Guest editorial by Vijay K. Gupta, University of Colorado, Boulder

Vijay K. Gupta

We experienced the evolution of an extreme rainfall event and the flood that followed during the week of September, 11-18, 2013. The devastation was widespread in the Front Range, Colorado, but Boulder got the brunt of it. It was a rare opportunity to experience first hand an extreme geophysical event.

Prof. P. E. O’Connell, University of Newcastle, UK, was invited as the Arthur Boase Lecturer to the University of Colorado in 2008. During his visit, he presented me with a poster of Boulder’s great flood of 1894. A historical description of the flood is given here. Scenes of the flood and its aftermath were captured by two Boulder photographers, Lawrence Bass and Joe Sturtevant. My poster is a photo by Bass. It appears from a visual inspection that the 2013 flood is comparable in its severity to the 1894 great flood.

Floods are among the extreme geophysical events. An AGU Chapman conference on “Complexity and Extreme Events in Geosciences” was held in India in February, 2010. The newly developing nonlinear geophysical science is providing an interdisciplinary forum for different sections of AGU to exchange common ideas. This perspective is explained in an overview article coauthored by 12 colleagues from different AGU sections, and published in an AGU monograph on Complexity and Extreme Events in Geosciences, 2012. My talk at the conference was devoted to a nonlinear geophysical theory of floods (Gupta et al., Geophys. Res. Lett., 37, L11402, doi:10.1029/2009GL041540, 2010.), but our paper in the AGU monograph addresses a long-standing open problem on droughts.

What caused such an extreme rainfall event that led to the flood? In a new article published in New Scientist quotes an atmospheric scientist from the National Center for Atmospheric Research in Boulder saying, ”[…] that huge volume was due in part to a lingering heat wave that for months blocked tropical moisture from the Gulf of Mexico from reaching the Rocky Mountains. When that heat wave began to move east last week, weak winds allowed the growing storm system to sit above the Colorado peaks for days. Once that deluge hit the ground, more trouble awaited. Because of Colorado’s mountainous terrain, the region is flood-prone anyway but recent wildfires exacerbated things near Boulder and Fort Collins, two areas hardest hit by floodwaters. The fires had cleared land of vegetation that would normally absorb rainwater.”

The USGS designated it as a 100-year flood. It means that a flood of this magnitude has a chance of 1/100 to occur in any year. Standard calculations show that a 100-year flood has about 2/3 probability to occur at least once in 100 years. How does USGS assign such probabilities is an important question because data on extreme events are few and far between? The science of floods remains a challenging and a fascinating topic despite many advances that have taken place in the last three decades. I hope that new graduate students would give it a serious consideration in selecting their dissertation research topic.

Vijay K. Gupta is a professor of Civil, Environmental and Architectural Engineering at the University of Colorado in Boulder. He developed the theoretical foundation of the flood forecasting models that the Iowa Flood Center is pursuing in order to provide alerts to over 1,000 communities in Iowa via the Iowa Flood Information System.