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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The lower portion of Fourmile Gulch was analyzed using HEC-2 computer model. The
100-year event in the lower reach of the Gulch is wholly carried within a storm sewer,
thus, there is no floodplain in this area of Fourmile Gulch. The approximate floodplain
delineated on the 1984 FIRM is used to estimate the 100-year floodplain upstream of the
floodplain~identified-by the~Jj0MR-~ nThis-floodplain~ is-also~shown-on~~Figures"~5 A~and~~~~. ~~~~~
5B

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Updated 100-year floodplain delineation in the City of Black Hawk for North Clear
Creek, Chase Gulch, and Gregory Gulch uses the HEC-2 (USACE, 1991) hydraulic
model and is presented in the recently submitted Physical Map Revision to FEMA. The
North Clear Creek model begins downstream at the Richman Street Bridge and extends
upstream to the City limits. The North Clear Creek floodplain extending downstream
from the Richman Street Bridge to the City boundary was addressed in two studies
submitted to FEMA, including FEMA Case No. 03-08-0439R (Mardi Gras Casino) and
98-08-258R (Riveria Casino); and a CLOMR prepared for Jackpot Springs (including an
HEC-2 analysis) which has not yet been submitted to FEMA. These data represent the
best available information on the 100 year floodplain along North Clear Creek
downstream of the PMR and are incorporated in the Flood Information Report. The 100-
year floodplain obtained from the detailed hydraulic analysis within the City of Black
Hawk is shown of Figures SA and 5B.

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Hvdraulic Anal

Flood peak discharges for the 10-, 50-, 100-, and 500-year events for North Clear Creek,
Gregory Gulch, and Chase Gulch used in the floodplain study is summarized in Table 1.

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The approximate floodplain for Fourmile Gulch identified in the 1984 FIRM was updated
near the confluence with a LOMR (Carroll & Lange, 2001). Flood peaks for the 100-
year event were estimated using SCS methodology and the HEC-1 computer model at
various design points. The flood peaks were approved by CWCB.

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Q
Q
A
A

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2

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2

= projected flood peak, cfs,
= known flood peak, cfs,
= drainage area at projected site, mi2
= drainage area at known site, me.

and,

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where;

QI =Q2 {A

/

A 2 } 0,79

0.5 <

{A

/

A

2 }

<2

(McCain et al

1976)

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Scattered development is occurring within the North Clear Creek valley downstream
Black Hawk. Flood peak estimates below City limits at the confluence with Clear
are calculated using the area ratio equation suggested by McCain et aI.,

of
Creek

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events are reduced by the 200 cfs which
of Gregory Gulch in Black Hawk.

is carried within the flume for the entire length