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

I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I

(flooding) was reported during five other years, most recently during 1998, when
flooding and debris transport along Unnamed Draw damaged parked vehicles as well as
the Golden Gate Casino, and Gregory Street experienced street flooding over the top of
the curb and debris deposition on the street. In addition, City officials have reported
street flooding at various locations within the City during the summer months. Flooding
along major streams within the City is reported along North Clear Creek, Gregory Gulch,
Chase Gulch, and Fourmile Gulch. Flood damage is most common along Gregory and
Chase Gulch.

Three possible snowmelt-related flood events are reported in February and May, while
eight are thunderstorm-produced events occurring in July to mid-August. Flooding was
reported to be produced by thunderstorms occurring alone and in succession.

Previous Studies

Flood peaks for the 10-, 50-, and 100-year events for North Clear Creek, Gregory Gulch,
and Chase Gulch were originally calculated by CWCB in 1979 using the graphically-
based triangular hydro graph method, and the 500-year flood peak was calculated using
frequency analysis. Using these flood peaks, floodplains were delineated under the
direction of CWCB and presented in Flood Information Report (Owen Ayres &
Associates, Inc., 1980). These flood peaks were used for floodplain delineation on the
Black Hawk FIRM (FEMA, 1984). The 1984 FIRM is reproduced on Figure 2.

The 10-, 50-, and 100-year flood peak estimates were updated under the direction of
CWCB using the TR20 computer model (Love & Associates, Inc., 2001). The analysis
identified the impact of two reservoirs, Dorothy Lee Reservoir on Gregory Gulch and
Chase Reservoir on Chase Gulch, which were constructed after 1980 FIS. Dorothy Lee
Reservoir was constructed in 1991 for flood control in Central City (RMC, 1991) and
was incorporated into the Central City FIRM. Chase Reservoir was constructed as a
water supply reservoir for Central City along Chase Gulch and was completed in 1996
(RMC, 1991). Chase Reservoir provides inadvertent detention storage above its
emergency spillway. The flood estimates in the hydrologic report (Love & Associates,
Inc., 2001) were officially adopted by the CWCB during a board meeting on May 21,
2001.

Recent channel improvement proj ects which have been approved by FEMA in this
corridor as LOMRs or CLOMRs include Jacobs Lodge Casino (Case No. 98-08-3l0P),
Jacobs Parking (Case No. 96-08-227R and 02-08-526P), Mardi Gras Casino (Case No.
98-08-309R and 03-08-0439R), and Hyatt Black Hawk Resort and Casino for Fourmile
Gulch (Case No. 01-08-089R). The City retained Love & Associates, Inc. to produce a
Physical Map Revision (PMR) request for FEMA. The PMR was submitted to FEMA in
2003 and its approval is pending.

6