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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I engineers. planners. surveyors

May 15, 1991

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Mr. Larry F. Lang, P.E., Chief
Flood Control and Floodplain Management
COLORADO WATER CONSERVATION BOARD
721 State Centennial Building
1313 Sherman Street
Denver, Colorado 80203

Section

RE:

Cent ra 1 City
RMC No. 1671.001.00

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Dear Larry:

We have received your letter dated April 24, 1991 regarding your review of the
hydrological and hydraulic feasibility study for Central City. You made four
(4) recommendations which we would like to address, itemized in the same
manner as your comments.

1.

It is our understanding that the concern about reshaping the
Bridge/Spring/Main Street intersection is to provide channelization of
floodwaters away from the existing downtown area and toward Gregory
Gulch, rather than rely solely on inlets and subsurface flows for flood
control. We concur 100% that this mitigation approach is normally
preferable because of increased reliability and decreased liability.
This was also our first choice of flood mitigation. However, after
observing the existing elevations, determining cover over a 12" water
main, the cost of raising and lowering portions of roadway and parking
lot, and alternatives available, extensive intersection rework was
determined to be outside of the best interest of the City.

The City would not be without an improvement over previous conditions,
however. Improvements are proposed upstream from the location which
will remove much of the surface flow and sediment which contributes to
the downstream area. The flume handling capacity will be increased by
improving the inlet approach and reducing hydraulic jump inside the
flume. Facilities will also be provided to divert flume overflow from
Nevada Street near the top of the paved parking lot to the flume. This
can be oversized beyond what the flume can handle, with excess flow
surfacing on and being conveyed over the parking lot. Runoff from the
parking lot can more easily be directed toward Spring Street and away
from Main Street than runoff corning down Nevada Street. An improved
drainage collection and conveyance system will also be provided in Main
Street to further reduce flooding potential.

Given the above improvements, the partial redirecting of surface runoff
upstream of the intersection, and the fact that the downtown does have a
gravity flow outlet in Gregory Street, the planned course of action
without significant reshaping of the intersection would provide
substantial flood mitigation.

Rocky Mountain Consultants, Inc.
8301 East Prentice Avenue NO.1 01. EnglE>wood. CO 80111
Offices also in Longmont . Boulder. and Estes Park, CO

[phone]