The issue is the velocity of the water coming over the top. Current design thinking would be aimed at slowing the water down as it hits the spillway (which would considerably reduce the spray you see in the photos)...Think series of "dragons teeth" or grouted large rip-rap instead of just a smooth paved apron. Drainage practical thinking and design (engineering) has evolved since the early 1960's state of the art, including the designs to control release speed of the water in a 500 year event.
Apparently, this type of catastrophic event was precisely warned about several years ago, and the experts that the State listened to said it could not happen:
http://www.mercurynews.com/2017/02/13/oroville-dam-update-evacuations-remain-in-place/
DSchmitt MidlandMike While there is a veneer of soil under the emergency spillway, its evident the core of the hill is rock, which would not erode at any alarming rate. Even at the Grand Canyon, the average downcutting rate was less than 1 foot per 1000 years. The unlined auxillary spillway was eroding at its top. The veneer of soil is apparently 30' deep at that location.
MidlandMike While there is a veneer of soil under the emergency spillway, its evident the core of the hill is rock, which would not erode at any alarming rate. Even at the Grand Canyon, the average downcutting rate was less than 1 foot per 1000 years.
The unlined auxillary spillway was eroding at its top. The veneer of soil is apparently 30' deep at that location.
There have been several questions posed about the quality of rock underlying the spillways, though more from resistance to erosion than compressive strength (which seems to be more than adequate). The channel for the main spillway had to cut through tens of feet of the bedrock, and the contractor had little problem doing that by using D-9's with large rippers (no blasting). Also keep in mind that the Feather River route was known for rockfalls.
As you said, the main spillway seems to be holding, though the Cal DWR folks are going to be watching it like a hawk.
One thing that caught my attention was realizing that the base of Oroville Dam is only 152' above msl and it is a good distance from the Sacramento River delta. Failure of the weir for the emergency spillway would have flooded a significant portion of the eastern Sacramento Valley.
I suspect the state will be drawing up a bunch of new regulations about monitoring and maintaining dams as was done after the close call with the Van Norman reservoir in the aftermath of the Feb 1971 Sylmar earthquake.
Wonder if a new rail spur will be installed to bring the huge quantities of repair materials to the site ?
Since the Oroville Dam was built 50- 60 years ago (!), design features, construction methods, on-site equipment, logistics, economics, and priorities (i.e., environment) have changed immensely. Large rip-rap rocks (>25 tons) would take unusual efforts and permits to move by highway - from where and how far away ? Thousands of cubic yards - and about twice as many tons - of other embankment fill (smaller rock, most likely) and then the materials for concrete* will all have to be moved in, plus likely some steel for new/ replacement control structures and reinforcing bars, etc.
*Cement, coarse aggregate (smallish rock), and fine aggregate (sand) - water they already have plenty of !
[Hindsight is 20-20 of course, but each major dam failure seems to have its own specific cause. This one may have been inadequate spillway design, plus recent mismanagement of the reservoir pool/ flood control storage capacity/ height/depth (too complex to explain here); erikem mentioned the seismic damage from the earthquake as it affected the Van Norman Dam; the Teton Dam failure was caused by crummy geology and inadequate preventative measures (see https://en.wikipedia.org/wiki/Teton_Dam#Cause), as was the St. Francis Dam near L.A. (see https://www.kcet.org/history-society/the-flood-st-francis-dam-disaster-william-mulholland-and-the-casualties-of-la ),etc.]
By the way, the original Johnstown Flood of 1889 was caused by erosion of the earthern dam (owned by the PRR for a while, then sold to the South Fork Club) after the spillway became clogged and the dam was overtopped . . . https://en.wikipedia.org/wiki/Johnstown_Flood
- Paul North.
erikem I suspect the state will be drawing up a bunch of new regulations about monitoring and maintaining dams as was done after the close call with the Van Norman reservoir in the aftermath of the Feb 1971 Sylmar earthquake.
For maybe 20 - 30 years now there have been nationwide regulations requiring studying and improving the stability of dams against overtopping and seismic events. That's led to some major retrofits, including even the Conowingo Dam across the Susquehanna River here on the East Coast. There, many steel rod and grouted 'tie-backs' (more accurately, 'tie-downs') were drilled from the top of the dam down through it and anchored into the bedrock below.
The wikipedia article on the dam has been edited to give a pretty good accounting of the current events.
https://en.wikipedia.org/wiki/Oroville_Dam
I tried to sell my two cents worth, but no one would give me a plug nickel for it.
I don't have a leg to stand on.
The Dakota and Iowa Railroad could probably ship out a unit train of big, pink rocks on short order from their quarry in Dell Rapids, SD. UP could pick it up in Sioux City, IA and send it on down the line. I wonder how you haul a 25 ton rock in a rail car. Boxcar with wide doors?
Thanks to Chris / CopCarSS for my avatar.
25 ton rock is about 12.5 cu. yds. To give you an idea, a cube about 7 ft. on each side would have a volume of about 12.5 cu. yds. So a 25 ton rock would fit comfortably in the width of a gondola car. But 5 of them would overload most cars: 5 x 25 tons = 125 tons = 250,000 lbs. + 60,000 lbs. tare = 310,000 lbs., so need the 4-axle car with the heaviest capacity, 315,000 lbs. ("315K" car), so 4 would be a maximum load. Even with 5, they would be only 5 x 7 ft. = 35 ft. long, which is way less than the usual length.
A boxcar with normal width doors would probably work - if you could find a forklift to lift one rock without going through the floor of the boxcar. I shudder to think what could be put into a high-cube boxcar, or on a TTX flat, or a set of double-stack well cars . . .
Perhaps I missed it but: has there been any discussion or analysis regarding silt accumulation which would allow water accumulation to a higher level than designed for?
At some locations the Corps of Engineers are dredging silt accumulation from reserviors because the designed capacity can no longer be attained.
PDN: dumped 33 and 40 yd air dumps with Volkswagen sized big rip rap - Do those short cars ever gallop! (Chained the carbody on the opposite side to the rail to keep the car from derailing or jumping off the truck centerpins. And yes, managed to derail a few anyhow)
DC: It's been odd how quiet/ invisible the USACE (Army Corps) has been so far in the national press. (Blissfully ignorant newsworkers?)The Sacramento District has to be scrambling right now. https://www.facebook.com/sacramentodistrict For all the noise being made, these guys call many of the shots here, especially with dam safety.
MC - looks like you and NDG could share some stories (see his post of Mon. Feb. 13, about 1/3 of the way down the 1st page of this thread).
By the way, if my theoretical 25 ton/ 12.5 cu. yds. rock were a sphere, it would be about 8.7 ft. in diameter. It'd still fit inside a typical gon body with a foot or so to spare. That VW 'beetle' might be about the same volume, too - a little longer, yes - but for sure neither 8'-8" high nor wide.
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