ROCKMAN wrote:H-man - You seem to be very familiar withe subject so I have some questions. Might sound like criticism but not...just curiosity.
A) Given the arid nature of the rergion did they delay an earlier release in order to save resources?
B) Not that I've read more about the deterioration/lack of maintence wss the evacuation called much later then it should have been? Evacuations themselves can be deadly but was the late call PR driven?
C) Some comments seem to indicate it would have been a prudent safety policy to have been holding a much lower reservoir long before the rains hit.
The situation reminds on the huge mudslide a few years ago that took out a whole community. Afterwards a geologist showed maps of numerous massive mudslides that happened in the same valley for thousands of years. IOW even though it was inevitable the homes were still built.
RM - A) The the "rainy" season is from October to April. Except when it it November to February. Except when it is just two days in December. So figuring out how to both provide adequate flood control space through March and have a full reservoir in June is as much art and guesswork as science. With 5 years of drought, I'm thinking many reservoir managers have had some "upstairs" pressure to hang onto more water sooner than perhaps their own personal best judgment dictated.
In California, we have at least 4 types of people who have a say about how our reservoirs and dams are managed. The flood control people want a reservoir that is always half-empty. The recreation people want a full lake year-round. Irrigation people want a full lake in the spring and an empty lake in the fall. And the fish/wildlife people want the dam completely removed. Let's not even get into what the hydroelectric power people want.
Now you know why Mark Twain said in California, whiskey is for drinkin' and water is for fightin'
B) People in Oroville have ALWAYS been scared about the reservoir. Picture living in a town immediately downstream of the nation's highest dam, and it is an EARTH dam. There were quite a few, when the concrete spillway started to break apart and the DWR was talking about letting the reservoir rise to spill over the "emergency spillway" directly onto the dirt, who decided on their own to get outta Dodge. The local government was taking their cues for evacuation from the DWR (state Department of Water Resources). It wasn't until AFTER the emergency spillway started to also fall apart that the DWR alerted local agencies and the evacuation order was quickly placed. The evacuation wasn't just for the town of Oroville (about 40,000 people). The Feather River that would get flooded included several moderate cities further downstream, lots of town, a huge chunk of rice farms...hundreds of thousands in total. If that spillway actually did fail, we're talking Katrina-level rapid flooding. A 30'x1000' wall of water. The east coast/midwest, with regular flooding and hurricane experience, is perhaps more trained for evacuation orders like this.
C) This is the first that I've really been familiar with the technical aspects of Oroville's dam. I'm much more familiar with Shasta Dam, which is federally-managed by the Bureau of Reclamation. At Shasta, once the level gets to within 30', the BOR starts bleeding off water to hold that level as best they can without flooding downtown Redding, Anderson, and Red Bluff downstream. They did a bleed like that 4 weeks ago and again started to ramp up that bleeding last week as soon as the last of the bad storms past. Still, they came within 5 feet of being "full". They are blowing 70,000 cfs now to get the lake back down to 30' of freeboard. Looking at the available Oroville inflow and outflow data, it looks like Oroville did not blow off as much, nor as soon as Shasta from several weeks ago, nor blew off as much before this new set of storms set in.
There is a fundamental engineering difference between Shasta and Oroville and the tools they have for water level control. Oroville has two primary tools - their hydro plant (18,000 CFS capacity) and their primary spillway. But the primary spillway can only take off the top 30 feet or so. They do not have any intermediate vents through the dam.
Besides their own hydo plant, Shasta has about 15 different vents through the dam at different elevations. The water level doesn't have to get near the top to blow off water. They can blow off water much earlier, and with all those individually-controlled vent valves, they have redundancy if something fails. As their final level control function, they have several drum-gates at the very top of the spillway, which they can, if everything else doesn't work, fully lower and spill up to 250,000 CFS of water from the top of the lake down the dam spillway. The dam's face itself is the spillway, consisting of 50 feet (at the top) to 500 feet (at the bottom) of reinforced concrete. Solid rock and massive concrete energy dissipaters at the bottom. The dam is considered the safest place to be in case of nuclear attack.
This incident appears to have involved a collision of maintenance failures, a heavy winter (but not the worst ever), some questionable reservoir level management, and perhaps some under-appreciated design flaws. I'm just thankful no one had to die to discover this. It may cost the state a few hundred $million to fix and cost farmers a few million acre-feet of water and the local residents a whole lot of hassle.
We have several other tall earth dams around the north State. I'm betting they will all get a very deep inspection + some re-engineering of their level control systems, along with a fundamental re-evaluation on the operational logistics to balance storage and flood control priorities.