Regional Frequency Analysis of Extreme Precipitation in Northeastern Colorado and Fort Collins Flood of 1997

Regional frequency analysis based on the index flood method has been utilized to analyze short duration annual maximum precipitation (AMP) for Northeastern Colorado. An extraordinary storm and flood that occurred in the City of Fort Collins, Colorado, in July 1997 prompted the study. The main objective of the study was to develop regional frequency curves that may be useful for that part of Colorado. In addition, we wanted to answer the question whether the City’s design criteria for storm drainage as of 1997 was adequate, and to determine to what degree the referred storm was indeed an extraordinary storm. We utilized up-to-date technology that has been widely popularized in literature, with some minor modifications. We conclude that the regional growth curves developed may be useful for the subregions specified. However, they should be used with caution for sites located near the boundaries of subregions close to the Colorado Front Range. This has become evident in determining the precipitation quantiles for Fort Collins, for which it was necessary to further narrow down the subregions. The study also showed that the city’s storm drainage design criteria were underestimated. Furthermore, the 1997 2 and 3 h storms appear to be 100 yr+ events, while the return period of the 6 h storm may have been of the order of 400 years. While the study provided us with some practical answers, it nevertheless also brought a number of questions concerning the assumptions underlying the index flood approach and several steps regarding the definition of regions and testing criteria thereof. These are discussed at length in the final section of the paper.