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<br /> <br />guidelines. The peak rate of runoff is the same for the 2- and the 6-hour storms because <br />the rainfall distribution for the 2-hour storm is identical to the first 2 hours of the 6-hour <br />storm. The 6-hour design storm was selected for use at Rocky Flats because it tended to <br />give larger volumes of runoff than the 2-hour design storm. Results are compared with <br />independent studies by UDFCD and Advanced Sciences, Inc. (ASI). <br /> <br />Detailed hydrologic work for this report included the drainages terminating at Indiana <br />Street. This analysis fulfills the needs of DOE for facility and channel planning on the <br />Rocky Flats Plant site. Additional detailed hydrologic studies to include drainages east of <br />Indiana Street are reserved for separate study associated with the ongoing project to protect <br />the water supplies downstream of Rocky Flats known as "Option B." <br /> <br />Six runoff-event frequencies were analyzed: the 2-, 5-, 10-, 25-, 50-, and loo-year events. <br />This range of frequencies is adequate to characterize the hydrology of the area as it pertains <br />to drainage planning. It is consistent with national practice and local standards. More <br />severe runoff events (e.g., probable maximum flood) are beyond the scope of this document <br />and were not analyzed. <br /> <br />A study area extension, including the basins tributary to Great Western Reservoir and <br />Standley Lake, is included in this Master Plan for the sole purpose of providing a tie-in to <br />existing UDFCD master plans. <br /> <br />PREVIOUS STUDIES <br /> <br />As an added measure of safety, special analyses for much longer precipitation events were <br />conducted. These included consideration of 24-hour storms and up to lO-day precipitation <br />events so as to reflect "upslope storms" similar to those of May, 1969. Long-duration storms <br />were specially considered in relationship to detention and retention reservoirs and ponds. <br /> <br />In preparation for this analysis, the flood hydrology of the Rocky Flats site and previously <br />prepared drainage and flood-related reports pertaining to Rocky Flats were reviewed. <br /> <br />The reports reviewed are as follows: <br /> <br />The UDFCD Manual has provisions for reducing the storm precipitation used in the analysis <br />of basins larger than 5 square miles. The major basins in the study area (Walnut Creek at <br />Indiana Street, Woman Creek at Indiana Street, Upper Big Dry Creek at Indiana Street, and <br />Rock Creek at the 'Plant boundary) are all smaller than 5.0 square miles; hence, no areal <br />reduction was made. <br /> <br />1. <br /> <br />"Hydrology of a Nuclear-Processing Plant Site, Rocky Flats, Jefferson County, <br />Colorado." United States Geological Survey (USGS) Open-File Report 76- <br />268. March 1976 <br /> <br />2. <br /> <br />"Instruction Manual for Site Drainage Computer Model - Rocky Flats Plant, <br />Golden, Colorado." Lee Wan and Associates. February 13, 1987 <br /> <br />The flood hydrology analyses were specifically designed to allow for study of alternate <br />scenarios, and therefore, sub-drainage basin boundaries and flood design points were <br />defined to allow for these special considerations. This included no considerations of storm <br />sewer (local drainage systems) water transfers. <br /> <br />3. <br /> <br />"Flood Hazard Areas Delineation." Big Dry Creek. Westminster Area. <br />UDFCD. Greiner Engineering, November 1986, revised December 1988 <br /> <br />The analysis was interfaced with completed master drainage planning of UDFCD for the <br />Big Dry Creek basin. Extensive and costly flood studies were conducted by UDFCD in <br />conjunction with the City of Westminster and Jefferson County. Downstream interests have <br />relied upon these previous UDFCD flood studies and master plans. Flood inflows to Great <br />Western Reservoir and Standley Lake as published herein are similar to those previously <br />adopted by UDFCD. <br /> <br />4. <br /> <br />"Outfall Systems Planning." Big Dry Creek (ADCO) and Tributaries. <br />UDFCD. Muller Engineering, August 1987, revised January 1989 <br /> <br />5. <br /> <br />"Review Report of the Great Western Reservoir Pre-Feasibility Study." Pre- <br />Decisional Draft. Interagency No. DE-AI04-89 AL32905. U.S. Army Corps <br />of Engineers. October 1989 <br /> <br />6. "Design Recurrence Intervals Study." Advanced Sciences, Inc. Task 9. <br />August 28, 1990 <br />