Basically, if the lake keeps filling up but the dam holds, then everything around it eventually becomes an emergency spillway in one form or another.
Dang, my youngest sister lives in Yuba City!
In the immortal words of Inspector Clouseau: “Not anymore!!”
Just kidding. Seriously I hope she and her family and neighborhood come out fine. Which they probably will.
Which, if you were paying attention to the reports, is exactly what the officials monitoring the situation thought was happening, thus resulting in the evacuation notice. However, that backwards erosion apparently slowed, before stopping (of course) when the water levels had lowered to the point that the water was no longer going over the emergency spillway.
The danger of the erosion of the hillside has long been a concern to many groups. Repeated requests to fix the problem before it was needed have been routinely turned down. Most recently in 2016.
The erosion below the emergency spillway is in no way normal nor expected. It is a considerable hazard.
What the gullies do further down the hill doesn’t matter. It’s the edges of the gullies moving up towards the lip that’s the danger. The lip is concrete. The material below it isn’t. It’s that material that will start eroding the next time the emergency spillway needs to be used.
the evacuations were lifted, which must be a relief to those affected
They’re still recommending that people stay away, but they aren’t enforcing it.
There are lessons here. Google the 1861-62 California flood. Such an event, which will eventually happen, could wash out all the dams in Northern California, flooding all the cities downstream and cutting off the water to Southern California. Also, there was a close call at the Glen Canyon dam on the Colorado River back in May/June of 1983. 100 degree temps were melting snow, filling Lake Powell to overflowing. There the spillways are concrete tunnels which plunge downward at a 50 degree angle and then suddenly become horizontal. The enormous amount of water tore up the concrete and started eating the soft sandstone. If that flow went on long enough, it could have eroded the stone which anchors the dam. It was a close call.
Sort of a race to the bottom, no?
No thrill-seeking kayakers sneaking in to try that run? Surprising.
Hoover Dam outside Las Vegas has similar diversion tunnels alongside the dam that serve as emergency spillways. The one on the East / AZ side is located near a tourist overlook spot. So you can see into this awesome skateboard park half-pipe like thing. But 100-plus feet across. Then the edge falls away and you can sight down into what looks like a 70+ degree descending that’s tube 50-plus feet across diving from the brilliant desert light of day into the Stygian gloom. There’s no sign of a bottom; the edges just disappear into the darkness.
It was simultaneously beautiful and downright terrifying to look into. I always imagined some skateboarders playing on the upper shallow slopes then slowly getting closer until suddenly the slope was too much and down they’d go. AAIIIIIIiiiiyyyyyyyyyyy!!!
Not when there is lots of undetermined debris in water, like uprooted trees (called 'strainers in the vernacular) and other such dangers. That is why they were cutting trees down in flooding areas - to keep them from blocking flow under bridges and such. Plus, at bottom the Aux spillway are big pillars of concrete to disrupt the straight flow of water into river (hence the huge amount of ‘spray’ at base into air on most of pics during the emergemency release).
Being a former whitewater’r myself (Snake, Salmon, different forks of Boise rivers. etc), it’d seem prime, but I know the hazards of flooding and what to expect/respect. You just don’t do it for safety’s sake…unless you wanna win a Darwin award :rolleyes:
I have the great, good fortune to have been in Las Vegas in July of 1983, and visited the dam at that time. It is the only year in the history of the dam that they actually have had to use the spillways. There was roughly 4’ of water going over on each side. I remember looking down at the AZ spillway and watching the fish (trout, probably) swimming facing the current right at the edge of the spillway, probably filter feeding. I kept waiting for one to go over the edge, but of course that never happened. When they were done, they simply flicked a tail and swam away quite easily against the current.
The sight of all that water going over the spillway was quite impressive. Looking down to the bottom of the gorge where the spillway was discharging the water back into the Colorado River was even more impressive! :eek:
Interesting L.A. Times article: Government severely misjudged strength of Oroville emergency spillway, sparking a crisis. The pictures there make a strong case that the primary spillway will have to be comprehensively rebuilt.
As this requires taking the main spillway out of service for quite some time, it can’t (safely) be done until an emergency spillway worthy of the name is completed. Since this will have to serve in the role of primary spillway while the real primary spillway is rebuilt, it will probably be a major piece of construction.
And nothing permanent can be done until the current rain / snowmelt season is past. It will be a real challenge to complete these projects before the next one arrives.
I think this is where they filmed the scene in Cherry 2000 where the car is lowered down that chute on a cable. Very memorable scene.
Dennis