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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 1O.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
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