FLOOD00608

• Designation Number: 516
• County: Denver
• Community: Denver
• Title: Flood Insurance Study - Denver, CO, Volume I
• Date: 3/1/1996
• Designation Date: 5/1/2000
• Floodplain - Doc Type: Floodplain Report/Masterplan

10-, 50-, 100-, or 500-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, average 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 (l percent chance of annual exceedence) in any 50-year period is
approximately 40 percent (4 in 10); for any 90-year period, the ri sk
increases to approximately 60 percent (6 in 10), The analyses reported
herein reflect flooding potentials based on conditions existing in the
community 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 the peak
discharge-frequency relationships for each flooding source studied
in detail affecting the community.

The hydrologic analysis for Clear Creek was performed by the COE,
Omaha District (Reference 23), Their analysis was based on fully
developed basin conditions. Discharge records for the Clear Creek
stream gages at Golden and Derby were analyzed using methods
presented in Bulletin No. 17 published by the U.S. Water Resources
Council (Reference 35). The results of these analyses were used to
calibrate to the following runoff models: the Massachusetts
Institute of Technology Catchment Model (MITCAT) (Reference 36),
and the U,S. Environmental Protection Agency (EPA) Storm Water
Management Model (Reference 37). MITCAT was used to model the 400-
square-mile mountainous area upstream from Golden; the Storm Water
Management Model was used to model the lower 175-square-mile plains
basin, Rainfall values used'in the models were obtained from the
Precioitation-Freauencv Atlas of the Western United States, Volume
II. Colorado, published by the National Oceanic and Atmospheric
Administration in 1973 (Reference 38).

The hydrologic analysis for Bear Creek was performed by the COE,
Omaha District (Reference 24). The Bear Creek Dam and Reservoir
intercepts flows from 239 square miles of the total 261-square-mile
drainage basin, The remaining square-mile drainage area below Bear
Creek Dam still has the potential of generating damaging runoff
flows during a cloudburst event over the lower basin, No
applicable runoff records are available through this reach because
of the recent construction of the dam. Therefore, discharges for
the 22-square-mile drainage area below the dam were developed by
using the EPA Storm Water Management Model (Reference 37) with
modifications by the Missouri River Division of the COE. These
discharges were computed assuming full basin development,

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