FLOOD08313

• County: Adams
• Community: Thornton
• Stream Name: South Fork Grange Hall Creek
• Basin: South Platte
• Title: LOMR - Affected Panel 08001C 0038 G - Case No. 99-08-377P
• Date: 8/23/2000
• Prepared For: Thornton
• Prepared By: FEMA
• Floodplain - Doc Type: Historic FEMA Regulatory Floodplain Information

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Federal Emergency Management Agency
Washington, D.C. 20472

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MAY 0 9 2000

CERTIFIED MAIL
RETURN RECEIPT REQUESTED

IN REPLY REFER TO:
Case No.: 99~08~3np

The Honorable Noel Busck
Mayor, City of Thornton
9500 Civic Center Drive
Thornton, CO 80229

Community: City of Thornton, CO
Community No.: 080007
Panel Affected: 0800lC0038 G
Effective Date of AUG 2 3 2000
This Revision: .

102-I-A-C

Dear Mayor Busck:

This responds to a.t:e~uest that the Federal Emergency Management Agency (FEMA) revise the effective
Flood Insurance Rate Map (FIRM) and Flood Insurance Study (FIS) report for Adams County, Colorado and
Incorporated Areas (the effective FIRM and FIS report for your community), in accordance with Part 65 of
the National Flood Insurance Program (NFIP) regulations. In a letter dated July 19, 1999, Mr. Frank D.
Casteleneto, City Engineer, City of Thornton, requested that FEMA revise the FIRM and FIS report to show
the effects of construction of the Thornton Town Center along South Fork Grange Hall Creek and updated
topographic infonnation from approximately 950 feet downstream of Grant Street to approximately 100 feet
downstream of Melody Drive. The construction of the Thornton Town Center included installation of a
7-foot-high by 14-foot-wide reinforced-concrete box culvert (RCB) under Grant Street; a storage pond just
upstream of Grant Street; and approximately 1,400 feet of7-foot-high by 10-foot-wide RCB from the storage
pond to just upstream ofInterstate Highway 25 (1-25). The construction has resulted in a realignment of the
profile baseline for South Fork Grange Hall Creek; therefore, the stationing and letter designations of the
cross sections from approximately 800 feet downstream of Grant Street to just downstream of Huron Street
were adjusted accordingly. In addition, this Letter of Map Revision (LOMR) includes a correction to the
cross-section letter designations on the effective FIS profile panels to make them consistent with those shown
on the effective FIRM and Floodway Data Table from 104th Avenue to approximately 900 feet upstream
of Washington Street.

All data required to complete our review of this request were submitted with letters from Mr. Jay L.
Carlson, P.E., Kiowa Engineering Corporation, and Mr. Casteleneto.

We have completed our review of the submitted data and the flood data shown on the effective FIRM and
FIS report. We have revised the FIRM and FIS report to modify the elevations and floodplain and floodway
boundary delineations of the floods having a I-percent chance of being equaled or exceeded in any given
year (base flood) and a O.2-percent chance of being equaled or exceeded in any given year along South Fork
Grange Hall Creek from approximately 800 feet downstream of Grant Street to just downstream of
1-25. As a result of the modifications, the base flood elevations (BFEs) for South Fork Grange Hall Creek
and the widths of the Special Flood Hazard Area (SFHA), the area that would be inundated by the base flood,
and the regulatory floodway increased in some areas and decreased in other areas. The base flood and
floodway are contained in the culvert and storage pond from just upstream of Grant Street to just upstream
ofI-25; therefore, the SFHA and floodway from the upstream end of the storage pond to just upstream of