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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5. Define flood hydrology for the 500-year storm event using graphically-based
probability analysis or other methods as approved by the CWCB,
6. Prepare a final report, and,
7. Coordinate with CWCB and perform a field review of the watershed.

Love & Associates, Inc., already has in its possession much of the information required
to perform the study. As a result of our long standing position as drainage and flood
control engineer for the City of Black Hawk, we have additional information which may
be useful in completing the study, which could be used at the discretion of the CWCB.
This information includes such items as:

I. Survey for the City obtained during 1999 (BCA Group, 1999),
2. Topographic mapping for the City flown during November 1998 with 2-foot
contours electronically filed on AutoCAD Version 14 (ACAD-14) (Landmark
Mapping, 1998),
3. USGS digitized topographic maps,
4. Draft soils maps obtained from Mr. Eugene Backhaus, Resource Conservationist
for NRCS, for Gilpin County, Colorado,
5. HEC-I analysis on floppy disc by J.F. Sato, Inc. for Gregory Gulch,
6. Flood Insurance Study (FIS) (FEMA, 1984) and Flood Insurance Rate Map
(FIRM) (FEMA, 1984) prepared for the City under the direction of CWCB, and,
7. Owen Ayers & Associates, Inc., 1980 study and Technical Appendix prepared for
the City of Black Hawk and the CWCS.

UNDERSTANDING OF THE PROJECT

The study area is located east of the continental divide in the foothills area of Colorado.
The study area is characterized by narrow valleys bordered by steep slopes ranging from
approximately 20 to 60 percent. Elevation within the study area ranges from
approximately 7,500 to over 12,000 feet msl. The steep upslope areas are covered with a
variety of rock outcrop, thin residual soils on bedrock, and thicker debris, alluvium, and
slope wash deposits and are vegetated with grasses, trees, and shrubs. Deeper soils and
wetland vegetation are found on alluvial deposits adjacent to streams. The study area has
been subject to underground and surface mining and residential and commercial
development. Roads, buildings, and mining-related features such as mine shaft openings
and tailings have sometimes altered historic storm water flow paths and runoff
characteristics.

The City of Black Hawk (City) is located within Gilpin County, Colorado. North Clear
Creek, Gregory Gulch, and Chase Gulch, part of the Clear Creek and South Platte River
Basin, flows through the City. North Clear Creek flows eastwardly through the City
adjacent to Highway 119, eventually joining Clear Creek near the intersection of
Highway 119 and 6. Gregory Gulch flows from City of Central into the City on State
Highway 279, passing through part of the business district. Chase Gulch flows through
the City along Chase Street, passing through a residential area. Gregory and Chase
Gulch join North Clear Creek within City limits.

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