FLOOD06608

• Designation Number: 352
• County: Gilpin
• Community: Central City
• Basin: South Platte
• Title: Flood Insurance Study - Central City, Colorado, Gilpin County
• Date: 2/1/1994
• Designation Date: 3/1/1994
• Floodplain - Doc Type: Floodplain Report/Masterplan

annual flooding due to very localized intense thunderstorms,
sometimes in combination with snowmelt.

2,4 Flood Protection Measures

In an effort to reduce flood damage and flood related maintenance
cost, the City of Central City established steps to remove the
downtown areas from the Special Flood Hazard Areas. Hydrologic and
hydraulic studies were conducted to determine the necessary
improvements, Flumes and pipes were placed along the major gulches
to convey flows underneath the developed areas.

3.0 ENGINEERING METHODS

For the flooding sources studied by detailed methods in the cODlDunity,
standard hydrologic and hydraulic study methods were used to determine
the flood hazard data required for this study. Flood events of a
magni tude which are expected to be equaled or exceeded once on the
average during any 10-, 50-, 100-, or SOO-year period (recurrence
interval) have been selected as having special significance for
floodplain management and for flood insurance rates. These events,
commonly termed the 10-, 50-, 100-, and SOO-year floods, have a 10, 2, 1
and 0.2 percent chance, respectively, of being equaled or exceeded
during any year. Although the recurrence interval represents the long-
term, averaRe period between floods of a specific magnitude, rare floods
could occur at short intervals or even within the same year, The risk of
experiencing a rare flood increases when periods greater than 1 year are
considered. For example, the risk of having a flood which equals or
exceeds the 100-year flood (1 percent chance of annual exceedence) in
any 50-year period is approximately 40 percent (4 in 10); for any
90-year period, the risk increases to approximately 60 percent (6 in
10). The analyses reported herein reflect flooding potentials based on
conditions existing in the cODlDunity at the time of completion of this
study, Maps and flood elevations will be amended periodically to
reflect future changes,

3.1 Hydrologic Analyses

Hydrologic analyses were carried out to establish peak discharge-
frequency relationships for each flooding source studied by
detailed methods affecting the cODlDunity,

The U,S. Army Corps of Engineers (COE) HEC-l computer model was
used to determine peak discharges for Gregory Gulch and Eureka
Gulch (Reference 1), Peak discharge-drainage area relationships
for each flooding source studied in detail are shown in Table 2.

3.2 Hydraulic Analyses

Analyses of the hydraulic characteristics of flooding
sources studied were carried out to provide estimates
elevations of floods of the selected recurrence intervals.

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