FLOOD01607

• County: Gilpin
• Community: Central City
• Stream Name: Eureka and Nevada Gulches
• Title: Master Drainage Study
• Date: 2/12/1988
• Prepared For: Central City
• Prepared By: HydroDynamics
• Floodplain - Doc Type: Floodplain Report/Masterplan

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CENTRAL CITY MASTER DRAINAGE STUDY

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onto tailing sites and use of erosion control blankets and some
other means can be implemented.

The alternative to stabilizing tailings (and reducing sediment to
downtown Central City) is to remove tailings. However, we are of
the opinion that it will be more cost effective to stabilize the
tailings assuming their composition is not detrimental to
establishment of grasses. Before costs estimates are formulated
on stabilization of the tailings, a preliminary investigation
must completed. From this information, proper slope
stabilization techniques can be developed. The same techniques
can be applied to the tailings which are currently used as the
Free Parking Lot during the tourist season.

5.2 Nevada Gulch Flume Inlet Improvement

This situation is similar to that of the Opera House flume inlet.
The capacity of the 60-inch square concrete flume (Figure 4.1.3)
beneath the Free Parking Lot is much greater than the inlet
controlled capacity estimated by CWCB. An improved inlet
condition would make it possible for this structure to convey the
entire 100-year flood.

The improved inlet would consist of wingwalls, a sloping apron
between the wingwalls and a modified headwall. All of these
items would be designed so that the existing structure capacity
is maximized.

5.3

Spring, Roworth and Nevada Street Inlets

Existing pipes would be used to drain additional drop inlets.
The inlets (Colorado Department of Highways Type C inlets) would
be placed in roadside swales. This would stabilize the flowline
of the roadside swales and significantly reduce street flooding
from rainstorm events. An ancillary benefit is the reduction of
the street erosion contribution to sediment problems in downtown.

Mine Tailing and Slope Stabilization

As noted above, material eroded from tailings piles contributes
significantly to sediment problems in downtown Central City.
These sediments are likely to cause as much damage as flood water
to property in its path. Two major sources of sediments are the
slopes of the Free Parking Lot and tailings in Spring Gulch.
Sediments from denuded slopes and tailings in Nevada Gulch will
be routed through the drainage system to Gregory Gulch below
downtown if the Nevada Gulch flume is properly maintained.

5.4

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Tailings piles are typically at or near the angle of repose with
flattening due mainly to erosion. Free Parking Lot slopes, for
example, range from 35 to 40 degrees (1.4:1 to 1.1:1) except for
eroded gullies. These steep slopes must be flattened, preferably
to a 3:1 slope, and revegetated to stabilize them. In the case
of the Free parking Lot, this would greatly reduce the number of
parking spaces. In the case of other tailings piles throughout

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HydroDynamics Incorporated