2018-02-07_PERMIT FILE - M1981148 (8)

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unit hydrographs developed by the CUHP and attenuation of same at various points in the model described below. Engineers are further guided by the Blue Book(Lakewood 1982)when evaluating storm drainage within city boundaries. This analysis follows UDFCD protocol with a check for variances introduced by the Blue Book. Colorado Urban Hydrograph Procedure As stated before, UDFCD recommends the use of the CUHP when catchments (term of art in the CUHP) are 90 acres and greater. Lakewood, in its Blue Book, recommends the use of the CUHP when the catchment is 130 acres or greater. Sub-catchments upgradient to the mine and sub- catchments contributing flow to the exterior drainage ditch bypassing the mine total 192 acres or so. The following discussion summarizes the input parameters and results for and from the CUHP. Rain According to UDFCD, Rainfall Chapter,the recommended 100-year storm duration for areas less than 15 square miles is 2 hours with no application of depth reduction factors. The CUHP software develops this storm automatically based on a point value derived from NOAA for a 1-hour 100 year storm. The NOAA value for the Lakewood gage is 2.22 inches (Attachment 1). The value for the Mt. Morrison gage is 2.21 inches (Attachment 2). Use 2.22 inches. The CUHP generates a two hour storm for a 100 year storm given a one hour 100-year point value. Attachment 3 documents the two hour storm generated by CUHP for a point value of 2.21. The Blue book also has a two hour storm that Lakewood prefers. This two hour storm was compared with the CUHP generated two hour storm in Attachment 3. The Blue Book storm has a greater peak 5 minute incremental flow and a greater total rainstorm than the CUHP NOAA Storm. A new rain gage was added to the CUHP program with the title Blue Book and contains the Blue Book storm. CUHP Model Mae 4 07/30/2017