I am going to summarize here the Hydrology Report used to justify the height of the tsunami wall at the San Onofre Nuclear Generator Plant (SONGS). I haven’t found out the date of the report yet, but from the references it looks like around 1974. It was amended in 2007, but without new references. The 52 page report is quite detailed in examining records, and making hydrologic analyses, and it looks hard to criticize. However, we are now 38 years into the future with more data and better hydrology codes and computers, so we are looking forward to a new analysis. The heights are measures from the mean lower low tide (mllw), meaning of the two low tides a day, it is the average of the lower of the two. The high tide level that contains 90% of high tides is 7.18 feet above that. To that one adds a storm surge of 1.98 feet, and an isostatic anomaly of 0.33 feet (expansion due to increased ocean temperature in the summer) giving a total of 9.3 feet. They consider the largest tsunami as coming from the Newport-Inglewood fault which is 5 miles off shore, and assume a 7 foot vertical displacement of this. This is computed to result in a 6.3 foot tsunami. Adding this to the 9.3 feet above gives a 15.6 tsunami plus high tide plus storm surge. What more could add to the perfect storm plus tsunami? Oh, yes, the maximum storm wave. More than 99% of waves have been less than 10 feet, so 11.4 feet is added in for wave height, giving a total tsunami plus perfect everything else of 27 feet. The height the base of the plant is 30 feet above mllw, giving a three foot leeway to the maximum everything that is individually likely.
When this is redone with modern statistical methods, what we be quoted are probability levels containing 90%, 95%, and 99% of the fluctuations in time. We can approximate that here by adding the high tide level containing 90% of high tides, or 7 feet, to the height containing 90% of waves, or 5 feet to get 12 feet. What about storms? It is only rarely stormy, so ignoring them might also describe 90% of the time. So with a typical day during which the height only gets to 12 feet, the remainder of the tsunami wall height of 30 feet means that it can handle an 18 foot tsunami.
Opponents of the plant poo-poo that the 30 feet is measured from low tide rather than high tide. It turns out that tides are measured from mllw as a base. If the set of numbers is accurate, it really doesn’t matter where you measure it from. The 30 foot height is built up from beach rise, from a cliff, and from a tsunami wall.
We look forward to the results from more recent trenching, and consideration of other nearby faults, and modern hydrodynamics and statistical methods. It has been well reported that the San Andreas and most other local faults are strike slip faults, not subduction faults that cause large area and height displacements like the Japanese trench earthquakes. The key thing that could change in a new assessment is the tsunami height, since the sea measurements would pretty much stay the same.
A good talk on the SONGS reactor safety update after Fukushima was given by Caroline McAndrews of SONGS at the Beckman Center of the National Academy of Sciences in Irvine in the Distinguished Voices series on December 1, 2011. She pointed out that the two diesel backup generators are at the 30 foot level, and have a 7 day supply of fuel, should external power be cut. There are also batteries with the electric switching gear at the 50 foot level. This was the problem at Fukushima, since the electric equipment was in flooded tunnels. There is also 5 million gallons of stored cooling water. The hydrogen venting valves, which couldn’t be opened at Fukushima can now also be opened electronically, or manually, if needed. Clearly, it is not enough to just update the reactor safety in response to failures at each previous accident, since that would take forever to get the safest plant. All sorts of scenarios have to be considered, and safety upgraded for each.
The plant was designed to resist a 7.0 magnitude earthquake from the nearby Newport-Inglewood fault, and is able to withstand an acceleration of 0.67 g, that is, 67 percent of the acceleration of gravity, which is 32 feet per second per second, or something we feel every time we jump up or fall down. This strengthening covers the catagory VIII shaking range that runs up to 0.65 g. The much feared magnitude 8 on the San Andreas fault, which is on the order of fifty miles away, would only give a 0.2 g acceleration at San Onofre, since much would be absorbed on the intervening earth. A recent survey by UCI’s Lisa Grant Ludwig found that the San Andreas slipped about every 80 years with an uncertainty of 40 years. It is overdue, since the last major slip in Southern California was in 1857, which is 155 years ago. Caltech is trying to install an instant warning system for the San Andreas earthquakes, and if this was present, the reactor could be shut down a good minute before the shaking arrives. People who worry about the reactor, or even surviving such an earthquake themselves, should call for this system to be funded and completed.