FLOOD03149

• County: Gilpin
• Community: Central City
• Stream Name: Eureka and Nevada Gulches
• Basin: South Platte
• Title: FEMA LOMR Application Hydrologic and Hydraulic Study
• Date: 10/1/1991
• Prepared For: Central City
• Prepared By: RMC
• Floodplain - Doc Type: Floodplain Report/Masterplan

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Eureka Gulch into two (2) separate watersheds in order examine the
effects of a. reservoir at the Dorothy Lee Placer. In this study and
the detailed RMC feasibil ity study, watersheds were divided into
seventeen (17) subwatersheds. Estimated runoffs from these areas,
along with flow routing and combinations, provides runoff rates at
points of specific interest.

B. Criteri a

Precipitation depths for storms of various return periods are published
in NOAA Atlas II. (See Reference No.3 in the Bibliography.) Some-
times more site specific information is available which may differ from
the NOAA Atlas II, and such i nformat i on should be cons i dered. For
Central City, no other suitable data was found, so used in this study
were the NOAA Atlas II values for May through October (whi ch time
period essentially el iminates snowfall). The SCS unit hydrograph
method was used to estimate storm runoffs, with data obtained by using
guidelines presented in SCS TR-55 and NEH-4. (See Reference Numbers
4 and 5, respectively, in the Bibliography.) Comparing estimated
runoff results using the 2-hour, 6-hour, and 24-hour type IIa rainfall
distribution, it was determined that the latter resulted in the higher
estimated peak runoffs, and therefore was used in this study. The SCS
unit hydrograph method was util ized in the U. S. Army Corps of
Engineers HEC-l computer program. (See Reference No. 6 in the
Bibliography.) The selected design storms for analyses were the 2-year
(for calibration purposes), 10-year, 50-year, 100-year, and 500-year
storm events. No reduction in precipitation depths were allowed for
size of watershed area, and the 500-year rainfall depth was determined
by extrapolation.

C. Procedures

1. Appendix "A", Rainfall Results and Distributions

Research was performed in an effort to establish realistic
precipitation data. The largest storm runoff event of recent years
was due to a series of July storms in 1975. According to newspaper
accounts researched by Brian Hyde of CWCB, on July 8, a downpour
hit the Spring/Nevada Gulch area resulting in water and sediment
flooding Main Street and entering some buildings. On July 20, a
slightly worse storm hit causing overflow from the Nevada flume
with water and sediment flooding Nevada, Main, and Gregory Streets.
On July 23, a severe storm event occurred which left 3'-4' of
sediment deposited on Main Street. The July 23rd storm is recorded
not only by newspaper but in the minds of local residents. Unfort-
unately, the precipitation station in Central City from 1956 to
1977 was not continuously in operation, and no data exists for July
of 1975. For comparative information only, records for Idaho
Springs and Nederland, the two (2) closest gauges, were researched.
It was 1 earned that there were no records for July in Idaho
Springs, and none or negligible rainfall amounts fell in Nederland.

Information was sought from NOAA, USGS, CWCB, the State climato-
logist, a meteorlogist, and a special hydrological project called
ENSTAR performed by the University of Colorado for Niwot and

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