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• L Introduction Westem Mobile, Inc., a subsidiary of Lafarge Corporation has proposed to expand their existing DMG permit (#M-74-015) for the Rockin' WP sand and gravel operation located along the St. Vrain Creek east of Lyons, Colorado. The subject property is located in Sections 20, 21, 22, 27, 28 and 29 of Township 3 North, Range 70 West of the Sixth Principal Meridian, County of Boulder, State of Colorado. The subject property runs from Highway 36 on the west to North 61" Street on the east and is bounded by Hygiene Road on the south and Highway 66 on the north. The main channel of the St. Vrain Creek is located adjacent to or traverses the northern portions of the proposed operation. The boundaries of the 100-year floodplain of the main stem and south branch of the St. Vrain encompass the majority of the site. Since the projected schedule of mining and reclamation is proposed to encompass approximately 30 years, it is likely that flood events of the St. Vrain of various return intervals will occw during the life of the mine. In addition, the proposed final reclamation pla~r will continue to effect and be subject to floods after the completion ofmining operations. CP Engineering has been retained by Westem Mobile, Inc. to prepare a Floodplain Impact Analysis report for the purpose of determining the effects of the proposed mining and reclamation plan on the St. Vrain Creek floodplain. The following analyses .are • included in the report: • Existing status of the St. Vrain Creek and floodplain. • Impacts on the St. Vrain floodplain as a result of mining and reclamation. • Off-site floodplain impacts. • Design of mitigation measures Please note that this report does not include an analysis of these issues for the existing Rockin' WP-North and Miller pits mining and reclamation operations. IL Analvsis and Design Guidelines This report utilizes recommendations and design procedwes from the "Technical Review Guidelines for Gravel Mining Activities" published in December 1987 by the Urban Drainage and Flood Control District (IJDFCD). These Guidelines were accepted for use in the review of new gravel mining proposals within the District and for the purpose of evaluating existing permits along the South Platte River and its tributaries (the ;~t. Vrain is a tributary of the South Platte). The Guidelines are intended to protect rivers and streams from erosion and degradation damages. Degradation has been observed when a • river or stream breaches a berm between the main channel and the gravel pit, o:; between individual pits.