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. .. . DRAINAGE CRITERIA MANUAl (V. 1) RAINFAlL 3.0 DESIGN STOI~ DISTRIBUTION! FOR CUHP The 1-hour point precipitation value described in Section 2 is distributed into 5-minute increments for use with CUHP model (i.e., temporal distribution). This is described in Section 3.1 and summarized in Table RA-2. The rainfall duration used with CUHP varies with the size of the watershed being aoolyzed. Also, for watersheds 10 square miles or greater, there is an adjustment made to the incremental precipitation depths to take into account the greater watershed size (i.e., area adjustment). This is described in Section 3.2 and summarized in Table RA-3. A sllJmmary of the stann duration and whether area adjustments for different watershed sizes are ne4!!ded is provided in Table RA-1. n~ RA-1 Storm Duration and Are WatellShed Area (square miles) Suggested Minimum Storm Area Adjusbnent Required? Duration Less than 10.0 2 hours No 10.0 to 20.0 3 hours Yes 20.0 and Iaraer 6 hours Yes 3.1 Temooral Distribution The cunent version of CUHP (see RUNOFF) wa; designed to be used with the NOAA 1-hour rainfall depths described in Section 2.1. The 1-hour rainfall depths for areas within the District are provided in FigUres RA-1 through RA-6. To obtain a temponil distribution for a design storm for use in the Denver region,1the 1-hour depth is transferred into a 2-hclUr design storm by multiplying the 1-hour depth(s) by the percentages for each time increment given in Table RA-2. The resultant design stormes) may then be used wiIIh CUHP. The total of aU the incremental depths for the first: hour of the design storm does not agree with the 1-hour depth used to develop the design stann. 00 not lbe alarmed. The temporal distribution presented in Table RA-2 represents a design stonn for use wDh a distributed rainfall-runoff routing model. The distribution is the result of a calibration process perfonned by the District to provide, in conJunction with the use of CUHP, runoff flow peak rates and volumes of the same retum period as the design storm. The NOAA Atlas values are -embedded- in the 2-hour and other duration design stonns. In order to develop the temporal distribution for the 3-hour design storm (for watersheds between 10.0 andl20.0 square miles in size), first prepare the first two hours of the storm using the 1-hoUlr storm point precipitation and the temporal percentage distribution shown in Table RA-2. The difference between the 3-hour point precipitation ~m Equation RA-3 and the 2-hour point precipitation (Table RA-2) is then distributed evenly over the period of 125 minutes to 180 minutes. In order to develop the temporal distribution for the 6-hour design stonn (watersheds greater than 20.0 square miles). first prepare the first 06f2001 lhban Or.3inage and Rood Control District RA-3