FLOOD08905

• County: Gilpin
• Community: Black Hawk
• Basin: South Platte
• Title: 1998 Black Hawk Flood
• Date: 9/8/1998
• Prepared For: Clear Creek County
• Prepared By: Consultants
• Floodplain - Doc Type: Flood Documentation Report

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The majority of basins upgradient of Bobtail Street are zoned for mInIng activities.
Lands immediately adjacent to Bobtail Street are zoned for core gaming, transitional
gaming, and residential use. Development is planned or has occurred in Basin BI
(Miners Mesa), Basin B2 (pump station), and Basin B4 and B5 (Crooks Palace Casino).
Additional development is likely to occur along portions of Bobtail Street once the Street
is constructed.

Since drainage basins upgradient of Bobtail Street are less than 70 acres in area, the
Rational Method is used to analyze stonn peak discharges.

Time of concentrations used in the analysis include both overland (Equation 3-3 in
USDCM) and channelized flow components. Flow lengths and channel slopes are
estimated using aerial topographic maps at a scale of 1":2,400'. Flow path modifications
by spoil piles, when significant, are incorporated in the study. Overland flow distances
are assumed to equal 250 feet for basins less than 60 acres in area and 500 feet for basin
areas larger than 60 acres.

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Average channel slopes of the basins all exceed 20 percent. Channel velocities are
calculated using the Manning's equation and a trapezoidal-shaped channel with 3:1 side
slopes. It is assumed the channels flow at critical velocity. Because concentrated flows
in the Black Hawk area historically transport large amounts of debris and sediment,
assuming critical flow rather than subcritical flow will yield shorter times of
concentration for the basins. Critical velocity is calculated by increasing the average
roughness coefficient in the Manning's equation until the Froude Number equals
approximately 1. Because of the steep slopes, the time of concentration for most basins
are on the order of 10 minutes, as shown within Table 1. Channel velocity calculations
used to calculate times of concentrations for channelized flows are presented on Table 2.

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Stonn peak discharges are calculated for the 5- and 100- year events. Rainfall intensities
were obtained from TimelIntensity/Frequency Curves representative of the City of Black
Hawk (Appendix). The basins are assumed to be zoned for mining activities and portions
of the basins are likely to be rezoned for other types of development. Basin soil
classifications have not been obtained. However, about half of the Fourmile Gulch/Silver
Gulch Basins (located, in part, within the City north of North Clear Creek) are reported to
be Hydrologic Soil Group D (Reference 3). Assuming a similar mix of soils in the basins
draining to Bobtail Street, undisturbed basin soils are represented as a combination of
Lawns, Sandy Soil and Lawns, Clayey Soil on Table 3-1 of the USDCM, and Rational
"c" coefficients for undisturbed soils equal approximately 0.09 and 0.35, respectively,
for the 5- and I OO-year events.

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The Rational "c" coefficients used for this study for the 5- and I OO-year events equal
0.47 and 0.65, respectively, as recommended in Table 3-1 of the USDCM for
undeveloped, off site flows for basins whose land use is not defined. These coefficients
are used, in part, because unknown portions of the basins draining to Bobtail Street have
been historically impacted by mining activities. These impacts include unvegetated spoil

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