Some info from the LA Times:
http://www.latimes.com/visuals/photography/la-me-lake-oroville-spillway-pictures-photogallery.html
What I can’t understand - How can any engineer THINK a unfortified earthen spillway can withstand the pressure of rushing water? Didn’t they ever look at the Grand Canyon?
Water is the most universaly destructive force that nature presents humanity and anyone that doesn’t believe it will pay the price.
The really scarey part is that this is the ‘tallest’ Dam in this country, an earthfilled body[frortified ?], something like 3,000 feet side to side.
Somehow, they engineered this dam, expecting that an overflow was unkilely (?), while providing it with asingle ‘concreted’ spillway, and an open. earthen, emergency spillway in a parallel position.
As BaltACD suggested:“…How can any engineer THINK a unfortified earthen spillway can withstand the pressure of rushing water? Didn’t they ever look at the Grand Canyon?..”
Makes one wonder what con$traint$ were placed on the Engineering Staff, during the Dam’s engineering phase; seems as if, maybe, one too many corners were cut?
There certainly is a major concern about back erosion leading to failure of the spillway gates.
The erosion for the auxillary spillway is much more serious that for the main spillway. The spillway is about 30 feet high and there is a lot of water in the lake between 871’ msl and 901’ msl.
It’s not clear how much danger is posed by the erosion on the aux spillway, but the pictures I’ve seen make me very glad I’m not downstream. The sheriff made the correct call in ordering the evacuation.
It has active gates. Auxillary spillway limits lake elevation to slightly over 901’ msl. Real question is how much lower can the lake be emptied by the main spillway.
The ‘fun’ began last week with the damage to the concrete on the main spillway channel. The flow through the main spillway was reduced in hopes of lessening damage and consequent erosion - reduced outflow was less than inflow, so lake level rose. Emergency spillway was topped about 8AM PST o
The terminology used has caused confusion… ‘They allowed water to pass over the auxiliary spillway’ (or something like that…) the key word being “allowed”. I don’t think they “allowed” that to occur, but rather it was a consequence of closing down the main spillway… it might have been expected, but there is no way they could have disallowed it.
As soon as the news media catches up with this story, we are going to see a flood (maybe even a torrent) of news coverage about the background history leading up to this dam disaster. This is not what is commonly called an "act of God." There is also going to be a forest of finger pointing.
1:48 pm Evacuation order changed to evacuation warning. Residents may return to homes but should be ready to leave again on short notice if the situation changes.
My home is in Marysville. The City was ordered evacuated although because of its levees (the tallest and strongest in the area) Marysville is shown surrounded by water but not subject to flooding from a spillway collaspe (unless the levees failed).
My family did not leave, but monitored the situation and was ready to go which could be done quickly since with most people gone there would be very little traffic on the roads. My home is less than 1/8 mile from EB Highway 20 which is on levee to beyond the flood danger area.
All businesses in Marysville closed. Some businesses accross the Feather River in Yuba City were open during the emergency. Apparently YC was not included in mandatory evacuation, which is strange. All of Yuba City is shown, on the map, as an area that could be flooded.
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.
The flow to the main spillway is controlled. Its concrete lining failed about half way down. When they closed it to invesigate the failure of the concrete lining, the water level of the lake rose and the flow started to wash out the auxillary spillway. They then reopended the main spillway, which lowered the lake level. The flow blew out the side at the damaged location, but because it is well below the top of the dam and the soil shallow over the bedrock, there is very little errosion.
It now appears, but is not certain, that repairs to the main spillway can wait untill a time when the spillway can be closed without raising the level of the lake.
Hindsight is 20 20.
It looks like the main spillway will hold up. It is, however, not certain that the lake level can be lowered enough ta accomodate water from storms and runoff coming in the next few days and weeks.
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/
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 . . .
Wunnerful - the political idiots that gave us PTC now wreak havoc on public watter projects. (then again, CA and it’s mismanaged mess is already sliding backwards)
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.
- Paul North.
The wikipedia article on the dam has been edited to give a pretty good accounting of the current events.
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? [:o)]
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 . . .
- Paul North.
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.