I'm not a civil engineer but here is my take on this: The emergency spillway will fail, and the DWR knew/knows it. Current Sierra Nevada snowpack is
173% of average and many of the northern California reservoirs are at almost 100% capacity due to heavy rainfalls. Based on some numbers I have seen and ran, the expected 4" of rainfall from this weeks storms alone presents a grave risk to the dam complex (although not the dam itself) with about a 4
foot surplus over what the weir can handle. Inflows have been well under 50 Kcfs for the past 48 hours, there exists peak inflows on record of well over 250 Kcfs. It looks like CA DWR has either not been posting the inflow rates or keeping them FOUO after the evacuation order. One thing to keep in mind is the downstream flow rates. If the dam can outflow 250 Kcfs but downstream can only handle 100 Kcfs, you have a large problem. There is also significant debris backing up the turbines so they can not be relied upon to provide power.
Dam structure - Low risk. Top of dam is 20ft above the emergency spillway. Although looking at this topo map, there looks to be a depression going downhill away from the #841. If the main spillway were to erode that high up, water could conceivably head downhill right on the dam itself.
Main spillway - Substantial risk of further erosion creeping up the hill towards the release gates. Based on what I have seen the main spillway can not handle flow rates above 65 Kcfs without furthering the erosion already present which presents a problem because they need to run outflow to its full 100 Kcfs to make any appreciable dent in water levels. In this picture you can see the bluish bedrock and the brown dirt. Anywhere there is danger of water flow should be straight bedrock or else you end up with our current erosion condition. It is much worse now than in the picture.
Emergency spillway - High danger of failing by the time full reservoir inflows arrive with full rain/snowmelt. We will very likely see a compromise of the weir and/or surrounding area leading to a total collapse and erosion of the emergency spillway and surrounding topography. Looking at these pictures I can see why at first the DWR thought the situation was stable but after they began to see the erosion channels creep toward the weir they freaked out and recommended the evacuation. Without looking at the engineering drawings there is no way to tell how deep the bedrock is underneath all the glacial shit rock. From looking at the second picture it is more than 30-40 feet. There exists a significant danger that the main spillway will not be able to be used to its full outflow rate which will cause water to flow over the emergency spillway which will then rapidly cause further erosion up to and even underneath the weir. It's only a matter of time before the whole thing collapses and with it 30
feet of reservoir. If it goes like that then the main spillway gate structure could be damaged as well causing a compounding lack of flow control.
Before
After
Thats what 30 feet and no bedrock? This is from the top of the frame between the trucks and where the road is washed away.
Worst case scenario: Further erosion causes weir to fail
and damages the main spillway gates causing uncontrolled outflow which erodes the depression by hill #841 and causes the dam to go.