Tuesday, February 21, 2017

Liveblogging the Deluge: Big Changes on the Tuolumne River

Source: http://www.capradio.org/articles/2017/02/20/don-pedro-reservoir-spillway-opens-for-first-time-since-97/
I've been away for the last five days, experiencing California's storms from an entirely different perspective, that of being exposed and out in the open country of Death Valley National Park. There will be plenty of information and pictures about our adventures soon enough, but there were plenty of events back home that have been rather significant as well. A great many people have been affected as the waters of the San Joaquin River and its tributaries have risen past flood level.

First and foremost, at Don Pedro Reservoir on the Tuolumne River the emergency spillway has been opened for the first time since the floods of 1997. This became necessary when the lake filled nearly to capacity, and a new storm battered the north state over the weekend. The channel downstream floods at around 9,000-10,000 cubic feet per second, but storm runoff threatened to exceed that level. The dam operators had to make a decision to cause some flooding downstream to avoid even greater flooding if runoff was too high.
Flows were ramped up to around 16,000 cubic per second over the weekend, and have since settled back to 13,500 cfs at the latest reading (below). Rain and snow are still falling in the region, so the concern about flooding remains for the time being. The lake fills at 830 feet, and the current level is 828.82 feet. There isn't much room for error.
Source: USGS (https://waterdata.usgs.gov/ca/nwis/uv?11289650)

People are understandably concerned about Don Pedro Reservoir as they compare notes with the events at Oroville Dam in the last few weeks. Both dams have an emergency spillway that flows over unlined and unreinforced rock rather than a concrete channel.

There are some important differences, however.

First, the emergency channel below Don Pedro is much less steep than Oroville's, and is therefore less subject to the headward erosion that threatened to undermine the spillway at Oroville. Second, the spillway was tested previously, during the floods of 1997. The runoff amounts in that event were almost astronomically higher than any flows expected in this year's event. A forty-foot channel was carved through the meadow, and the flowing waters are unlikely to do anything worse this time around.
Still, these are nervous times. I had my first chance to see the higher flows this morning as I headed to work. I stopped at the head of the Tuolumne River Parkway Trail to have a look. The picture below was taken on February 5th when flows were around 10,000 cfs. Note that the entire trail was above water level.
The picture below shows the same scene this morning with flows closer to 14,000 cfs. As you can see, the entire trail is under water now, which means I'll have to find another place for my birdwatching for a few weeks or so.
One senses that authorities are just barely keeping ahead of events, which is not surprising given the nearly nonstop barrage of atmospheric river storms battering California. The situation has not quite spiraled out of control, and the storms are letting up for a few days, which will allow for more floodwaters to flow into San Francisco Bay. I hope we'll have a breather.

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