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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constructed, prohibiting useful comparison. The third previous study
was performed by RMC, with data identical to that presented herein, and
does not allow for comparison. Consequently, the runoff values
presented in the CWCB report are all that are available for comparison
purposes.

Differences in results between the CWCB analyses and that presented
herein are due primarily to three (3) issues.

1. eN Values. The criteria and procedures used by RMC in eN selection
is provided in this report. Based on detailed field
reconnaissance, field borings, and a determined SCS hydrologic soil
group of "B", CN values used herein are lower than those used in
the CWCB report. The result is lower estimated runoffs by RMC than
CWCB.

2. Detenti on Facil i ty. The CWCB report was performed pri or to
construction of a 14 acre-feet detention facility on Eureka Gulch.
The detention provided is below 60% of the entire Eureka Gulch
watershed, and was des igned to opt imi ze runoff reduction. The
facility results in significant reduction in peak runoffs not
incorporated in the CWCB report.

3. Runoff Routing. The CWCB report presented a look at the overall
runoff picture of the watersheds, whereas this study is concerned
with more specific detail as well. Surface flow diversions to
underground conveyance facil ities were made internal to HEC-l,
which must be added back to surface flows to provide meaningful
comparisons. The surface and subsurface peak flows do not coincide
with respect to time, but simple addition of peaks may be close
enough for reasonable comparison. Additionally, runoff from Nevada
Gulch, through more detailed investigation, flows over into the
Eureka Gulch watershed which was not accounted for in the CWCB
report. As best as possible, these differences were accounted for
without duplication or elimination of flows in the presented RMC
Eureka and Nevada Gulch total watershed runoff rates.

Gi ven the above 1 imi tat ions, Tabl e No. 1 presents a compari son of
estimated 100-year runoff rates obtained by CWCB and RMC.

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