With the evacuation order lifted last evening, and ~185,000 residents returning to their homes in Oroville and surrounding communities, emergency repairs continue around the clock on the damaged emergency spillway at Oroville Dam. Yesterday morning (Feb. 14), a concrete pumper and boulders were being used to patch the main erosion features that developed when the emergency spillway was overtopped:

The water level of the lake is being reduced by 8 or 9 feet per day, and now stands at 880 ft., which is 21 feet below the top of the 901 ft.-elevation concrete weir that was overtopped a few days ago by 1.5 ft. The rapid lowering of the lake is being accomplished with a continuous 100,000 CFS flow over the main spillway (the same as the average flow over Niagara Falls) which was heavily damaged over a week ago:

Little more than one day of only about 10,000 CFS flow over the concrete weir of the emergency spillway caused unexpected erosion down into the highly fractured bedrock, which is what led to the evacuation orders as the continuing erosion back to the concrete weir led to fears that structure could be compromised:

I’ve been following a continuing discussion by outside experts here, and also following the forecasts of a series of new storms that will sweep through California over the next week.

Based upon everything I’ve seen, I’d say that for the foreseeable future, the dam facility is out of danger. Here’s why:

1) The 100,000 CFS flow through the (damaged) main spillway will, in all likelihood, keep the lake level below the 901 ft. elevation of the emergency spillway. Even though the next week of storms will cause as much as 7-14 inches of liquid-equivalent precipitation to fall, much of that will fall as snow except over a relatively small portion of the watershed. The 100,000 CFS flow rate through the main spillway could be increased to 150,000 CFS if needed.

2) As of yesterday, the main spillway damage was not eroding uphill, suggesting that portion is on bedrock. It is possible that the remaining concrete is actually more stable at higher flow rates, which creates a waterfall out and away from the bedrock the concrete flume is sitting on.

3) Spring snow melt usually occurs slowly enough that the main spillway will be able to keep up with it.

4) Even if the concrete weir is over-topped again, and erosion of the bedrock extends back to the concrete weir, that structure is very thick, and it is not at all obvious that it would fail. This is a big uncertainty, though, and if it ever gets to that point, evacuations would again be announced.

Note that all of this positive outcome depends upon the undamaged uphill portion of the main spillway remaining stable. This is why repairs to the hillside below the emergency spillway are being performed as fast as possible, in case the main spillway fails and the lake level rises uncontrollably, and then once again overtops the emergency spillway weir.

I believe that the use of the previously-untested emergency spillway, and the resulting damage, was actually a blessing in disguise. That structure was designed to handle 10-15 feet of water flowing over it, but it only took 1.6 ft of flow to cause substantial hillside erosion in less than 2 days. Now that the geology of that hillside has been revealed, engineers will be able to come up with an engineering solution so that if there ever is a catastrophic flooding event, the emergency spillway will be in a much better state to handle it without failure of the concrete weir, which could result in a ~30 ft. wall of water to cascade downstream.