Report: California's Oroville dam’s main spillway must be repaired by November

Hotair ^ | 03/23/2017 | John Sexton

[SeekAndFind]

The Oroville dam has largely faded out of the news in the past few weeks but the crisis is not over yet. A report on the safety of the dam concluded: “A very significant risk would be incurred if the Gated Spillway is not operational by November 1.” November 1 is the start of the next rainy season. From the Associated Press:

Officials with the state Department of Water Resources, which operates the dam, fear a huge rupture that opened in the main spillway could expand to cripple the flood gates that send out controlled releases of water and keep water from spilling over uncontrollably.

In a statement, spokeswoman Maggie Macias said the agency’s objective is to have a fully functional spillway before the start of the next storm season.

“We’ll be working round-the-clock through spring, summer and fall to make that happen,” she said.

In case you’ve forgotten what the main spillway looks like, here it is:

Note the size of the helicopter in this image for scale:

So California has about 7 months to shore up all of this damage and repair the spillway in time for the next rainy season. And there is concern that repairing the visible damage will not be enough. Inspectors found water was leaking between the apparently undamaged sections of concrete, meaning even the sections of the spillway that look salvageable may need to be replaced or repaired before it can be used safely. The cost to repair all of this damage is expected to be in the tens of millions of dollars.

The report also says the emergency spillway should not be used under any circumstance and needs to be redesigned. If you recall, authorities briefly allowed water to flow over the emergency spillway last month and then saw evidence the ground below it was in danger of undermining the concrete. Though the Oroville dam itself was never in danger, 188,000 people were evacuated over concern the spillway could fail catastrophically, sending a wall of water into downstream cities and towns.

[EarthResearcher333]

For those following the Oroville spillway situation, there are new posts on the big thread that may be of interest. (links below)

DWR's 1960's Split spillway "engineering political solution" coming back to haunt the future? (new Failure mode of the entire dam)

Unexpected waterflow observed in places in upper spillway - Why?

Expert Board of Consultants swizzled flow number specifications for the New Main Spillway and the New Emergency Spillway. Why?

A "flip bucket" temporary fix proposal to "jump" spillway flow over the damaged section "plunge pool"

[Grampa Dave]

Thanks for the excellent way for us non engineers, non rock people as Seldon calls geologists and non physic majors to decide what to look at and keep up to date on this boon doggle.

[blueplum]

March 27 - they are see-sawing between full spillway / closed power plant, and full on power plant / closed spillway to try to maintain levels and avoid scouring

https://www.youtube.com/watch?v=0DJpwr57pAY

[EarthResearcher333]

Thanks for the link.

[EarthResearcher333]

New information on an unusual situation: A well known long term "leakage" or "wet area" on the backside of the big main dam. Dam Inspectors have noted this in 2014 and 2015 reports and have recommended "investigations" and "actions" to be taken. So far, no evidence this has been investigated. See more at post links to the big thread below.

2014/2015 Dam Inspection Reports: "Long Standing..Wet Area" - midslope dam backside seepage (being ignored?)

More Info: Mid-slope Main Dam Erosion channels from unaddressed "Weepage" inside of dam

[Grampa Dave]

Thanks for the update.

So how long and how much bad data was swept under the cracking concrete?

[EarthResearcher333]

"So how long and how much bad data was swept under the cracking concrete?"

IF you are asking about post 65, the leakage on the backside of the earthen dam would be inferred from penetration of the compacted clay wall in the middle of the dam core. If a seam or crack formed in the clay water barrier, then this could allow "piping" of water from the pressure of the higher reservoir water through this seam or crack and then out the backside of the dam embankment.

It is normal to have some "weepage" in dams. However, if the source is not accurately investigated, this creates an unknown regarding a risk factor. What is surprising is that this seems that it was not investigated.

The wide area affected could represent a volume of water that is either weeping in a thin strata line, or could be a focused piping leak that widens into a strata line on the backside embankment. The latter issue is more dangerous as a focused piping leak could cause hydraulic erosion much quicker within the clay core - triggering an escalating situation.

So far, I've been able to get sat pic evidence of this condition back to 2013.

[EarthResearcher333]

More info: How should this be addressed..

= = copy of post in big thread:

"What’s the fix.."

The first step must be to investigate it. Weepage does occur at dams**, but what is unusual about this situation is that it is along a "seam area". If there is water leakage along a thin crack like defect (in the compacted clay water wall barrier), then the hydraulic erosion effects are spread. If the leakage source is a focused area in the compacted clay, then the hydraulic erosive potential notably escalates the risk factor.

The immediate prudent action to the existence of an unknown "risk factor" - in a proven long term existence of a "weepage" area with downhill erosion marks - is to lower the reservoir level below the elevation of the leak. This means, that to eliminate an unknown risk, the water level would be lowered to 660ft or below.

Then investigative procedures would be enacted to identify the saturation profile within the dam (probes). With this data, remedial repairs would be performed accordingly in the breaching area(s). (some dam remedial repair on notable leaks required excavation and replacement of the core layer defect using large volumes of mixed clay including Bentonite).