FLOOD03554

• County: Gilpin
• Community: Black Hawk
• Basin: South Platte
• Title: Hydrologic Analysis for the 1980 Black Hawk Floodplain
• Date: 2/2/2001
• Prepared For: Gilpin County
• Prepared By: Love & Associates, Inc.
• Floodplain - Doc Type: Flood Documentation Report

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The FIS, performed by Owen Ayers & Associates, Inc. in 1980 under the direction of
CWCB, reports the North Clear Creek drainage basin is approximately 34.3 sq. mi. in
area at the City water treatment plant, and Gregory and Chase Gulch are approximately
3.5 and 4.2 sq. mi., respectively, at their confluence with North Clear Creek. The three
streams are capable of transporting large volumes of water and debris at dangerously
high velocities and cause widespread damage. Historic flooding is reported within City
Limits along the three streams. Flood damage in the City is most common along
Gregory and Chase Gulch.

The watersheds are largely undeveloped; most of the area is forested. Steep channel
slopes result in extremely high velocities in North Clear Creek and its tributaries during
flood events. North Clear Creek passes through the City flowing adjacent to State
Highway 119.

North Clear Creek, Gregory Gulch, and Chase Gulch in the City are regulated under the
FEMA NFIP program.

North Clear Creek, Gregory Gulch, and Chase Gulch flood peaks were analyzed in 1980
under the direction of the CWCB using graphical methods. A 6-hour design storm was
used for the analysis.

Since 1980, Gregory Gulch hydrology was revised using an HEC-l hydrologic model
developed for City of Central in 1991 which included Dorothy Lee Reservoir, and flood
peaks developed for the model were adopted by FEMA. This model was extended
through Black Hawk to North Clear Creek in 1996 for a CDOT project. A water supply
reservoir for Central City was constructed within the Chase Street watershed. The City
has expanded its boundaries along North Clear Creek since 1980.

In the 1980 study, 100-year flood peak estimate for North Clear Creek are 2050 and 2670
cfs at upstream and downstream Black Hawk City limits, respectively. The 100-year
flood peak estimate for Gregory and Chase Gulch within City Limits are 1240 and 1160
cfs, respectively, in the ]980 study. The Gregory Gulch 1991 HEC-] study estimated the
100-year flood peak was 969 cfs at the city boundary between Central City and Black
Hawk. The 1996 revised hydrologic analysis for Gregory Gulch reported a 100-year
discharge of 991 cfs at the city boundary and 1006 cfs for the outfall at North Clear
Creek.

PROJECT MANAGEMENT SUMMARY:

At Love & Associates, Inc., all projects are managed by one of the principals. The lines
of authority and coordination are clearly defined, not only internally, but between Love
& Associates when we are the prime consultant and our subconsultants, where
appropriate.

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