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