FLOOD10485

• County: Arapahoe
• County: Douglas
• Stream Name: Big Dry Creek
• Basin: South Platte
• Title: Big Dry Creek ‎(Arapco)‎ & Tributaries - Major Drainageway Planning - Phase B Preliminary Design Report
• Date: 4/1/1998
• Prepared For: Arapahoe County, Douglas County, Englewood, Greenwood Village, Littleton, UDFCD
• Prepared By: WRC Engineering, Inc.
• Floodplain - Doc Type: Flood Mitigation/Flood Warning/Watershed Restoration

E. BASELINE MODELLING RESULTS
A summary of the baseline peak runoff values at primary design points is presented in Table ITI-4.
It can be seen from this table that the peak flows in this study are substantially less than those
computed in the 1975 Major Drainageway Planning report prepared by VTN (Reference 10). The
difference in peak runoff rates may be attributed to several factors. The hydrology for the VTN
study was developed in 1975 and used an earlier version ofCUHP, used a 6-hour design storm and
divided the study area into only five watersheds. In addition, the conservation area which now exists
in the upper part of the Big Dry Creek watershed was considered to be fully developable in the 1975
study. The reduced values in WRC's study for the imperviousness of the sub-watersheds within this
conservation area resulted in lower peak flows. Another major factor which reduces peak flows is
the inclusion of constructed publicly owned and maintained detention ponds. Routing the flows
without consideration of detention resulted in a marked increase in peak flows at various design
points, which can be seen from Table 1II-5. The VTN study did not include any detention in the Big
Dry Creek watershed. Also included in Table ITI-4 are the 100-year peak runoff values currently
uti I ized by the Federal Emergency Management Agency (FEMA) (Reference 11) for flood insurance
administration purposes. These values were obtained from the VTN study's existing land use
conditions hydrologic analysis results. The VTN (1975) values represent the full build out
hydrology as estimated in 1975. The WRC (1995) values represent the current hydrologic analysis.
Development ofthe undeveloped areas within the Big Dry Creek watershed is currently occurring
at a rapid pace. Projections indicate that at the current rate of growth, the small remaining
undeveloped areas will be fully developed within a few years, if not sooner. A more complete
discussion of the hydrologic analysis and results is included in Reference 16.
A large num ber of conveyance elements were required to route the generated runoff hydrographs.
Since there is a limited number of elements allowed in the SWMM program, an element was not
assigned to the outflow point of each sub-watershed. However, an additional S WMM run was made
with no routing to prepare a concise sub-watershed peak flow summary of the CUHP-generated sub-
watershed flows. This summary of sub-watershed peak flows is presented in Table III-6
111-14

F, MASTER PLAN MODELING RESULTS
As previously mentioned, the modelling prepared for the master planned improvements differs only
in that new detention facilities are proposed in the upper reaches of Tributary A and an additional
detention facility is proposed on an unnamed tributary to Big Dry Creek. These facilities are more
fully discussed in Chapter vn of this report and were included in the SWMM model as Design
Points 125, 143, and 285. Considering these detention facilities, typical runoff hydrographs at
primary design points are presented in Figures IU-3A thru 3D. A summary of peak flows for the 2-,
5-, 10-,50- and I OO-year precipitation events is presented in Table ITI-7, and shown graphically in
the Peak Flow Diagrams, Figures III-4A thru 4C. Routing schematics for the Big Dry Creek
watershed including the three new master planned detention facilities are included in Figures II1-5A
thru 5F

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