NOTE:
Image is not from today ... just trying to put the parking lot rock dump in perspective.
All these aerial shots are showing the truth, that, as Mr. Cahill pointed out is in short supply. There’s more holes, more potential problem areas than I think they’re saying.
Earlier I heard that temps will be in 60’s and 70’s..snow melting will start adding to the mess.
It really depends on how far the bedrock is beneath the emergency spillway. The Department of Water Resources contends it is about 30 feet underneath, but we don’t know whether that is the high point or the low point along the 1700 foot length of the concrete weir on the spillway, not to mention the width of the asphalt parking lot to the west (left) of the weir.
Once any part of the weir, or the parking lot, is undercut as water flows over the emergency spillway in the week-long storm starting on Wednesday, the whole thing will go due to soil erosion. That is likely.
Then the reservoir will dewater through some (more likely eventually all) of the extent from the east edge of the weir through the parking lot on the west. How much comes out depends on the lowest point of the bedrock underneath the area.
DWR says the dewatering will go down to about 30 feet below the upper edge of the emergency spillway (909 feet by my recollection). How much that is depends on where the reservoir level is when the breach occurs. My wild-assed guess is that this is a minimum of 300,000 acre feet of water.
That’s enough of a pulse to flood Oroville. The Butte County Sheriff is absolutely right to insist that all the evacuees stay out. IMO it won’t be safe for them to return until the big snowmelt season is over, unless the reservoir breaches earlier, like next weekend.
But the pulse from breach of the emergency spillway might be a million acre feet. The Oroville Dam has a maximum capacity of 3.5 million acre feet at the 909 foot level. At that point the ability of downstream dams to handle such a pulse comes up. The closest big downstream dam has (again my recollection) a capacity of a million acre feet. If it is as empty as possible when the Oroville Dam emergency spillway breaches (this dam collects runoff from all the streams between it and Oroville), it might hold.
Plus the pulse won’t hit the downstream dam all at once – it will come in over a period of at least several hours. Hopefully it will take the Oroville emergency spillway at least half a day to unzip horizontally when it breaches, and the breach pulse will extend over a day or more. That would give the next big dam downstream another day of releases to handle the pulse.
The big question, as DWR acting chief Bill Croyle has noted from the beginning, is the erosive effect on downstream levees from massive releases, intentional or not, of water from the Oroville Reservoir. Those could be overtopped by a major flood. But they could also be eroded out by massive fluctuations in water flow and pressure just from rapidly rising and dropping river water levels as big pulses of water go by that aren’t big enough to overtop them.
This is a special problem in the Sacramento area as so many previously reserved, empty floodplains, have been filled with residential subdivisions. 100,000 people immediately downstream of the Oroville Dam have been evacuated. Many times that number live in the Sacramento area floodplains now.
Not only will they be flooded out for a month or more (the floodplains drain slowly), and some killed, but they won’t be able to rebuild in those areas until the Oroville Dam and its related structures are rebuilt. Because the latter are required for flood control against normal winter storms.
Marcie McKenna
@McKennaSG1
3h
#orovillespillway Watch this live stream youtube.com/watch?v=JxbDJl… The emergency spillway is Overflowing again. Erosion in the wall continues
This tweet was 3 hours ago.