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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TABLE NO. 1
COMPARISON OF CWCB AND RMC HYDROLOGICAL RESULTS'
100-YEAR EVENT

WATERSHED AREA

RMC

CWCB

700 cfs
430 cfs
1240 cfs

385:t cfs
449:t cfs
969 cfs

Eureka Gulch2
Nevada/Spring Gulches
Gregory Gulch, City Limit2

'For various reasons, an absolute direct comparison is
not possible. See text of report for explanation.

2The CWCB value is for runoff before detention; the RMC
value is after construction of a 14 acre-feet detention
facil ity.

Note that the estimated 100-year peak runoff leaving Central City is
271 cfs less than the CWCB rate used in the Flood Insurance Study
performed for Bl ack Hawk downstream.

B.

Hydraul i cs

Exhibit "D" is a flow diagram which shows surface and subsurface
drainage patterns and rates for the 100-year event. Exhibits "E" and
"F" graphically show the results of flume analyses (subsurface flows)
with water surface profiles or hydraulic grade lines plotted. It can
be seen that for the 100-year event, Opera House flume is not
surcharged in the arched rockways, and at no point in either flume
should the 100-year flow rate present structural problems.

Exhibit "E" indicates a maximum flow velocity less than 14 feet per
second (fps) in the Opera House flume for the 100-year event, which is
deemed acceptable. For Nevada flume, 100-year flow velocities reach
20 fps. Normally one would avoid designing for flow velocities in
this range, particularly when sediment laden flow is expected.
Nonetheless, consideration of several conditions has resulted in our
acceptance of flow velocities as analyzed.

I. The flume is not being designed for a proposed condition, but
already exists;

2. Although additional flows beyond previous conditions were directed
to the flume at the parking lot from Nevada Street by summer/fall
1991 improvements, culvert overflow from Spring Street was
prevented from entering the flume, with a net result of little
flow change overall from previous conditions;

3. Due to the watershed soil type being quite pervious, frequency of
abrasive flows in the flumes is limited; and

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