the Feather River nor whether downstream towns are flooded or not. Any water not going down the main spillway will go down the emergency spillway and into the river. (edit for typo)
- AnimalMuppet 10y ago
  They can choose to use the emergency spillway. They just can't choose not to use it. That is, if water rises high enough, it will go out the emergency spillway, and they can't necessarily prevent the water from rising that high.
wyldfire 10y ago
Why does water passing over the dam destroy the dam?
- AnimalMuppet 10y ago
  Because it's an earthen dam, not concrete. Water going over the top will erode the dam, lowering the height, causing more water to go over, which causes more erosion...
- mikeash 10y ago
  It's an earthen dam. If water goes over the top, it will start to erode the dam, resulting in more water going over the top, resulting in catastrophic feedback.
- deleted 10y ago
  [deleted]
- Zardoz84 10y ago
  erosion
- maxerickson 10y ago
  The main dam structure isn't built to withstand the water flow. Wikimedia has a nice picture of the area: https://upload.wikimedia.org/wikipedia/commons/d/d5/OrovilleDam.jpg https://upload.wikimedia.org/wikipedia/commons/d/d5/Oroville... The emergency spillway is on the far left, it is lower than the rest of the dam. So if water is flowing over the main structure, it's anyway already failed.
- nomel 10y ago
  It's [1] an embankment dam, so basically just a pile of dirt/rocks. When the water goes over, the material of the damn goes with it [2]: > Even a small sustained overtopping flow can remove thousands of tons of overburden soil from the mass of the dam within hours. [1] https://en.wikipedia.org/wiki/Oroville_Dam https://en.wikipedia.org/wiki/Oroville_Dam [2] https://en.wikipedia.org/wiki/Embankment_dam#Safety https://en.wikipedia.org/wiki/Embankment_dam#Safety
WildUtah 10y ago
3. The emergency spillway is 21 feet lower than the dam crest and very, very wide. It should be impossible for the main dam to overtop from water inflow. But there was some concern during design about tsunami. It's important not to overtop even in a brief event like that.

crisdux 10y ago

Regarding #2, they can choose to decrease flow out of the main spillway and let it go over the emergency spill way - From my understanding, that is exactly what happened over the weekend. They were trying avoiding continued damage to the main spillway.

maxerickson 10y ago
Quoting from one of the LA Times updates linked from the link: The result is a balancing act — drain as much water as possible as quickly as possible while trying to minimize further damage to the spillway. The main spillway was returned to action, but at reduced capacity compared to what it could normally do under the current conditions. After all, the spillway needs to last for the remainder of the rainy season. The stated capacity of the main spillway is 250,000 cubic feet per second, once it became clear the emergency spillway was not holding up, they increased the main spillway to 100,000 cubic feet per second. So limiting damage to the main spillway is an ongoing concern.
- mintyfresh 10y ago
  The capacity of the intact spillway is ~250,000 cfs - the capacity of the Feather River through Oroville and Yuba City below, currently in the lower flood stages with 100,000 cfs outflows from the main spillway, is much lower than that. In addition to attempting to minimize further erosion of the main spillway, they set an outflow level that wouldn't flood the towns downstream.
  - stuckagain 10y ago
    It's difficult to describe the hydrological situation of Yuba City and Marysville to people who have never been there. The rivers run through the towns way up above the street level, owing to mining debris washed down from the mountains in the Gold Rush era, and later reinforced by Corps of Engineers works. So if the rivers overflow their banks in Yuba City, the city will be ruinously flooded. It's really freaky to stand in downtown Marysville and look up at the river bank 15 feet above you.
    - heartbreak 10y ago
      On Street View that doesn't appear to be the case at all. It looks like there's a 15-20 foot levee between Marysville/Yuba City and the Feather River.
    - flukus 10y ago
      Do they have release valves where they can release water and potentially cause minor flooding over a wide area instead of potentially catastrophic flooding wherever a "natural" breach occurs?
  - friism 10y ago
    Both the main and the emergency spillway send water into the Feather river. When the reservoir is at capacity, the dam operator has no control over how much water goes into the Feather River nor whether downstream towns are flooded or not. Any water not going down the main spillway will go down the emergency spillway and into the river. (edit for typo)
AnimalMuppet 10y ago
They can choose to use the emergency spillway. They just can't choose not to use it. That is, if water rises high enough, it will go out the emergency spillway, and they can't necessarily prevent the water from rising that high.

wyldfire 10y ago

Why does water passing over the dam destroy the dam?

AnimalMuppet 10y ago
Because it's an earthen dam, not concrete. Water going over the top will erode the dam, lowering the height, causing more water to go over, which causes more erosion...
mikeash 10y ago
It's an earthen dam. If water goes over the top, it will start to erode the dam, resulting in more water going over the top, resulting in catastrophic feedback.
deleted 10y ago
[deleted]
Zardoz84 10y ago
erosion
maxerickson 10y ago
The main dam structure isn't built to withstand the water flow. Wikimedia has a nice picture of the area: https://upload.wikimedia.org/wikipedia/commons/d/d5/OrovilleDam.jpg https://upload.wikimedia.org/wikipedia/commons/d/d5/Oroville... The emergency spillway is on the far left, it is lower than the rest of the dam. So if water is flowing over the main structure, it's anyway already failed.
nomel 10y ago
It's [1] an embankment dam, so basically just a pile of dirt/rocks. When the water goes over, the material of the damn goes with it [2]: > Even a small sustained overtopping flow can remove thousands of tons of overburden soil from the mass of the dam within hours. [1] https://en.wikipedia.org/wiki/Oroville_Dam https://en.wikipedia.org/wiki/Oroville_Dam [2] https://en.wikipedia.org/wiki/Embankment_dam#Safety https://en.wikipedia.org/wiki/Embankment_dam#Safety

WildUtah 10y ago

3. The emergency spillway is 21 feet lower than the dam crest and very, very wide. It should be impossible for the main dam to overtop from water inflow. But there was some concern during design about tsunami. It's important not to overtop even in a brief event like that.