We're all fast becoming experts in dam spillways over here in Northern California, as Lake Oroville fills to capacity and water pours out over the "emergency" or auxiliary spillway. This is not the be confused with the dam itself (i.e. the power plant and the main route for the water to flow) or the spillway, which is basically another set of gates that don't generate power but provide relief if there's too much water in the lake.
Part of the problem is that last week, during a break in these historic rains and snows that we've been getting, engineers discovered a hole in the spillway that has now grown into a 250ft chasm in the concrete chute. In an effort to mitigate further damage to the spillway, they reduced the releases from the lake, but due to warmer temperatures and even more rain, the lake filled up too much again and the emergency spillway had to be called into the action.
But the emergency spillway is nothing more than a glorified hillside with a slightly lower reservoir wall above it. Here it is in action earlier today.
This part of the structure has never been used in the dam's 48-year history so nobody really knew what was going to happen. While the water was evacuated as designed, the hillside underneath the emergency spillway eroded away much quicker than anyone had anticipated and had started to weaken the stability of the spillway wall itself. If the emergency spillway wall fails, the lake would drain about 30 feet essentially instantly into the towns below. The dam itself (770 feet, tallest in the USA) isn't threatened, as that's a separate structure. Still, 30 feet of water off the top of Lake Oroville is a LOT of water.
On Sunday afternoon, officials ordered the mandatory evacuation of about 130,000 residents of several towns downriver, while many more are under an evacuation warning should anything else go wrong. This effort is still ongoing and nobody knows when people will be allowed to go back home.
The good news as of just about an hour or two ago is that after turning up the flows in the damaged spillway (which will have to be repaired at some point, ideally) the emergency spillway is no longer needed as the level of the lake has fallen below the height of that wall. They're continuing to drain the lake as quickly as possible in anticipation of another storm coming later this week and to reduce the pressure on the weakened auxiliary spillway wall.
There are some plans afoot to try to shore up the hillside underneath this spillway, but they won't know for sure until daylight tomorrow.
FOOTBALL (all times GMT):
19.45: Serie A, 6th vs. 7th: Lazio vs. AC Milan
20.00: Premier League, 14th vs. 5th: AFC Bournemouth vs. Manchester City