FLOOD10386 ‎(2)‎

• Designation Number: 576
• County: Gilpin
• Community: Black Hawk
• Stream Name: North Clear Creek, Gregory Gulch and Chase Gulch
• Title: Floodplain Information Report - Black Hawk
• Date: 7/1/2004
• Prepared For: Black Hawk and CWCB
• Prepared By: Love and Associates
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Stream flow discharge is reduced by underground hydraulic conduits for segments of
North Clear Creek and Gregory Gulch. Along North Clear Creek, an 8-foot diameter
bypass pipe constructed near the Jacobs Casino was approved by FEMA in LOMR Case
No. 98-08-310P (Futura, 1996). During the 100-year flood, the bypass pipe will carry
approximatelY'~ 950~cfs, ~and~surface~flows~estimates~are-reduced,by~this-amount.~ ~It~was
assumed the pipe carries no discharge during the 500-year event. A closed flume is
located in Gregory Street. The Black Hawk FIS indicates the flume can carry 200 cfs
during the 100-year event (FEMA, 1984). Flood peaks for the 10-, 50, and 100-year

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During the 500-year event, it was assumed there is no reservoir storage, and the 500-year
flood peak discharges listed in the 1984 FIS have not been changed.

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The hydrologic analysis ncorporated storage/discharge curves for Dorothy Lee
Reservoir, a detention reservoir in Gregory Gulch, and Chase Gulch Reservoir, a water
supply reservoir located in Chase Gulch. Since Chase Gulch Reservoir is used for water
supply, it was assumed the reservoir is full and storage of floodwaters occurs only above
the spillway. The stage/storage/discharge relationship for Chase Gulch Reservoir was
obtained from as-constructed plans on file with the Colorado State Engineer's Office
(RMC, 1996). The stage/storage/discharge relationship for Dorothy Lee Reservoir was
taken from the Central LOMR (RMC, 1991).

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North Clear Creek watershed is dominated by Hydrologic Soil Group "D" based on
recent NRCS soils mapping (NRCS, 1998). Land use information was obtained from
CWCR Land use was subdivided into high density forest, low density forest, meadow,
high density residential, low density residential, and commercial. This information was
compiled and is shown on Figure 4. Using these data, average curve numbers were
calculated for each subcatchment.

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The study area was divided into catchments as shown on Figures 3A, 3B, and 3C,
Catchment Areas. Design points are defined at various points in the study area and were
placed at major confluences and at detention storage areas. Point precipitation estimates
for the 24-hour storm were obtained from the NOAA Atlas 2 (Miller et aI., 1973) and are
3.3, 3.0, and 2.2 inches for the 100-, 50-, and 10-year storms, respectively, for non-
snowfall events. Precipitation distribution used a SCS 24-hour "Type IA" precipitation
distribution for North Clear Creek and a SCS 24-hour "Type II" precipitation distribution
for Gregory and Chase Gulch.

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The update of the hydrologic analysis for North Clear Creek, Gregory Gulch, and Chase
Gulch was performed under the direction of CWCB and uses the SCS computer program
TR20. The model uses information on precipitation, soils, land use, geometric
parameters of the watershed, and hydraulic structures to estimate flood hydro graphs and
flood peak discharges.

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ENGINEERING METHODS