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. . . CHAPTER 5 HYDROLOGY SECTION 1.0 RAINFALL COLORADO STATEWIDE DRAINAGE AND FLOODPLAIN MANAGEMENT CRITERIA MANUAL with the depth-area adjustment application procedures, UDFCD developed an adjustment factor table for drainage basins between 10 and 75 square miles. The UDFCD table is included in this section as Table CH5-T104. The 3- and 6- hour storm distribution values can be adjusted by multiplying each incremental rainfall depth by the appropriate adjustment factor from Table CH5- T1 04 for a given time increment and the size of the drainage basin. An example showing the development of a CUHP design storm distribution is given in Section 1.7.3 of this chapter. 1.6 RAINFALL DISTRlBTION FOR SCS UNIT HYDROGRAPH METHOD IHEC-1 or HEC-HMSl The HEC-1 computer program was developed by the US Army Corps of Engineers Hydrologic Engineering Center to assist engineers in performing rainfall-runoff analyses for both short and long duration storms based on various hydrologic analysis methods. The SCS Unit Hydrograph method within the HEC-1 program 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 rainfall depth data used in the HEC-1 model should be a centrally distributed storm event with depths at time intervals of 5-minutes, 15-minutes, 60-minutes, 2- hours, 3-hours, 6-hours, 12-hours, and 24-hours for the desired recurrence frequency. The NOAA procedures to determine these rainfall depth values were discussed previously in Section 1.3. The 24 -hour rainfall distribution is centered around the midpoint of the design storm (time = 12 hours), and is commonly known as "Balanced Storm" distribution. These rainfall values are input into the HEC-1 model using the PH record. When using the PH record, a value of 0.001 should be input into Field 2 to prevent the program from using an intemal point rainfall reduction adjustment. 1.6.1 DEPTH-AREA REDUCTION FACTORS Figure CH5-F115 provides the depth-area reduction curve for the 24-hour storm event (NOAA, 1973). Depth-area values are input into the HEC-1 model using the JR record. The peak flow value at a given point should be determined using the depth-area value for the total watershed area tributary to the subject point of interest. 1.7 EXAMPLES The following examples are provided to lead the reader through the rainfall distribution development procedures outlined in this section by analyzing a hypothetical drainage basin. 1.7.1 EXAMPLE 1: RAINFALL DISTRIBUTION CALCULATION FOR HEC-1 INPUT Problem: Develop the 100-year, 24-hour design storm distribution for Basin A. Solution: AUGUST 2002 RAINFALL CH5-108