FLOOD03281

• County: Boulder
• Community: Boulder County
• Stream Name: Boulder Creek
• Basin: South Platte
• Title: Hydrology Addendum to FIS Boulder County and Incorporated Areas
• Date: 10/1/2000
• Prepared For: Boulder County
• Prepared By: Montgomery Watson
• Floodplain - Doc Type: Floodplain Report/Masterplan

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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 a short duration, There is usually a very short window of
opportunity for flood waming 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 drainage ways, 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,

The most extensive documented flood occurred in 1894 and is generally considered a loo-year
event on Boulder Creek. The flood damaged most bridges in the City of Boulder. Since that
time, the most significant widespread flooding event occurred in May 1969. The event affected
most of the streams within the County, and caused widespread damage. The event was caused by
three days of heavy rain and snow, Several other floods have occurred on the various creeks
within the town causing damage in several locations. Many streams within the City have a
history of debris blockage and flow diversions. The AS includes a discussion of historic
flooding events for each of the entities within the study area.

Previous Studies
The U,S. Army Corps of Engineers (US ACE) first developed Floodplain Information Reports
(FPIRs) for Boulder County and the City of Boulder in 1969. Since that time, FEMA, the
USACE and private consultants have conducted several Flood Hazard Studies for the City and
County. The first AS in the county was prepared for the City of Louisville in 1973. This FlS
was followed by an AS for Longmont in 1977; the City of Boulder in 1978; Boulder County,
Superior and Nederland in 1979; Lafayette and Lyons in 1980; and Jamestown in 1983.

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

Community Response to the CWCB Desilmation and Approval Process
A letter from the CWCB was transmitted to Boulder County and each of the incorporated
communities regarding the Board's proposed action for designation and approval of the Boulder
County and Incorporated Areas FlS. The County and all municipalities were receptive to the
Board's proposed designation action and indicated that they would return an official letter
requesting that the CWCB designate and approve the floodplain information currently available
to the communities. However, letters from the City of Boulder, Town of Louisville, Town of
Lyons and City of Superior were not received prior to publication of this report. Copies of the
request letters or a are included in the Appendix of this report.

All of the entities were contacted by phone to discuss the designation process, review the FIRMs
and to solicit comments regarding the maps and general flooding condilions in the County, In
addition, Lafayette and Louisville requested that meetings be held with CWCB staff. A field tour
of several Boulder County floodplains was also conducted at this time, No contradictory

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