FLOOD07708

• Designation Number: 317
• County: Boulder
• Community: Unincorporated Boulder County
• Stream Name: Bullhead Gulch and Tributaries
• Basin: South Platte
• Title: Flood Hazard Area Delineation - Bullhead Gulch and Tributaries
• Date: 7/1/1991
• Designation Date: 11/1/1991
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Hydrology was developed using future land use imperviousness and existing channel conditions,
The 5,790-acre basin was divided into 70 sub-basins for an average size of 82 acres. These sub-
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basins were delineated from the 200 scale, 2-foot contour interval topography provided by
Landmark Mapping, Ltd. (1990), Figure 6 gives sub-basin delineations, routing elements and
design point location, The SWMM flow chart, Figures 7 and 8 are included which schematically
depicts the routing model. Table 5 is a tabulation of the input parameters used in the CUHPE
computer model, and Table 6 provides an input data listing for the SWMM computer model.

For the purposes of this study, the roads, railroads, reservoirs and irrigation ditches were
neglected as obstructions to the drainage path, These structures were not analyzed as "flow
attenuation" facilities. Only the two detention ponds located along the southwest side of the
Burlington Northern Railroad embankment were included in the SWMM model. These ponds
are owned and maintained by the City of Louisville.

For the future conditions hydrology, the CUHP and SWMM models were developed to reflect
the proposed changes anticipated within Bullhead Gulch watershed, For the CURP model, a
land-use percent impervious value reflecting wning map projections was used, The time of
concentration (for basins of 90 acres or less) was determined using the formula 1" = L/l80 + 10
= minutes. For the SWMM model, the channel geometry for the routing elements was
established based on discussions with the District. An incised low flow channel was added to
the model in order to simulate the natural erosion process of channel bottoms as a sub-basin
becomes urbanized, The geometry of the low flow channel was approximated, based on a
carrying capacity of 10 - 20 percent of the future conditions loo-year runoff. In addition, the
overflow channel side slopes were set at a maximum of 20 (horizontal) to I (vertical),

FLOOD HAZARD AREA DEUNEATION
BUWlEAD GULCH AND lRIBUTARIES

FINAL
7/31/91

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