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. COLORADO STATEWIDE DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL . . CHAPTER 5 HYDROLOGY SECTION 1.0 RAINFALL CHAPTER 5 HYDROLOGY SECTION 1.0 RAINFALL 1.1 INTRODUCTION Due to the complexity of the natural terrain, orographic effects of the Rocky Mountains, and semi-arid climate of the region, the type and duration of rainstorms can vary substantially within the State of Colorado. However, rainstorm events can be generally defined as short duration convective storms or long duration general rainstorms. The short duration convective storms (cloudbursts/thunderstorms) can produce high rainfall intensities for a short period and generally cover watershed areas of less than 200 square miles. Convective storms are commonly known to be responsible for high peak flows and flooding problems for many small drainage basins. The long duration general rainstorms can produce rain coverage over a large watershed area for a period in excess of six hours up to several days. General rainstorms can produce large amount of total rainfall runoffs and sometimes generate higher peak flows than the convective storms. Depending on the purpose of the hydrologic analysis, it may be necessary to analyze both types of rainstorms in order to estimate the high peak flow rate and the high runoff volume for a given drainage basin. The information presented in this section is the state-of-art information available at the time of preparation of this manual and should be updated as better techniques and new rainfall data become available in the future. 1.2 RAINFALL DATA The rainfall data published by National Oceanic and Atmospheric Administration (NOM) in their "Precipitation-Frequency Atlas of the Westem United States, Volume III - Colorado, 1973" should be used to perform necessary hydrologic calculations within the State of Colorado, unless site-specific rainfall studies have been performed and adopted by the local govemment agency having jurisdiction over the study area. The NOM Atlas 6-hour and 24-hour precipitation frequency maps for various storm events for the State of Colorado are included as Figures CH5- F101 through CH5-F112. The 6-hr and 24-hr point precipitation values can be estimated directly from Figures CH5-F101 through CH5-F112, and these point rainfall values can then be used to develop 5-minute, 10-minute, 15-minute, 30-minute, 1- hour, 2-hour, 3-hour, and 12-hour rainfall depths using the procedures outlined in this section. AUGUST 2002 RAINFALL The Rational Method can be used for drainage basins with a total contributing area of less than 160 acres. For drainage basins with an area greater than 90 acres, CHUP and/or HEC-1 computer models should be used to estimate the runoff data. CH5-1 03