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Secondly, peak flows and runoff volumes were compared to previous studies by ASI, USACE, and UDFCD. The other studies used some different assumptions and also different modelling techniques than were used in this study. However, comparison between studies can still be made by careful analysis of the data from the previous studies. The UDFCD-ca1culated peak inflow to Standley Lake compares closely with the value calculated in this Master Plan (4 percent to 22 percent higher). The relevant different assumptions used in the previous studies are as follows: FUTURE BASIN HYDROLOGY 1. ASI used the SCS TR-20 computer model for the hydrologic analysis. ASI assumed that Coal Creek would spill to ditches that cross the plant site and contribute to peak flows on site. Site surveys for this Master Plan indicated that this would not occur, and therefore Coal Creek flows were not included in this study. Spill flows were subtracted from ASI flows before comparison to Master Plan flows. Because the total spill volumes are not provided by ASI, runoff volume cannot be compared for design points that receive spill flows. This 1992 Master Plan, to be consistent with the previous UDFCD master plan for the Big Dry Creek basin, uses similar future projections of basin development. UDFCD assumed that future urban development would be modest. To follow up on this assumption, the 1991 zoning maps of Jefferson County were examined as they relate to a new development, Jefferson Center. Meetings were held with Arvada, Jefferson County, Westminster and the UDFCD. A meeting was also held with Mr. Howard Lacy, President of the Jefferson Center Metropolitan District. Mr. Lacy said that the Jefferson Center will most likely utilize a complete stormwater detention system and would thereby have little effect on Rocky Flats' drainage planning. 2. The USACE (USACE, 1989) study used a different design storm precipitation distribution than the present study. It used the HEC-l model for hydrologic analysis. The study included only the scenario of the ponds being empty at the outset of the storm. As a result, a timeline was established for development projection to the year 2015 (25 years) and it was concluded that the UDFCD future land use projections as used in their Master Plan for Big Dry Creek (Greiner Engineering, 1986) would be applicable, at least through the year 2015. In the event that these projects did not materialize, this Master Plan could be modified. 3. The UDFCD study was based on the CUHP model, but used future development conditions for the plant area with a higher impermeable area than used in this Master Plan. This tended to slightly increase the UDFCD peak flow and runoff volume for Walnut Creek. INFLOW TO STANDLEY LAKE/GREAT WESTERN Standley Lake and Great Western Reservoir are located downstream and east of the Master Plan study area. It should be noted that the previously approved UDFCD plans are not affected. Table IV-ll presents a summary of flood magnitudes adopted for this Master Plan. ASI-ca1culated 100-year peak flows compare well (5 percent higher to 23 percent lower) with those calculated for this report with two exceptions. First, runoff values at the Woman Creek Diversion Dam and to Pond C-2 are lower in the ASI analysis. The Coal Creek Spill flow was subtracted directly from the ASI value before comparison was made. No spill flow hydrographs were presented by ASI; therefore it is not possible to verify that the peak spill flow and peak basin flow are directly additive. Second, peak flows below the Core Area in the Walnut Creek basin are higher (40 percent to 170 percent) than those calculated by AS!. The Master Plan flows show a rapid routing of high Core Area runoff to correspond to slower runoff peaks from the undeveloped areas farther upstream.