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<br /> ' • 1 <br /> Memorandum URS Ureiner Woodward Clyde <br /> p ~r P .e~9y ~~i l~i~ <br /> ~To: Steve Gallup, Therese Walch ~ ~ p (S~f/~ ~ wa-c'dfn- ~ <br /> ~ TN> <br /> From: Binhong Wu, John Holroyd, P.E. <br /> Date: February 25, 2000 <br /> Subject: FEMA Floodplain and Floodway Analysis for the Lower Amazon Creek <br /> At the City's request, URS has developed a preliminary cost estimate for completing a <br /> Federal Emergency Management Agency (FEMA) Letter of Map Revision (LOMR) for the <br /> Lower Amazon Creek from the Southern Pacific Raikoad crossing to Green Hill Road. <br /> Completing the LOMR will require a detailed hydrologic/hydraulic analysis of the <br /> floodplain and floodway for this stream reach. Several different methods can be used to <br /> perform the hydraulic analysis. This memorandum presents three modeling options, <br /> including a discussion of the advantages and disadvantages of each option and a <br /> recommendation of a preferred option. A brief description of the procedures and tasks <br /> needed to complete the LOMR, along with the preliminary cost estimate for each task, is <br /> included in the memorandum. Also included in the memorandum is background information <br /> on the previous Amazon Creek FEMA study and specific FEMA requirements to complete <br /> the LOMR. <br /> Background Information and FEMA Requirements <br /> In 1979, FEMA contracted with CH2M Hill to conduct a Flood Insurance Study (FIS) for <br /> the City of Eugene. As part of the 1979 FIS study, Amazon Creek from Martin Drive to <br /> its crossing with the Southern Pacific Railroad (approximately 9.1 stream miles) was <br /> studied using detailed hydraulic methods. The final version of the FIS and its associated <br /> floodplain and floodway maps was published in 1986. The Lower Amazon Creek was <br /> not included in the detailed study since it was located out of the City limit at that time. <br /> Due to the expansion of the Urban Growth Boundary (UGB) in the 1990s, the Lower <br /> Amazon Creek was designated by FEMA as Zone A, which is the flood insurance rate <br /> zone that corresponds to the 100-year floodplain that is determined in the FIS by <br /> approximate methods. <br /> Since a detailed flood insurance study does not exist for the Lower Amazon Creek north <br /> of the Railroad, the City is interested in conducting a detailed floodplain and floodway <br /> study for the Lower Amazon Creek to implement floodplain management regulations. In <br /> accordance with FEMA regulations, any request for revisions to the existing FIS <br /> information needs to demonstrate that the revised information (e.g., hydrologic and <br /> hydraulic analyses and the resulting floodplain and floodway boundaries) is consistent <br /> with the existing FIS information. Since the area of revision (i.e. the Lower Amazon <br /> Creek) in this study is located at the downstream limits of the previous study, a so-called <br /> "effective tie-in" must be included to ensure the continuity of the FIS. An effective tie-in <br /> is obtained by matching the 100-yeaz flood water surface elevation at the upstream limit <br /> of the new study azea to the 100-yeaz flood water elevation at the downstream limit in the <br /> previous Amazon Creek FIS study (difference must be less than 0.1 feet). Matching of <br /> \\Wr2\PROD-W~.1945042na\TASK0300\AMAZ.OMFEMA\fema.doc 4:08 PM 02/25/0() Page 1 <br /> <br />