FLOOD02883

• Designation Number: 266
• County: Broomfield
• Community: Broomfield
• Stream Name: Quail Creek & Tributaries & McKay Lake Basin
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Quail Creek and Tributaries and McKay Lake Basin, Broomfield, Westminster
• Date: 7/1/1986
• Designation Date: 11/1/1986
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Table 3. Table 4 provides similar information for the McKay Lake basin. For the
Quail Creek basin, the impervious percentage according to projected land uses
from planning and zoning maps, were taken from Figure 2-1, and Table 3-1 of the
runoff section of the "Urban Storm Drainage Criteria Manual" (USDCM) (Reference
6). The percent imperviousness for subcatchments in the McKay Lake basin was
obtained from a future land use map for the Big Dry Creek basin. The Big Dry
Creek watershed study was in progress at the time of preparation of this FHAD.
The surface area, width, and ground slope of the subcatchments were measured on
U.S. Geological Survey 7.5 minute quadrangle mapping of the area. The remainder
of the parameters are based on the soils and ground cover characteristics in the
basins, and were estimated from field inspection and from the 1979 Broomfield
FHAD, since the basins are similar in nature.

Existing irrigation ponds and ditches along the drainageways were assumed to be
full to capacity for this hydrologic analysis, therefore flows were assumed to
flow directly across the irrigation ponds and ditches.

Five types of conveyance elements can be used in the UDSWM2 program, a
trapezoidal channel, a circular pipe, a direct now element, and a channel or
pipe with an overflow section. The length of the element, width of channel or
diameter of pipe, invert slope, side slope of channel banks, the Manning's "n"
value, and maximum flow depth is described for each conveyance element. Tables 5
and 6 show the conveyance elements, and their dimensions, used for the Quail
Creek and McKay Lake basins, respectively. The conveyance element dimensions for
the Quail Creek and McKay Lake basins were based on future channel improvements
meeting established criteria for flow depth, velocity and channel slope as given
in the USDCM.

Detention Pond Routing
There is only one designated detention pond within the Quail Creek basin. It
lies within the Tributary "An sub-basin. This pond was incorporated into the
UDSWM2 model as a detention reservoir conveyance element. The detention storage
in acre feet versus outflow in cubic feet per second, was calculated for the pond
and input into the model. With this information, the UDSWM2 model was used to
route flows through the detention pond. The pond is located just upstream of
Zuni Street in the Westlake Subdivision, and is a retention/detention pond. For
this study the pond was assumed to be filled to the invert of the outlet when the
storm events occurred. Flows from the developed basin upstream of the pond are
significantly reduced below Zuni Street.

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