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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STUDY AREA DESCRIPTION

Community Description

The City of Black Hawk is located within Gilpin County, Colorado, on the eastern side of
the Rocky Mountains in central Colorado. Please refer to the Vicinity Map, Figure 1.
Central City abuts Black Hawk to the west. North Clear Creek, Gregory Gulch, Chase
Gulch, and Fourmile Creek flow through the City. The climate is relatively dry and
averages approximately 17 inches of precipitation per year. Mean monthly temperatures
range from a low of approximately 280 F in January to a high of 630 F in July.

Watershed Description

Black Hawk is located east of the continental divide within the North Clear Creek
watershed, part of the Clear Creek and South Platte River Basin. The watershed is
characterized by narrow valleys bordered by steep slopes ranging from approximately 20
to 60 percent grade. Elevations in the City range from approximately 7600 to 9600 feet
ms!. 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. High density development within the watershed is
concentrated in the valley areas, and intermittent low density developments are scattered
throughout the watershed. The watershed is largely undeveloped, covered primarily with
sub-alpine forest vegetation of varying density, and meadows are scattered throughout the
forested area. The City 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 altered historic stormwater flow paths,
runoff characteristics, and surface water quality. Many of the alluvial deposits adjacent
to North Clear Creek have reportedly been completely re-worked by mining activities.

The North Clear Creek drainage catchment is approximately 34.9 square miles in area at
the lower City boundary and 61.3 square miles at its confluence with Clear Creek. The
drainage catchments for Gregory Gulch, Chase Gulch, and Fourmile Gulch are
approximately 3.5, 4.2, and 1.6 square miles, 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 occasionally cause widespread damage.

Black Hawk is experiencing rapid commercial development. Most new development is
occurring adjacent to North Clear Creek, although infrastructure is being constructed for
future upslope commercial and residential development.

Floodin2 Problems and Floodin2 History

A comprehensive history of flooding in the City prior to 1980 is presented in Flood
Information Report (Owen Ayres & Associates, Inc., 1980). Briefly, floods in Black
Hawk are reported during 1864, 1881, 1883, 1898, 1910, 1942, and 1975. High water

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