People living below Lake Oroville are eyeing the Pineapple Express, an atmospheric river pounding Northern California. The 188,000 downstream residents (from Oroville and south for 50 miles) remember last year’s spillway disaster that resulted in their evacuation. Now, one year later, if inflows cause the lake level to reach 830 feet the newly reconstructed spillway must be used for the first time to release water from Lake Oroville. Whether or not this happens depends on how much water flows into the lake and how fast it arrives.
The “Pineapple Express” name means the rain originates in the subtropical and tropical Pacific Ocean and is warm. Snow levels were over 11,000 feet (above the highway passes) at the start of the storm Thursday evening. Saturday afternoon, the snow level is expected to lower a few thousand feet. “Atmospheric river” means lots of water with 3 to 7 inches of rain forecasted for the Sierras (the lake’s watershed). Because most of March’s heavy precipitation was cold, it fell as snow above 3,000-5,000 feet elevation and the high Sierra snowpack is 3 to 8 feet deep. Now some of that snow will be hit by warm rain and begin to flow downslope into the lake. This storm should end by Sunday, but another might hit late next week.
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The combo of heavy rain and snowmelt has triggered an alert from the Department of Water Resources. DWR may need to allow water to flow down the mostly-reconstructed Oroville spillway for the first time. Spillway use is required if the lake level reaches 830 feet in April (full is 901 feet) so DWR began releasing more water to lower the lake level ahead of the storm. Before the increased releases began at noon on Wednesday, the lake level was 794 feet, having been allowed to rise 70 feet during March. At 4 pm Thursday the level was 793 feet.
Thursday’s DWR update reported increased releases from Lake Oroville through the Hyatt Powerplant (at base of dam). The total release possible there is 12,500 cfs. The river valves (tunnels bypassing the dam) can release an additional 4,000 cfs. Inflows will continue after the storm passes as rivers and streams in the watershed continue higher flows. Using the powerplant and tunnels, 16,500 cfs can be removed from the lake. At times during the March storms, over 80,000 cfs inflows were recorded from the rain below 3,000-6,000 feet elevation. Above that, the precipitation was snow and didn’t flow into the lake.
Forecasted storms expected in the Feather River basin this weekend may require using Lake Oroville’s flood control outlet spillway (also known as the main spillway) this week or next.
After last year’s spillway incident, the Department created the 2017/18 Lake Oroville Winter Operations Plan to ensure public safety in the event of major storm events. . . The current forecasts show the potential for inflows to raise the reservoir to near the 830-foot trigger elevation by the middle of next week. [...]
In anticipation of the incoming weather, DWR is increasing outflows from Hyatt Powerplant. Outflows were increased from 9,500 cfs to 11,500 cfs at noon, and will be increased to 13,000 cfs at 1PM.
After last year’s disaster, a siren like those used for tornado warnings was installed in Oroville to warn about possible failure of structures holding back the lake water. But if a problem occurs at the lake, the safest route from danger is to the west, across the Sacramento River. The route north goes closer to the dam/spillway before being out of the flood zone and south is still in the flood zone for many miles. During the 2017 evacuation, the route west was impassable due to flooding of the lowlands near the river. Now, the National Weather Service has issued a Flood Watch for the same area and all of northern California from Friday through Sunday. For the next week or so, residents will be thinking about what was built upstream and listening for the siren.
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I reported on the repair work and the disaster’s forensic report earlier this year. At the end of that article are links to other stories from before the evacuation when Lake Oroville was filled to the brim on February 11, 2017, the ensuing catastrophe, and the aftermath including repairs and recriminations.
I’m happy to be above all this mess listening to the rain that began Thursday before sunset. What’s your nature story today?