2011-12-16_REVISION - M1977572 (71)

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3. Increases in the 100 -year water surface elevations are causing a decrease in the bridge capacity and inducing overtopping of Highway 50 north of the bridge. 4. The erosion potential between the effective condition and existing condition tends to decrease using the assumption of fixed bed hydraulics due to slower velocities associated with higher water surface elevations. The purpose of the Phase II study is to develop a set of design improvement to mitigate impacts to the 100 -year floodplain, minimize erosion potential at the Highway 50 Bridge, and meet Colorado Division of Reclamation Mining and Safety requirements for reclamation. 1.3 Previous Studies The City of Lamar's FEMA Flood Insurance Study (FIS) was published November 17, 1982. It should be noted that Prowers County and the City of Lamar have separate regulatory Flood Insurance Rate Map (FIRM) panels showing different floodplain zone designations for the Arkansas River. An approximate 100 -year floodplain is shown on the Prowers County FIRM panel [FEMA, July 12, 1977] and a detailed 100 -year floodplain with base flood elevations (BFEs) is shown within the city limits on the City of Lamar's effective FIRM panel [FEMA, November 17, 1982]. The detailed 100 -year floodplain and BFEs shown on the City of Lamar FIRM panel supersedes the approximate designation shown in Prowers County and is considered to be the effective regulatory floodplain for which this study will be compared. It should also be noted that a regulatory floodway was not found on the Arkansas River within the study reach. Furthermore, the City of Lamar's FIS states that "a floodway was not computed for the Arkansas River because it would be totally outside the City's corporate limits ", [FEMA, May 17, 1982; Pg. 8]. The detailed 100 -year floodplain, with base flood elevations, is based upon modeling and mapping provided by Camp Dresser & McKee, Inc. (CDM) while under contract with FEMA between 1979 and 1981. The topography utilized in the CDM study was produced in 1980 by Landmark Mapping Ltd. A request to obtain the original CDM modeling and mapping as well as any additional information pertinent to the Arkansas River in the study area was submitted to FEMA by ACE. FEMA responded to the request stating that a search of their files did not produce any information in the current study reach of the Arkansas River. During the late 1980s several analyses were conducted by WRC Engineering, Inc. (WRC) for Valco on the Arkansas River in support of a 404 permit application for realignment of the river and expansion of gravel mining operations. A 100 -year floodplain analysis in the study reach was performed and documented by WRC in a letter report dated June 30, 1987. The WRC letter report documents a 100 -year floodplain analysis using the U.S. Army Corps of Engineers (USACE) HEC -2 hydraulic modeling program and the 1980 topography utilized in the effective FEMA study. The WRC HEC -2 100 -year water surface elevations provided a favorable comparison to the 100 -year water surface elevations published COVALCOO1_Phase 11 Report_082211.docx 1.4 ANdERSON CONSULTiNC ENC{iNEERS, INC.