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

22

Local drainage facility recommendations for Eureka Gulch are:

1. County Road Street inlets and piping

2. High Streets and Register Alley drains
and pipes.

These items will be discussed in more detail below.

4.2

Dorothy Lee Placer Detention Pond

The existing embankment location on the Dorothy Lee placer site
was investigated to determine if there was sufficient storage
(Figure 1.1.1). The raw water storage plans designed by Rocky
Mountain Consultants in 1979 were also investigated. The raw
water storage plans call for an extensive dam roughly 33 feet in
height. This is a jurisdictional dam with design and construction
subject to review by the State Engineer's office.

A lower, less extensive dam, has been investigated at the
existing embankment site. Design parameters for this lower dam
include a maximum vertical dimension between outlet works
,flowline and overflow spillway of less than 10 feet. This will
result in a non-jurisdictional dam and impound less water. The
height and volume of the raw water storage design is not needed
for a significant reduction in flood peaks.

Flood reservoir routing for 24 and 36 inch diameter, outlet pipes
wi th 25 and 15 foot wide uncontro 11 ed emergency spi 11 ways,
respectively were computed. The 24 inch outlet pipe was
selected for conceptual design due to its greater impact on more
frequent flood discharges such as the 2 and 10-year events.
Table 3.2.1 summarizes the hydrologic changes (i.e., reductions
in peak flow) that will be caused by construction of this
detentio).1 pond. '

4.3

Opera House Flume Inlet Improvements

Since the existing pipe underneath the Opera Association parking
l~t (Figure 4.1.2) is a 60-inch corrugated pipe, the capacity of
this installation can only be increased by improving the inlet
configuration. The 60-inch pipe drains into higher capacity
culverts beneath the Opera House, Spruce and Pine Streets, and
other buildings adjacent to Main Street. Under existing
conditions, 'since the flume is on, a hydraulically steep slope,
the inlet controls the capacity. Improved inlet opportunities
consist of a side-tapered inlet with concrete and steel end
sections installed to transition water from the Eureka Gulch
channel, which is roughly triangular in shape, into the pipe
inlet without losing velocity head. The design required to
transition water from the Eureka Gulch channel to the pipe' so
that the throat controls headwate,r, would consist of installing a
circular to elliptical shaped transition pipe with a 4:1 to 6:1
side taper and a head wall with rounded edges (radius of
curvature greater than or equal to 5 inches) where the channel

HydroDynamics Incorporated