|
<br />
<br />4,
<br />
<br />The 16,75 square mile area upstream of Standley Lake has an existing
<br />imperviousness of 19 percent which increases to 23 percent for future
<br />development conditions, Initial infiltration loss rates for this basin are 3,1
<br />inches per hour, Other detailed data are provided,
<br />
<br />"Outfall Systems Planning." Big Dry Creek (ADCO) and Tributaries.
<br />UDFCD. August 1987, revised January 1989. Muller Engineering.
<br />
<br />Subiect:
<br />
<br />This master drainage plan was prepared for UDFCD to provide preliminary designs for
<br />drainage and flood control for Big Dry Creek and tributaries,
<br />
<br />Tl.~ U'n'_..' r'~441, h~<;n "ndr4~m Of Great Western Re-ervol'r has <; 0 square
<br />.l.lJ~ .yaJJ1UI. '-'l....."'.n. V~111 Uy.,...l......U...... ...... ~ ... .. ....,0 ..
<br />
<br />miles of area with both an existing and developed imperviousness of 34
<br />percent. This basin has an initial infiltration loss rate of 3,1 inches per hour.
<br />
<br />The substantive data and findings of the report have been approved by UDFCD,
<br />Sponsoring governmental agencies include Adams County, Jefferson County, Jefferson
<br />County Airport, City of Broomfield, and the City of Westminster.
<br />
<br />5.
<br />
<br />The report includes Table 7 which provides sub-basin parameters for Big Dry
<br />Creek tributaries as prepared by Muller Engineering Company, Inc, in the
<br />parallel study for drainage master planning, However, the Muller data for
<br />basins upstream of the reservoirs is not included in the Greiner report,
<br />
<br />Background data and calculations for the report are included in a Technical Appendix, The
<br />report is to be used in conjunction with a 1988 parallel study for Big Dry Creek entitled
<br />"Flood Hazard Areas Delineation Big Dry Creek, Westminster Area" by Greiner
<br />Engineering,
<br />
<br />6,
<br />
<br />The future conditions 100-year discharge profile assumes the proposed future
<br />spillway configuration for Standley Lake and a base flow from the outlet works
<br />of 430 cfs upstream of the confluence with Walnut Creek at the Denver-
<br />Boulder Turnpike, A combined base flow of 480 cfs from both Standley Lake
<br />and Great Western Reservoir would exist downstream of the confluence with
<br />Walnut Creek,
<br />
<br />Muller Engineering assumes in this report that the Great Western Reservoir Spillway will
<br />be enlarged, The present spillway is too small to pass the 100-year flood, The State
<br />Engineer of the State of Colorado has a "hold order" on Great Western Reservoir,
<br />restricting its operation as a darn safety precaution,
<br />
<br />Assumntions
<br />
<br />7,
<br />
<br />Figure 2 of the report shows a discharge below the Standley Lake spillway for
<br />the 100-year flood of about 1850 cfs and 100 cfs for the 2-, 5-, and 10-year
<br />floods,
<br />
<br />The study and report were prepared by experienced engineers using accepted and approved
<br />criteria .for the Denver metro area.
<br />
<br />8, The future conditions 100-year discharge profile on Walnut Creek assumes the
<br />proposed future spillway configuration for Great Western Reservoir and a
<br />base flow from the outlet works of 50 cfs, The configuration is not defined,
<br />
<br />Limitations
<br />
<br />1. The study area extends from the outlets of Great Western Reservoir and
<br />Standley Lake to 144th Avenue, It does not include the basins upstream of
<br />the two reservoirs.
<br />
<br />9, Table 8 of the report is taken from the parallel Muller report, The Muller
<br />discharge for the upstream limit for the study of Walnut Creek, as presented
<br />in Table 8, is 1840 cfs,
<br />
<br />2, The flood discharges out of Great Western Reservoir and Standley Lake were
<br />used as input to the upstream end of the hydrologic model. There is little
<br />reference to the basins tributary to the two reservoirs,
<br />
<br />10, Sheet 3 of the report presents a map of the Walnut Creek and Woman Creek
<br />basins upstream of Standley Lake and Great Western Reservoir. Walnut
<br />Creek is identified as Basin 672 and Woman Creek is combined with Big Dry
<br />Creek as Basin 075.11
<br />
|