FLOOD08330

• County: Gilpin
• Community: Central City
• Stream Name: Gregory Gulch, Nevada Gulch
• Basin: South Platte
• Title: Flood Hazard Identification Report - Gregory Eureka Gulch and Nevada Gulch
• Date: 2/1/1987
• Prepared For: Central City
• Prepared By: CWCB
• Floodplain - Doc Type: Floodplain Report/Masterplan

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HYDROLOGIC AND HYDRAULIC DETERMINATIONS

Flood History

A partial list of flooding events is tabulated as follows:

Date of flood Cause

Remarks

July,1863

rainstorm extensive damage to property

May, 1864 unknown caused $5000 of damage to road in town

August, 1881 rainstorm caused over $100,000 of damage to Central
City alone. Several lives were lost.

July, 1885 rainstorm 3 lives were lost (not known if lives
were lost in Central City or Black Hawk).

July, 1975

rainstorm culverts plugged. Water ran down streets.

Flood Characteristics

Flooding in Central City is caused by lDcalized intense
thunderstorms, sometimes in conjunction with snowmelt.

Hydrologic Analysis

Hydrology for both Gregory Gulch and Nevada Gulch was obtained
through the Soil Conservation Servi ce' s (SCS) un it hydrograph method.
Results of this analysis were consistent with hydrology for Gregory Gulch
in the Federal Emergency Management Agency's report for Blackhawk (Ref.
13). The Bureau of Reclamation's Type B distribution of a 6 hour
rainstorm was employed (Ref. 3) with rainstorm totals derived from the
Precipitation-Frequency Atlas of the Western U.S. (Ref. 15). Results of
this analysis also proved consistent with CSM's (cfs/sq mil of adopted
peak flows from FEMA studies in the vicinity of Central City. An SCS
curve number of 79 was adopted for both gulches. Total rainfalls of
various frequency events were 1.85" for the 6 hour-10 year event, 2.55"
for the 6 hour-50 year event, and 2.75" for the 6 hour-100 year event.
(The 500-year peak flow was obtained by extrapolation). No reduction was
made to these rainfall totals because of elevation or drainage area size.

Resu lts of this analysis are summarized as follows:
Peak Discharge
Location Drainage area ~ fMf msf ~OO r
(sguare mil es) cfs cfs cfs cfs
Gregory Gulch at the down- 3.43 370 920 1240 1800
stream corp. limit
Gregory Gulch just upstream 1.85 300 610 700 1090
from Nevada Gulch
Nevada Gulch at the mouth 1.1 190 380 430 640
Figure 1 is a plot of the three frequency-discharge curves.

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