An article in the Sacramento Bee last Sunday reported that a proposed Auburn Dam on the American River “could be an expensive mistake.” This article couldn’t have come at a better time, considering all the renewed momentum that dam proponents gained from fears sparked by New Orleans and Hurricane Katrina, and Rep. John Doolittle’s recent attachment to an energy and water appropriations bill calling for a new 1 million dollar dam study. (See earlier post about Katrina Drawing Ill-Founded Support For the Auburn Dam.)
The Bee article, titled ‘Tempting Fate: A Torrent of Doubts,’ pointed out some very important myths and inherent inefficiencies in the dam’s location and design with regards to its supposed purpose as a multiuse dam. Among these are the limited amount of water the dam would have available to sell, the high price that this water would cost, the structure’s dangerous location on a fault line, and the liklihood that it would decrease, rather than increase, area recreational usage.
If all of this is true, then why is the Auburn Dam still on the table? And why do Doolittle and other dam proponents still have such a large base of support?
The answer lies in a single argument made over and over again. The main premise of this argument goes as follows: Auburn Dam would provide “500 year” flood protection as opposed to the “212 year” protection gained from improvements on Folsom Dam and American River Levees. The argument then concludes that we should spend money on the most complete safeguards that we can obtain, instead of “waste” any money on dam and levee repairs that will one day fail to an “imminent” 4 or 5 hundred year event.
It is easy to see how alluringly simple this idea of maximum protection can be. Unfortunately, it dangerously relies on gross misunderstandings of scientific data.
Let’s take a look at the numbers. First, what exactly is a 500 year flood, and where does this statistic come from? When one first reads the term, “500 Year Event,” s/he is likely to interpret it as meaning an event that occurs once every 500 years. This is one of the single most important misconceptions driving support for the Auburn Dam– “500 year event” does not refer to a flood that is certain to occur at some point in time. Rather, a 500 Year Event refers to a projected statistical probability… a water level and volume that, in one particular model, has an extrapolated 1 in 500 chance of occuring in any given year. The same applies for the term 200 year flood/event. In other words, a 200 year event refers to river flows with a modeled probability of 0.005, and a 500 year event refers to river flows with a modeled probability of 0.002.
We all know that statistical probabilities don’t exist without human analysis and without assumptions being made. So in order to better examine this argument for “500 year protection” in the form of the Auburn Dam, one needs to first understand how, exactly, these statistical figures are being calculated. Where does the data come from, and what kinds of presumptions are being made?
The data question is an easy one to answer. There exists about 150 years of consistent and reliable data on American River flows and floods in California, and 100 years of actual stream guage readings (flow data from an actual guage in the American River.) Mathematicians took those readings and plugged them into a programmed computer model, which in turn produced a probability distribution that assigned frequencies to the given data points. This probability distribution curve was extended to include flows and floods that have not appeared in recorded history. Included in this extrapolation are all flows above those that have been assigned a 1 in 80 chance of occuring. A 500 year event has thus never been recorded, and is instead only a statistical extrapolation. In fact, the American River has never, in recorded history, exceeded flows larger than those that correlate to a modeled 100 year event!
Considering that the justification for Auburn Dam is thus based upon flow volumes that have never actually been recorded and are rather just an assigned frequency on a particular modeled probability curve, it is rather important to analyze the accuracy of the model. We move on then, to asking what presumptions these particular extrapolations are based upon. In other words, we must look at the parameters within which the model was operating when it computed the liklihood that a non-recorded flow event will ever occur.
In this case, it is not so much assigned parameters but rather a lack thereof that raises cause for concern. The model assumes that for every reduced frequency or probability (a 500 year event being the SMALLEST in discussion), there exists a higher flow. This means that the model must also assume no limit to the possible size of a storm (and thus flow volume) in California. Within this set of (non-)parameters, one could assign a probability that the American River rage at 10 million cfs at some point in time! This does not mean that this event is actually possible. Or, as the saying goes, statistics can lie.
The reality is actually much more likely to be that California’s weather system does have a limit on its ability to produce certain amounts of water in any given storm. And in fact, government scientists from the National Research Committee have suggested that any extrapolation is dangerous and likely faulty beyond flows assigned a 1 in 200 chance of occuring; that in fact these larger projected flows with smaller modeled probabilities (such as a 500 year event) may actually have virtually NO probability at all.
What is the reasoning behind this suggested limit to to possible storms and American River flows?
Well, there is only ONE storm system that causes the American River to flood: the Hawaiian Tropic that we so recently experienced in December and January this winter. And as far as that storm is considered, it is extremely consistent and not of the size capable of dumping enough rain for flows as high as those assigned a 1 in 500 chance of ever occuring. (Or even a 1 in 200 chance of occuring for that matter.) There is NO GOOD REASON to assume that anything will change in this weather pattern. Therefore, there is also no good reason to assume that the American River will ever exhibit flows much higher than those recorded in the past 150 years. For such extrapolations to be at all valid, one would have to also propose the possibility of a radical change in California’s weather patterns. As far as researchers at the US Geological Survey, Pacific Institute and NRC can tell, such a proposal has virtually no grounding in scientific knowledge or reality.
So to get back to the argument we started with: Should we spend the extra money, incur the environmental damage, assume the earthquake risk, and ignore recreational losses, all for the difference between a probability of 0.002 and 0.005 that may have been incorrectly calculated to begin with? I’m going to go with no on this one. If a 500 year event were, based upon scientific study, actually a valid possibility, then the fight for Auburn Dam would carry much more weight. The facts, however, do not support this premise. That in mind, the incredible economic inefficiency and environmental loss (including theMiddle Fork and North Fork of the American River ) associated with Auburn Dam, are surely not worth undertaking. Rather, the Dam would be, as the Sacramento Bee article so eloquently put it, an expensive mistake.