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CITY OF GREELEY WATER POLLUTION CONTROL FACILITY FLOODPLAIN/FLOODWAY LETTEFl OF MAP REVISION Introduction This report was prepared at the request of the City of Greeley, Colorado, to provide the City with documentation needed to apply for a Letter of Map Revision (LOMR) from the Colorado Water Conservation Board (CWCB) and the Federal Emergency Management Agency (FEMA). The LOMR is for the reach of the Cache La Poudre (Poudlre) River adjacent to the City of Greeley's Water Pollution Control Facility (WPCF). The WPCF is presently located within the regulatory floodway of the Poudre River between Ash Avenue and U.S. Highway 85. The floodplain and floodway are defIned by the Flood Insurance Study (PIS) of FEMA, Community Panel Numbers 080184 0002B, 080266 0636C, and 080266 0637C. By City ordinance and FEMA regulations, improvements requiring new construction within the floodway that adversely impact said floodway are not allowed. This ordinance jeopardizes required improvements to the WPCF. In 1986 the City contracted with Warzyn Engineering, Inc. to design river channelization improvements to contain the 100-year flood within the channel. This study was performed for the river reach from Ash Avenue to 2300 feet upstream of 11th Avenue. The Warzyn improvements have not been constructed. Presently, the U.S. Army Corps of Engineers (CaE) is studying improvement alternates along the Poudre River including the Warzyn design. The alternates prepared or studied by the CaE will be analyzed and a benefIt-cost-ratio analysis performed to determine the recommended program. The preliminary CaE study is scheduled for submittal in March 1996. The earliest possible date for channel improvement construction is 1997 at which time the WPCF could be completely removed from the floodplain by implementation of design improvements. Authority Brown and Caldwell (BC) has undertaken a program to analyze the existing floodplain and floodway of the Poudre River adjacent to the WPCF incorporating the considerable changes that occurred since 1979 and 1982, the dates of the City and County PIS studies, and to determine floodway regulatory compliance concerning construction of new facility improvements. This task was authorized under Brown and Caldwell's contract with the City dated March 1, 1993. Hydrology The flood hydrology used for this LOMR was developed by the CaE and used in the PIS Community No. 080184 on July 16, 1979 and Community No. 080266 on September 28, 1982. No changes have been made in the flood hydrology for this LOMR. ilffi'li\ln ami Caldwell ~~O' 1S'J!Li:-i'li.s 21/7387