St. Malo, the 1000(?) Year Flood and Climate Change

St. Malo and Rocky Mountain National Park have been changed by the recent rains, and I was absolutely shocked by the power of nature. Cabin Creek which runs through St. Malo became a massive wall of water, trees, rocks and debris that completely reshaped the geography of Mt. Meeker’s eastern basin. The pictures above and video below gives an idea of the devastation. Having lived there for so long and known the trails and forest, the effect of the devastation is  shocking. The creek bed which was at most 10 ft wide is now over 50 ft wide. The pile of water, rocks and debris funelled out of the Meeker eastern face carving its way down the mountain and spread at St. Malo as an alluvial fan over 150 ft. wide, carving a new path for Cabin Creek in the process. The creek flow has now changed. One building is seriously demagaed and the other has disappeared. It appears that the mudslide began at the top of Mt. Meeker as a slide is visible from the distance.

See the video here: http://www.9news.com/news/article/356169/222/Chapel-on-the-Rock-survives-massive-rock-and-mudslide

Now, much debate has followed about the proportions and causes of the flooding in Colorado at large. Impressed by the power of this event, it is easy to want explanations. Many have used the term 1000 year flood, and Roger Pielke Jr. here and here explains why the term is not helpful at best, and in fact incorrect. The flood in fact is a 25-50 year flood:  Colorado has had a lucky streak, the last time Boulder flooded like this was 1894. He explains:

As is often the case in the aftermath of extreme events and disasters, people look for some way to put them into a bigger perspective. With respect to floods, a common way of establishing this perspective is through the N-year flood, which is defined as a flood with 1/N probability of occurring in any given year. So the 100-year flood, used in floodplain regulations, is a flood with an expected 1% chance of occurring in any given year.

Earlier this week, I presented some of my objections to the utility and meaningfulness of the concept of the N-year flood. In this post I show how the concept of the N-year flood can be used to turn fantasy into fact.

In an article titled “The Science Behind Colorado’ Thousand-Year Flood” Time magazine explains:

Parts of Boulder are experiencing a 1-in-1,000 year flood. That doesn’t literally mean that the kind of rainfall seen over the past week only occurs once in a millennium. Rather, it means that a flood of this magnitude only has a 0.1% chance of happening in a given year.Time is a fixture of the mainstream media and what is written there is widely read and repeated.

A big problem with Time‘s article is that Boulder did not actually experience a “1,000-year flood.” In fact, according to an analysis presented by fellow CU faculty member John Pitlick yesterday, using standard hydrological methods, Boulder experienced between a 25- and 50-year flood.

Apart from being incorrect, imprecise statements like this allow for implications, such as the association with climate change as a cause. Pielke Jr. explains here  why in this case, no particular cause can be identified by looking at the effect. However, one possible avenue of exploration for the uniqueness of this episode of rainfall, could be geological research on Cabin Creek. I am no geologist, but given the previous river bed width and depth, perhaps some interesting conclusions could be drawn about the proportions of the rainfall in this very localized and specific place.Furthermore, as part of Rocky Mountain National Park, this area is unaffected by human impacts. Geologically speaking, perhaps it has been 1000 years since Cabin Creek flooded like this, if ever ?

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