|
<br />,
<br />
<br />.
<br />
<br />.
<br />
<br />4
<br />
<br />
<br />NATIONAL FLOOD INSURANCE PROGRAM
<br />FEMA MAP COORDINATION CONTRACTOR
<br />
<br />Summary of Additional Data Required to Support a
<br />Letter of Map Revision
<br />
<br />Case No.: 00-08-383P
<br />
<br />Requester: Ms. Kallie E. Sampish, C.F.M.
<br />
<br />Community: Larimer County, CO
<br />
<br />Community No.: 080101
<br />
<br />In a letter from you and Mr. Daniel P. Giroux, P.E., also with Tuttle Applegate, Inc., dated December 7,
<br />2000, you stated that the widths of the regulatory floodway along Interstate Highway 25 (1-25) Divided
<br />Flow Path, as shown in the Floodway Data Table (FWD!) in the effective Flood Insurance Study (FIS)
<br />report for the unincorporated areas of Larimer County, Colorado, dated March 23, 1999, do not match
<br />those shown on the effective Flood Boundary and Floodway Map (FBFM) for Larimer County dated
<br />November 17, 1993. In your December 7 letter, you also stated that you believe that the floodway
<br />widths in the effective HEC-2 computer models (included in your August 23, 2000, submittal entitled
<br />"Backup Data, 1-25 Split Flow Output Data" as filename "I 25flood.out") are accurate. Your letter
<br />concluded that during the flood having a I-percent of being equaled of exoeeded in any given year (base
<br />flood), a flow of 2,032 cubic feet per second (cfs) that leaves the main channel of the 1-25 Divided Flow
<br />Path between Cross Sections C and D, as shown on the effective FBFM, does not flow over the 1-25
<br />roadway north of Larimer County Road 36 (LCR36) but rather flows primarily under the I-25/LCR36
<br />bridge. Therefore, you believe that the property just east ofI-25 and north of LCR36 should not be
<br />shown as within the regulatory floodway.
<br />
<br />We have reviewed all the submitted information and the backup information used to produce the
<br />effective FIS report, the Flood Insurance Rate Map (FIRM), and the FBFM for Larimer County and can
<br />offer the following clarifications and background data.
<br />
<br />. The cross-section labels as shown in the effective FWDT are incorrect. The cross sections labeled B
<br />through I in the table are incorrectly labeled. They should be labeled D through K, respectively, and
<br />the data for Cross Sections B and C are missing. Cross!!sections B and C in the FWDT actually
<br />correspond to Cross Sections 175088 (B) and 176343 (C), respectively, as listed in the effective
<br />HEC-2 model. Their floodway widths are 385 feet for Cross Section Band 1,011 feet for Cross
<br />Section C and match the corresponding widths shown on the FBFM.
<br />
<br />. Your reference to the I-25/LCR36 bridge describes the location of the LCR36 overpass ofI-25. The
<br />east-west width of this overpass opening is approximately 120 feet, as shown on Sheet No. 112 for
<br />the LCR36 plan and profile submitted in the Backup Data with your request and as measured from
<br />the original FIS work maps (City of Fort Collins, Engineering Division, aerial topographic maps,
<br />scale I" = I 00', contour interval 2 feet, based on aerial photography taken April 12, 1984). The
<br />elevation of the 1-25 road surface is approximately 4,832 feet at the LCR36 overpass.
<br />
<br />. Cross Section 176343 (C), as depicted in the effective model, reflects both flow that overtops the
<br />LCR36 roadway (approximate minimum elevation 4,830.2 feet; Station 5060 on the GR record) west
<br />of the I-25/LCR36 overpass and flow that passes under the I-25/LCR36 overpass (approximate
<br />minimum elevation 4,831.5 feet; Station 5950 on the GR record).
<br />
<br />3601 Eisenhower Avenue, Suite 600, Alexandria, Virginia 22304-6425 PH: 703.960.8800 FX: 703.980.9125
<br />
<br />Michael Baker Jr. Inc., under contract with the FEDERAL EMERGENCY MANAGEMENT AGENCY, Is a
<br />Map Coordination Contractor for the National Flood Insurance Program
<br />
|