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I I I I I I I I I I I I I I I I I I I I. INTRODUCTION A. INTRODUCTION: The current hydrologic criteria manual used by the City of Fort Collins utilizes depth-duration- frequency data obtained from NOAA Atlas 2 (1973). The current design criteria utilizes a 2 hour event based on NOAA Atlas 2 with a temporal distribution that was developed from historic rainfall patterns that places the peak rainfall intensity near the beginning of the storm event. NOAA Atlas 2 presents a series of maps and regional regression equations for use in estimating 2-year to 100-year, 5 minute to 24 hour precipitation depths. NOAA Atlas was prepared using the available precipitation gage data in a regional analysis for the period up to approximately 1967 to 1969. Daily rainfall data for the City of Fort Collins was available at this time, but little or no hourly data was available before 1940 with the exception of the Denver and Cheyenne, Wyoming areas. Since that time. an additional 30 years of record has been collected at many of these gage sites and some additional gages have been added to the network. A small number of gages have been relocated or deleted from the network. The problems associated with precipitation frequency analyses usually stem from the lack of sufficient data to model a T year event. or an event that has a probability of occurring every year of liT. For this reason. the substantially additional record could result in a significant impact on the predicted depth-duration-frequency data that was published in NOAA Atlas. In addition to the extended period of record. the City of Fort Collins experienced an extreme rainfall event in July 1997 that caused significant flooding damage to the Colorado State University campus and surrounding areas. The Fort Collins precipitation gage recorded a daily total for that rainfall event of 4.63 inches ending at 1900 MST on July 29. The highest 24 hour rainfall event that corresponds to that storm was recorded at 6. 18 inches. Based on data obtained from radar and other area gages. the maximum 24 hour depth at the center of the storm may have exceeded 10 inches (Doesken and McKee, 1998). Based upon commitments made by several Colorado agencies. the National Weather Service has indicated that they are likely to embark on a precipitation study similar to the one currently being conducted for the southwest United States to replace NOAA Atlas 2. Since the City of Fort Collins is currently updating their hydrologic criteria manual and the revised NOAA study would not likely be published for at least 3 to 6 years, there is a current need to assess the impact of the additional -1-