Tuesday, September 17, 2013

What caused the Colorado flood?

Many people have reached out to my family about the recent flooding in Colorado. Thankfully, we live on a hill away from any stream or river, so we stayed relatively un-flooded. We were isolated for only one day, but we did venture out to view some of the flooding damage and high water marks. Many people who reached out asked questions like "don't people know to not build in an area that floods?" While these questions are innocent, they are a bit misguided, for floods like this rarely happen and could actually happen anywhere. The real question that should be asked is "how did this happen?" Though we can't pin-point a specific cause, many experts are indicating that it was probably a combination of many factors and just a spell of bad luck. Let me 'splain....

Colorado is the following: dry, many times too dry. On average, Colorado as a whole only receives 17 inches of measurable precipitation per year. "But Colorado gets so much snow!" you say. Well, part of that is correct. The mountains do get a lot of snow. The plains do not. Also, on average, 10 inches of snow (not slush) equals 1 inch of precipitation. So when the city of Denver receives an average of 30 inches of snow per year, that only accounts for about 3 inches of precipitation.

Though most of it is dry, what prevents Colorado from becoming a desert state is actually quite dramatic... the Rocky Mountains. Many locations above 10,000 feet receive 25 to sometimes 50 inches of precipitation annually. Monarch Pass, considered one of the snowiest places in Colorado, receives on average 350+ inches of snow, or roughly 35 inches of precipitation from snow per year. That means Monarch Pass receives double the average precipitation for the state in snow precipitation alone. All of this precipitation that falls in the mountains conveniently fill the plethora of reservoirs throughout Colorado and is then pumped and drained based on historical water rights (which is a whole separate disaster). Without the engineering behind those reservoirs holding the precipitation from the mountains, the major cities along the Front Range would not exist. It is simply too dry. This, in itself, is a blessing and a curse.

So, as you can see, Colorado is dry. Even with the precipitation in the mountains being held in the reservoirs, Colorado has a history of drought. For the past few years, Colorado has had drought conditions that have lead to water rationing and some of the worst wildfires on record for the state. This parched landscape was begging for water, but like all things, too much of a good thing isn't always a good thing.

Last week, Boulder experienced something it hasn't in recorded history. As of 7am on Monday, September 16th, Boulder has experienced 17.07 inches of precipitation for the month of September. The September average? Just 1.7 inches. This 17.07 inches literally crushes the previous record of 9.59 inches in May of 1995. On September 12th, Boulder received 9.08 inches of rain in a 24 hour period. This also literally smashed the single day record set on July 31st, 1919 when a measly 4.8 inches fell. So to put that simply, a lot of water fell, doubling the previous records for the area. To bring this all home, Boulder has already broken its yearly record for precipitation with over 3 months left to go. What is even worse is that Boulder got as much rain in a week as it typically gets in an entire year.

That, is a lot of rain. That is so much rain that multiple reservoirs overflowed, with some even bursting, which added to the flooding. Due to all the water, many dams are now structurally damaged and in need of drastic repair. Lyons, CO became completely cut off after a dam burst in the mountain behind it. Helicopters have been airlifting people to safety.

You may have heard on the national news that Colorado is experiencing the 100-year or 1,000 year flood, or a "flood of biblical proportion." This is very misleading, for this doesn't mean this flood actually happens every 100 or 1,000 years (and no, there is no Noah's Ark in the Rockies). What it actually means is that there is a 0.1% chance that this can happen in a year. A low pressure system literally trapped a tropical Southwest monsoon system against the Front Range and Rocky Mountains. The rain had to fall, meaning it did so in buckets. Luck be (not) with us, it just happened at a time and place where there is 1) drought conditions so the ground system was not ready, able or willing to hold that water and 2) steep valleys and creeks that are lined with solid rock walls creating a literal funnel for the water to go through.

Other things could have added to this "perfect scenario". There have been a series of tree kills because of the pine borer beetle and forest fires over the past years. These leave slopes of hills and mountain barren, increasing water run-off, which in return increase any flood or mud slide size.

Overall, it is a disaster. Entire towns are isolated and will stay isolated for months. Estes Park, the gateway to Rocky Mountain National Park, has one road open. That road will close soon when snows in the mountains will make it impassible. Homes are destroyed and entire bridges and roads have just disappeared. Colorado will rebound, but thanks to all the rain, we are officially not in drought conditions anymore (cue in dark humor)!

If you want to read more about Colorado's drought history and how that could affect the future, check this out.

Here are some areal photos posted by the Denver Post. To see more, click here

Here is a popular photo used of an overpass flooding at CU Boulder. I currently do not have a citation source for this. If you know it, let me know.

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