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Colorado’s Big Flood: Arctic Oscillation (5)

By: Dr. Ricky Rood, 4:36 AM GMT on September 17, 2013

Colorado’s Big Flood: Arctic Oscillation (5)

I am going to continue with my series on the Arctic Oscillation, but I am going to take a diversion. That’s why this one is labeled number 5 – numbers 3 and 4 will be forthcoming. The first two in the series have definitions of terms and references to more materials (link to number 1 and link to number 2). I will, ultimately, bring this flood back to climate change and the Arctic Oscillation discussion – I think with plausibility.

I am writing this at the time of a historic flood in Colorado (Denver Post Photos). This flood has been labeled a 100 year flood in Boulder (Boulder Creek) and a 500 year flood in Longmont (St. Vrain River). The geographical reach of the flood is large, reaching from Colorado Springs in the south to Fort Collins in the north, about 130 miles. This geographical span also distinguishes this flood as compared to, for example, the Big Thompson River flood of 1976. Chris Burt wrote a blog about the history of flash floods in Boulder. The last flood that rivals the 2013 flood was in 1894.

I live in Colorado and I have a sizable garden that leads me to doing all sorts of things in the ground. Once digging a hole three feet deep to find an old pipe, at about 18 inches, I ran across a layer of rounded, smooth rocks. A local, who I tend to believe, told me that this was the wash from the 1894 flood. That put me in mind of Death Valley where there are massive alluvial fans of similar rounded rocks at the mouth of every canyon. You walk in these rocks and there are times they have an almost fluid character; you sink into them a few inches. Death Valley is dry and it does not hold soil. So these deep accumulations of rocks are visible. If you think about the flash floods and the years needed to build the piles it is staggering. After running across this likely layer of the 1894 flood, I looked around, and indeed, one does see the same form of alluvial fans in Colorado, only they are covered with soil and plants.

The normal flow for Boulder Creek is around 200 cubic feet per second, and in the 2013 flood numbers closer to 5000 cubic feet per second were measured. In the town of Boulder, the most obvious damage was on the western side of town on the steep slopes of the Foothills. Further up into the mountains, where there is great devastation, are the scars of many forest fires of the past few years – again, extending from Colorado Springs to Fort Collins. In these places the ground, which is a combination of soil and plant and those smooth rocks, liquefies. The ground first saturates with water, then turns into a mixture of dirt, rock and water. This slurry turns into slides and gushers of heavy suffocating material. The ground under houses essentially melts, and houses join in the flow. Roads crumble, join the fluid along with cars. There is danger from being overwhelmed from above and undercut from below. People die.

As these slides of water, rock, and detritus of the forests and the city move down stream, the heavier material drops out and flotsam, dirt and water flows into the channels of the gullies, creeks and ditches. In places, like the city of Boulder, flood-control fortifications channel the flood, and then it spews and spreads as it gets to more open land in the plains.

Most people who come to Boulder and the cities and towns of the Front Range think of mountains. However, most of the people and buildings sit away from the mountains. Traveling east from the mountains there are first mesas, and it is not far before one is in the high plains. Towns such as Longmont view Long’s Peak; they don’t sit on Long’s Peak. Longmont, Loveland and many others are more of the plains than of the mountains. If you drive north and south just five miles from Boulder, the straight roads go up and down the mesas, no more than about 100 feet in elevation. Between the mesas are the wide valleys of Boulder Creek, Left Hand Creek and the St. Vrain River. These valleys are the historical flood plains, and in a flood like the 2013 flood, they fill up. In 2013 they are also full of farms and houses and towns. The creek channels and ditches are engineered to carry water where it is wanted, but in a flood like this, the type of flood that made this wide valley, the water and the suspended soil and the rolling rocks fill up the valley like it has for thousands of years.

As I write this most of the water has moved down to the South Platte River and a diminishing flood is pulsing north towards Nebraska and Omaha and the Missouri River. The Front Range of Colorado is wet and wounded. The air and ground feel and smell like the aftermath of a hurricane. There is a surge of rot as late summer fields start of compost. Today I found crushed crayfish a hundred yards from what I presume is their home in an irrigation ditch, giving a coastal fishy smell. There are clumps of sodden pink insulation. Plastic bags of rotting garbage.

Climate: With the Pakistan flood in 2010 I started climate case studies. From a weather point of view, there are some similarities with Pakistan. There is moist air coming from a warm southern ocean, there are high mountains, and there are high and low pressure systems steering a river of that moist air, warm over cold, up the mountains. As the air rises it rains. Because these weather patterns are stuck, persistent, the rain is relentless. From a point of view of scale, the geographical expanse, the Colorado flood is much smaller than the Pakistani flood. From a preparedness and infrastructure point of view, there is no comparison. The resilience of Boulder, Longmont, Estes Park and Lyons will prove much more robust than that of Peshawar. As a climate case study, this mix of geography, weather, built environment and resilience are all part of the mix.

But what of climate change? It is reasonable to pose that climate change is playing into the Colorado flood and its impacts in at least two ways. The first is a change of the land surface. There has been change in the forest due to drought. There has been change due to destruction of forest by pine beetles. A large amount of land has burned in the past few years. The fire season has been long, the fires intense.

The second possible impact of climate change is in the weather pattern that has caused the flood. Let’s step back a few months. In March of this year much of the region that has been flooded was in extreme and extended drought. Then in April, in quite a localized region, there was every week a record snowstorm, providing four feet of snow. The monsoonal rain was pretty regular during the summer, but we came into September below average precipitation by quite a bit. Tuesday a week ago, the predicted rain was said to be enough that we might get up to average. A little later that week it was the largest rain event ever. The one day total passed the previous record by a factor of two. A couple of days later we have the wettest year ever.

This pattern of rain is hard to ignore. Persistent patterns of weather, with weather systems moving from east to west. This is where I will make the link to climate change and the Arctic Oscillation.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.