There appear to be 3 overarching design concepts which have been proposed for rebuilding the main spillway:
1) Rebuild/replace the existing alignment back up to grade the way it was, 2) Strengthen/reinforce the (broken) hanging lip of the current spillway to better channelize water into a drop pool, or 3) build a new main spillway and control structure over the location of the current emergency spillway.
The first scenario seems by far the most likely likely. This would require massive volumes of backfill over the deep drop pool areas, but a million cubic yards or so of material has already been stockpiled locally. A significant concern would be the long-term consolidation of such very deep backfill lifts, in that this could eventually cause a dip in elevation of the spillway across these areas, yielding more potential ”issues” in the future.
The second “hanging lip” scenario would appear to be the fastest and cheapest, which could become a significant political issue this summer for a project which might not be used again for 10 or 20 years, if ever. Although it is difficult to envision anchoring a structure on the existing spillway lip sufficiently strong to “flip” the flow into the air, it does seem plausible to better stabilize the lip so that it wouldn’t back cut upstream across a wider range of flows. A larger or differently shaped drop pool could be blasted out of the bedrock to contain the flow and channelize it through a series of natural cyclopean drops down the (now extra wide) bedrock channel. It should be be faster/cheaper to blast out more bedrock than to place and consolidate hundreds of thousands of cubic yards of backfill and concrete. This option probably wouldn’t work for all flows, and would be very difficult to analytically model, but might be able to be empirically scale modeled using Lidar of the channel bedrock contours to generate 3d printer molds. It seems worth considering.
The third option, building a new gated spillway over the existing emergency spillway, would be the most technically, politically, and fiscally expensive. DWR’s first visible response to this disaster was helicoptering bags of rock out to the emergency spillway, which they followed up with another hundred million dollars worth of truck-hauled rock and concrete. It is hard to envision them saying: “oops, sorry, we didn’t need that”, and ripping it all out to replace it with something new. This option only seems plausible if the existing spillway outlet structure is or becomes broken beyond repair.
Are there other main spillway design options beyond these three?
One additional consideration: the “probable maximum flood” design criteria for both spillways is more than 600,000 CFS. With 600 feet of head, this would require both spillways to dissipate around 30 GW of power, approximately equal to the instantaneous power consumption of the entire state of California. It seems unrealistic to expect this amount of power to be dissipated in such a small area without some significant collateral damage.
As one who is not paying for it (maybe), I would vote for the new spillway in the place of the emergency spillway, followed by a complete rebuild of the old spillway in the fashion of an emergency spillway. A well-made emergency spillway.
Hi jpal, Query: Where have these 3 design concepts been proposed? Are these early designs that have been released by DWR engineers? Or are they from discussions?
Your insight in the description of the situation is accurate to the engineering challenges they face. I would add that there are additional dimensions to the choices & difficult tradeoffs that are faced. If you were to create a cross-matrix array of the interdependencies of design, funding, politics, time crunch, and the "hand of God via nature - weather", you would end up with a dynamic project that must be adaptable.
On one hand, they must protect the public from catastrophic event(s) - thus the need to emergency repair & armor the Emergency Spillway as there is an unknown to the integrity of the upper main spillway's health.
On the other hand, they must plan for the ultimate restoration to adequate safety factor ratings to the Maximum Probable Flood levels. How they get there from now to the ultimate restoration almost assuredly requires flexibility in interim options.
I wouldn't be at all surprised if an option develops where a "flip" is used to a secondary lower spillway where the secondary lower spillway is built to dissipate the "capture" of the lift and transport it to another form of a dissipation block system @ the Feather river.
In any case, keep the wheels turning... we'll see what is going to be proposed soon. Let's hope the hand of God in Nature will be amenable (for the sake of many below the dam).
and Keep posting...
Don’t overlook option 4 - decommission the entire system.
“....both spillways to dissipate around 30 GW of power,”
Mind boggling.
It seems unrealistic to expect this amount of power to be dissipated in such a small area without some significant collateral damage.
I would take your final statement a step futher: I am CERTAIN most people vastly underestimate just how hard it is to dissipate that much energy.
As an example, and just for starters, you run a high risk of creating your own weather from the enormous amount of spray you are certain to generate. With that much energy to dissipate, it seems highly likely the spray will be energetic enough to generate lightning.
I'll even go so far as to guess this question was never even asked when the dam was designed. I'm not a civil engineer, but it's easy to see how they may not think of it now either.
REAL WORLD EXAMPLE: Way back in ancient history (only slightly before my time), a volcano erupted in Iceland. The lava flow threatened a developed area. Some genius decided the lava could be stopped by dropping water on it.
The Air Force loaded up some aircraft with water and they took off for Iceland.
Then someone spent a few minutes calculating how much energy would be released if all that water vaporized (it would have). The energy release would have been equivalent to a nuclear bomb. Not released in a single split second, but fast enough to be devastating none the less.
The planes were called back while in flight. I learned this from one of my engineering profs while I was a cadet at the Air Force Academy. I have no idea if it ever made it into the news, but it certainly made a permanent impression on me.