Many people that have contacted us ask the question I will paraphrase here: “Why, after 38 years when PG&E breached the dam and it was dropped from the jurisdiction of the State, has the lake become an issue. Why can’t people just leave it alone?” We knew that the current hoopla started in the summer of 2011 when the Department of Safety of Dams (DSOD) came out and inspected the dam and put it back under their jurisdiction because it was larger than 50 acre-ft of water. We also heard that the State was responding to a request from Nevada County Emergency Services. What we didn’t know was the mystery of what prompted that request.
This week I was doing some Googling research on the Van Norden Dam and I came across the following newscast from Channel 10 news in Reno in June of 2011. As you will see, this solved the mystery.
http://archive.news10.net/news/local/article/143947/2/Sierra-dam-cracks-worries-residents
PRETTY SCARY, huh?
I know what many of you might be thinking after watching this, “Maybe the dam is a danger!” Before you decide that the sky is falling, let’s just take a look at the facts surrounding the events and what actually was going on with the dam.
As anyone who was up here in the winter of 2011 knows, it was and epic snow year. It was rated in the top 5 snowfalls in the area which is saying a lot. The run off that year was also huge. One might think that the added flow intensity could have caused the crack in the front of the spillway. But let’s take a look at the dam during the runoff just a year before (a relatively normal year) in Figure 1.
Figure 1. Van Norden spillway on June 15, 2010
Remember the spillway was 35 years old in 2011 and had very little maintenance. As shown in Figure 1, even in 2010 you could see a crack had developed during that time on the front of the spillway and water was escaping from the north front side of the spillway. In 2012 when the crack was repaired by Holdredge & Kull, they discovered that the crack had allowed water to enter under the front of the spillway and create a lateral void that channeled the water out the north front side of the spillway. In 2011, the heavy melt flow was diverted into the void channel where the added pressure caused the expanded crack that you see in the newscast.
While dramatic, the diverted flow in 2011 did not really pose a threat to the dam as the “untrained” newscasters implied. The crack in the dam was localized to the front north side of the spillway. You can see the location of the crack in the overhead view of the dam in Figure 2.
Figure 2. Overhead sat map of Van Norden Dam spillway
Of course, this is not a view that was available to the news crew that was shooting the 2011 story or to Mr Burton who first reported it to Channel 10. Even if the entire front north side of the spillway had failed, there was still almost 100 ft of dam behind it. Remember, this dam was originally built to hold over 50 times the water that it does now and 100 times the amount of a 50 acre-ft lake. This is why Mr Ferrera from Nevada County Emergency Services stated that he did not think the dam was a threat.
Now I am not trying to say that the dam does not need work. The episode in 2011 made this quite clear. In fact, in the summer of 2012, Holdredge and Kull repaired the crack and void to satisfy the concerns of the state and county. Even that repair was just considered temporary. Rebuilding the spillway has always been in the plan to fix the dam. No matter which plan is executed, a 2.2 ft notch to leave 50 acre-ft of lake or a 5 ft notch that will drain the lake, the spillway will be rebuilt to avoid further leakage problems in the foreseeable future.
The mystery however, is solved.
