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. . . CHAPTER 5 HYDROLOGY SECTION 1.0 RAINFALL COLORADO STATEWIDE DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL The methodology used to generate the rainfall-runoff data will depend on the size of the drainage basin to be studied. The Rational Method for determining runoff is widely accepted as providing a sufficient level of detail for generating runoff from relatively small basins and can be used for drainage basins with a total contributing area of less than 160 acres. The Rational Method utilizes rainfall data in the form of time-intensity-frequency curves. Since the assumptions used in the Rational Method become less valid for larger areas, larger basins require a more rigorous analysis to generate runoff data. For drainage basins with an area greater than 90 acres, CUHP and/or HEC-1 computer models should be used to estimate the runoff data. For drainage basins with an area between 90 acres and 160 acres, either the Rational Method or the computer models may be used. For drainage basins with an area between 90 acres and 160 acres, either the Rational Method or a computer model (HEC-1 or CUHP) may be used. The CUHP (Colorado Urban Hydrograph Procedure) method has been used widely within the Urban Drainage and Flood Control District (UDFCD) jurisdictional area. The CUHP method was developed and calibrated to effectively model short duration convective storms within the Denver Metro area. Therefore, the CUHP method should only be used for urban areas that have similar hydrologic characteristic as the Denver Metro area. The CUHP model can be used to generate sub-basin hydrographs and the UDSWM (Urban Drainage Storm Water Management) computer program can be used to route and combine hydrographs. For detailed discussions on the CUHP and UDSWM methods and programs, please refer to the latest UDFCD Drainage Criteria Manual. The HEC-1 computer model developed by the US Army Corps of Engineers is a commonly used rainfall-runoff simulation model. HEC-1 model can be used to generate runoff hydrographs for both short and long duration storms based on various hydrologic analysis methods. HEC-1 model can route and combine flows of multiple drainage basins. The SCS Unit Hydrograph method within HEC-1 model is recommended for use in the State of Colorado. The SCS Unit Hydrograph method has been widely used and accepted in the westem United States. The US Army Corps of Engineers' Window-based hydrologic program HEC-HMS may be used in place of HEC-1 when appropriate. 1.3 RAINFALL DEPTHS The 6-hr and 24-hr point precipitation values for 2-, 5-, 10-, 25-, 50-, and 100-year storm events can be estimated directly from Figures CH5-F101 through CH5-F112. The point precipitation values for each storm duration (6- and 24-hr) obtained from the isopluvial maps should be plotted on the return-period diagram (Figure CH5- F113), and a straight line of best fit should be drawn. If any rainfall value deviates substantially from the best-fit line, the value read from the line should replace the original point precipitation value from the map. Once the 6- and 24-hr rainfall values have been obtained and adjusted (if necessary), the rainfall depths for other durations can be estimated using the following procedures from the NOAA Atlas II, Volume III, 1973. The State of Colorado has been divided into four (4) geographic regions by NOAA and they are shown on Figure CH5-F114. Before applying the empirical methods outlined below, it AUGUST 2002 CH5-104 RAINFALL