I appreciated your Jan. 20 story on the Anderson Dam Retrofit, in which you summarized the report that the Santa Clara Valley Water District staff presented to their board Dec. 9, 2016.
Because I live less than a mile from this huge structure and have a good view of it from my bedroom windows, I have developed a keen interest in this project.
The prospect of a massive tidal wave coming from the direction of the dam has a way of getting your attention.
I must admit that I was disappointed by the lack of any mention in your recent story of the catastrophic consequences of a failure of this structure, which has now been found to be so seriously compromised that the SCVWD has decided to tear it down and virtually replace it with a new dam.
Since you have viewed the Dec. 9 staff presentation, I assume you know it has been determined that a large earthquake would cause the alluvial material that forms the base of the dam to undergo liquefaction, and this could cause the dam to “slump” by as much as 25 feet. The assumption here is that, if the dam is only two-thirds full, it would still have enough structural integrity to retain the contents of its reservoir. This seems to me to be a stretch.
I have since confirmed with the chair of the SCVWD Board, John Valera, that a failure of this massive structure when it is completely full would create a 35 foot “wall of water” in downtown Morgan Hill, in 14 minutes. I have yet to determine how high the wall of water would be if the dam were to fail when it is only two-thirds full (between 60,000 and 70,000 acre feet of water), but I think it is fair to assume the water that would descend on Morgan Hill in such a scenario would be catastrophic.
I think the people of Morgan Hill deserve to be informed of these potential dangers. But for reasons about which I can only speculate, there seems to be an unofficial wall of silence among city leaders. I have a meeting with Mayor Steve Tate coming up, and I plan to ask him why we have not heard more about these possible dangers.
According to the U.S. Geological Survey, there is a 26 percent chance of a 6.7 earthquake on the Calaveras Fault (which passes just 2 kilometers east of the Jackson Oaks neighborhood), and a 37 percent chance of a quake of the same magnitude on the Hayward fault by 2044. What is more, a group of geophysicists at U.C. Berkeley recently demonstrated that there is a deep connection between the Hayward and Calaveras Faults which could result in an interaction that would create an earthquake as large as the one on the San Andreas Fault in 1906.
The most obvious way to dissipate the dangers this dam and these seismic findings provide would be to have the SCVWD commence the “dewatering” of Anderson Reservoir now, rather than three years from now, as is envisioned in the latest plans.
I would like your help in getting the word out regarding the dangers this compromised dam presents, along with the obvious solution for making them go away.
Dave Elliott, Ph.D.
Morgan Hill
