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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Table 5. Flood Peaks for the 10-,50-, and 100-Year Events at Selected Design Points
Including Storage Effects of Detention Along Chase and Eureka/Gregory Gulches.

Cl
C2
G1
G2

NCl
NC2
NC3
NC4
NC5
NC6

Note: See Figure 4 for design points.

Table 5 indicates the reservoirs slightly reduce flood peaks along North Clear Creek. At
design point NI at the City limits, for example, the reservoirs reduce 100-year flood
peaks from 2702 to 2520 cfs.

Table 5 indicates the time of concentration for North Clear Creek is less than the time of
concentration for Gregory and Chase Gulch, which is counter-intuitive given the relative
size of the watershed areas. The time of concentration difference is partly due to the
different precipitation distributions used for the model. The ''Type IA" distribution is
less peaked and, as a result, causes a later peak than the "Type II" distribution. Flood
peak attenuation by Chase Gulch Reservoir also increases the time of concentration for
Chase Gulch.

In the 1979 study, the SOO-year flood peak for North Clear Creek equals 3800 cfs
upstream of Chase Gulch and 4950 cfs at the wastewater treatment facility, and the 500-
year flood peak for Gregory Gulch and Chase Gulch equals 1790 cfs and 2100 cfs,
respectively, at their confluence with North Clear Creek. These flood peak estimates
were identified using frequency analysis. Since the reservoirs are assumed ineffective
during the SOO-year event, the 500-year flood estimates are not modified by the TR20
model study.

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