FLOOD05980

• County: Gilpin
• Community: Black Hawk
• Stream Name: North Clear Creek, Gregory Gulch, Chase Gulch
• Basin: South Platte
• Title: Reconstitution of 1979 Flood Peaks, North Clear Creek, Gregory Gulch& Chase Gulch
• Date: 4/26/2001
• Prepared For: CWCB
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Flood Documentation Report

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CONCLUSION

Flood peaks developed during 1979 for North Clear Creek, Gregory Gulch, and Chase
Gulch for the City of Black Hawk were reconstituted using the SCS TR20 computer
model. Similar to the 1979 analysis, the TR20 model uses a SCS "Type IA" distribution
for North Clear Creek and a SCS "Type II" precipitation distribution for Gregory and
Chase Gulch. Assuming Hydrologic Soil Group "B" soils, an ARC (AMC) of II, and
1979 land use, an average Curve Number of 79 was calculated for the study area. The
curve number was used to calibrate the TR20 model to 1979 flood peak calculations, and
the calibration resulted in a slight lowering of the average Curve Number in the study
area to 75.5. The Curve Number of 75.5 is within the range of curve numbers used for
other studies in the catchment which, ignoring the unrealistically low curve number of 55
used by Futura, ranged from 69 to 85_

The construction of reservoirs within Chase and Gregory Gulch watersheds reduce the
magnitude of flood peaks downstream of the reservoirs. Chase Gulch Reservoir reduces
the magnitude of historic lOO-year flood peaks by approximately 50 per cent at the outlet
of Chase Gulch. Chase Gulch Reservoir was modeled assuming the reservoir is full, and
inadvertent flood detention storage is provided above the emergency spillway only. The
emergency spillway elevation/discharge relationship developed by the design engineer
for the Reservoir was used in the TR20 model. Dorothy Lee Reservoir, a detention
facility, has only a minor effect on the magnitude of lOO-year flood peaks at the outlet of
Gregory Gulch. At the downstream limit of the study area along North Clear Creek, 100-
year flood peaks are reduced by approximately 200 cfs.

RECOMMENDATIONS

Flood peak discharges for the 10-, 50-, and IOO-year flood events in North Clear Creek,
Gregory Gulch, and Chase Gulch calculated by the TR20 model with Chase and Dorothy
Lee Reservoirs summarized in Table 5 should be adopted by the Colorado Water
Conservation Board. The flood peaks shown in this table represents current (2001)
conditions in the watersheds. Since the reservoirs are assumed ineffective in reducing
flood peaks during the 500-year event, 500-year event flood peaks developed in 1979
using frequency analysis will not change.

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