FLOOD07738

• Designation Number: 538
• County: Arapahoe
• Community: Arapahoe County
• Basin: South Platte
• Title: Hydrology Addendum to FIS Arapahoe County and Incorporated Areas
• Date: 1/1/2001
• Prepared For: Aarapahoe County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

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PIS. However, none of the gaging stations contain enough data to calculate peak discharges for
the approximate streams, which are the subject of the hydrology calculations in this report.

F100dinl!: Problems and F1oodinl!: History
Floods in the subject watersheds normally occur during the summer months and are caused
primarily by intense, localized cloudbursts or thunderstorms. Typically, these floods have high
peak discharges with low volumes over short duration. There is usually a very short window of
opportunity for flood warning and evacuation once a major storms hits the area, Less damaging
floods can also result from rapid melting of mountain snowpack in the spring and early summer
months, especially on the larger drainageways. A combination of rainfall and snowmelt can
sometimes prove to be the worst flooding scenario for Colorado's mountain streams, Generally,
it is assumed that for streams with headwaters below 9,000 feet in elevation, snowmelt does not
influence peak discharges.

Many smaller flooding events occurred throughout the South Platte basin in the late 1800's and
early 1900's. The most extensively documented flood in the South Platte basin occurred in 1965
as a result of severe downpours over several sub-basins. The flood caused over $500 million in
damages in the Denver metropolitan area, Most of the severe flooding events in the County
occur due to heavy thunderstorms in the Monument Divide area. The PIS includes a discussion
of historic flooding events for each of the entities within the study area.

In response to the severe flooding problems within the South Platte and tributary basins, several
flood control reservoirs have been constructed to protect urbanized areas in the western portion
of the County. Cherry Creek reservoir, located on Cherry Creek, was completed in 1950. There
are on-going studies regarding flood frequency analysis and flood forecasting to determine the
existing dam's adequacy for flood protection. Chatfield Reservoir, completed in 1975, is located
on the South Platte in the southwestern portion of the Denver metro area. Bear Creek Reservoir,
completed in 1982, is located on Bear Creek in the western portion of the Denver metro area.

Previous Studies
Due to severe flooding problems within the South Platte basin, several unpublished studies were
conducted within the South Platte basin regarding flooding and floodplain issues as they related
to design and construction of Cherry Creek, Chatfield and Bear Creek Reservoirs. The first PIS
in the county was prepared for the City of Englewood in 1974. This PIS was followed by an PIS
for Sheridan in 1976; unincorporated Arapahoe County in 1977; and Cherry Hills Village,
Columbine Valley and Greenwood Village in 1978. Glendale was included in the original
unincorporated Arapahoe County PIS. In addition, due to significant urban development, basin
planning studies and Letters of Map Revision (LOMR) are continually being developed for
floodplains in the County.

A summary of information available regarding floodplains in the county available from the
CWCB is shown in the Appendix,

Communitv Response to the CWCR Desil!:nation and Approval Process
A letter from the CWCB was transmitted to Arapahoe County and each of the incorporated
communities regarding the Board's proposed action for designation and approval of the

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