FLOOD04442

• County: Boulder
• Community: Longmont
• Stream Name: Lefthand Creek
• Basin: South Platte
• Title: LOMR - Affected Panel 08013C 0269 F - Case No. 97-08-394P
• Date: 5/14/1999
• Prepared For: Longmont
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

CHANGES ARE MADE IN DETERMINATIONS OF BASE FLOOD ELEVATIONS FOR THE
UNINCORPORATED AREAS OF BOULDER COUNTY, COLORADO, UNDER THE NATIONAL
FLOOD INSURANCE PROGRAM

On June 2, 1995, the Federal Emergency Management Agency identified Special Flood Hazard Areas
(SFHAs) in the unincorporated areas of Boulder County, Colorado, through issuance of a Flood Insurance
Rate Map (FIRM). The Mitigation Directorate has determined that modification of the elevations of the
flood having a I-percent chance of being equaled or exceeded in any given year (base flood) for certain
locations in this community is appropriate. The modified base flood elevations (BFEs) revise the FIRM
for the community.

The changes are being made pursuant to Section 206 of the Flood Disaster Protection Act of 1973 (Public
Law 93-234) and are in accordance with the National Flood Insurance Act of 1968, as amended (Title XIII
of the Housing and Urban Development Act of 1968, Public Law 90-448), 42 U.S.C. 4001-4128, and
44 CFR Part 65.

A hydraulic analysis was performed along Lefthand Creek to incorporate updated topographic information
and revised bridge modeling at the State Highway 119 and Burlington Northern Railroad bridges. The
revised analysis defined two channels along the overflow area shown on the effective FIRM. Lefthand
Creek North Overflow Channel is created by floodwater that leaves the main channel just upstream of the
southbound lane of State Highway 119. Lefthand Creek South Overflow Channel is created by floodwater
that leaves the main channel between the northbound lane of State Highway 119 and Burlington Northern
Railroad. Both overflow channels rejoin the main channel just upstream of South Hover Road. The
revised hydraulic analysis has resulted in a decrease in SFHA width, and increased and decreased BFEs
for both overflow channels and the main channel of Lefthand Creek from just upstream of South Hover
Road to just downstream of North 87th Street. The table below indicates existing and modified BFEs for
selected locations along the affected lengths of the flooding source(s) cited above.

Location

Existing BFE
(feet)*

Modified BFE
(feet)*

North Overflow Channel:
Approximately 800 feet downstream
of divergence from main channel
Approximately 750 feet upstream
of South Hover Road

5,015

5,016

4,996

4,993

South Overflow Channel:
Approximately 700 feet downstream
of divergence from main channei
Approximately 2,300 feet downstream
of divergence from main channel

5,012

5,011

5,008

5,009

Main Channel:
Just upstream of State Highway 119
Approximately 1,350 feet downstream of
Burlington Northern Railroad

5,017

5,016

5,002

5,003

*National Geodetic Vertical Datum, rounded to nearest whole foot