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<br />, <br /> <br /> <br />i <br /> <br />An Analysis of Rainfall for the July 28, 1997 Flood <br />in Fort Collins, Colorado <br /> <br />Purpose <br /> <br />Colorado is known for abundant sunshine, low humidity and a relatively mild climate. But <br />from time to time through history, extreme rainstonns have occurred. Analysis of the largest <br />reported rainstonns in Colorado has shown that the areas where the Great Plains meet the <br />Front Range of the Rockies is particularly prone to extreme rainfall events and associated <br />flooding (McKee and Doesken, 1997). Since 1920 at least 312 people have died in Colorado <br />as a result of flash floods (Tunnell, 1998), with most of these deaths occurring along the Front <br />Range. As the population of Colorado continues to grow, and as more and more people live <br />and work in or near urban areas along the Front Range, the vulnerability of this region to <br />catastrophic flash floods also grows. For large urban areas to exist in a flood-prone region <br />and still provide relative safety to their residents, wise and careful planning is required that <br />takes into account how much it has rained in the past and how hard it might rain in the future. <br />Wise planning is especially important where dry weather is so often the rule and the potential <br />for heavy rains is quickly forgotten. <br /> <br />On July 27-28th, 1997, very heavy rains fell over portions of eastern Larimer County in north <br />central Colorado. Extreme urban flooding conditions developed on July 28 in Fort Collins <br />that swept :live residents to their death and caused over $100 million dollars in property <br />damages. The purpose of this report is to thoroughly document the area, duration and <br />intensity of the rains associated with this Fort Collins flood. By defining the rainfall patterns <br />as precisely as possible, it is our hope that many can learn from this experience. Hydrologic <br />modeling and design, urban planning, emergency management, public policy and education are <br />just offew of the applications that stand to benefit. <br /> <br />Many of the extreme storms of the past along the Front Range have been studied (Follansbee <br />and Sawyer, 1948, Maddox et ai, 1977, Glancy and Daselar, 1986, Hansen et a1, 1988, and <br />others). However, many of these historic storms occurred in areas where precipitation data <br />were sparse. Results of "bucket surveys" produced estimates of rainfall patterns, but <br />confidence in some of the analyzed rainfall patterns from past storms has been low. <br />Interestingly, there had just been a meeting in Denver on July 17, 1997 of the Colorado <br />Extreme Precipitation Committee, a small group of predominantly state and federal officials <br />with common concerns. At that July meeting, the importance of thoroughly documenting <br />future extreme storms was discussed. While no group is currently funded to survey and study <br />extreme stonns, several groups agreed that if a huge storm were to occur, they would attempt <br />to gather as much information as possible. Coincidentally, in less than two weeks, the <br />opportunity presented itself. Since the Fort Collins storm occurred over an urban area where <br />many individuals and organizations took gauge rainfall measurements, and since remotely <br /> <br />1 <br />