FLOOD03840

• County: Adams
• Community: Thornton
• Stream Name: South Platte River
• Basin: South Platte
• Title: LOMR - Affected Panel 08001C0330 G - Case No. 04-08-0577P
• Date: 5/11/2005
• Prepared For: Thornton
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

CHANGES ARE MADE IN DETERMINATIONS OF BASE FLOOD ELEVATIONS FOR THE
CITIES OF COMMERCE CITY AND THORNTON AND THE UNINCORPORATED AREAS OF
ADAMS COUNTY, COLORADO, UNDER THE NATIONAL FLOOD INSURANCE PROGRAM

On August 16, 1995, the Department of Homeland Security's Federal Emergency Mauagement Agency
identified Special Flood Hazard Areas (SFHAs) in the City of Commerce City, the City of Thornton, and
the unincorporated areas of Adams County, Colorado, through issuance of a Flood Insurance Rate Map
(FIRM). The Mitigation Division 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
these communities is appropriate. The modified Base Flood Elevations (BFEs) revise the FIRM for the
communities.

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 to incorporate the effects of Tigers Reservoir along the South Platte
River. Tigers Reservoir was constructed along the east overbank of the South Platte River in an area
bounded by East 88th Avenue to the south, Colorado Boulevard to the west, and Interstate Highway 76 /
U.S. Highway 85 to the east. The analysis also included the effects of additional cross-sectional data and
the Tigers Reservoir dam along the South Platte River from approximately 2,300 feet upstream of the
McKay Road bridge to East 88th Avenue. A flow split along the east side of Tigers Reservoir was
analyzed from the upstream side of Tigers Reservoir to approximately 2,000 feet downstream, where the
flow rejoins with the main South Platte River. This has resulted in a revised delineation of the regulatory
floodway, increases and decreases in SFHA width, and decreased BFEs for the South Platte River. 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).

South Platte River:
Approximately 3,550 feet upstream of McKay Road
Approximately 1,000 feet downstream of East 88th Avenue

5,064
5,077

5,063
5,076

Tigers Reservoir Split Flow:
Approximately 4,000 feet upstream of McKay Road
Approximately 1,500 north of East 88th Avenue

5,066
5,072

5,064
5,069

.National Geodetic Vertical Datum, rounded to nearest whole foot

Under the above-mentioned Acts of 1968 and 1973, the Mitigation Division must develop criteria for
floodplain management. To participate in the National Flood Insurance Program (NFIP), the community
must use the modified BFEs to administer the floodplain management measures of the NFIP. These
modified BFEs will also be used to calculate the appropriate flood insurance premium rates for new
buildings and their contents and for the second layer of insurance on existing buildings and contents,

Upon the second publication of notice of these changes in this newspaper, any person has 90 days in
which he or she can request, through the Chief Executive Officer of the community, that the Mitigation
Division reconsider the determination, Any request for reconsideration must be based on knowledge of