2006-06-12_REVISION - M1982015 (23)

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• CONDITIONAL LETTER OF MAP REVISION (CLOMR) FOR ST, VRAIN LAKES I. INTRODUCTION The purpose of this report is to present hydrologic data, hydraulic data, and applications to acquire a Conditional Letter of Map Revision (CLOMR) from the Federal Emergency Management Agency (FEMA). The St. Vrain Lakes development will use the existing lakes as irrigation storage, recreational amenities and for stormwater detention purposes. The reach of the St. Vrain River under investigation is located in Sections 35 and 36, Township 3 North, Range 68 West, of the Sixth Principal Meridian, County of Weld, State of Colorado. The site is principally bounded by Highway 66 to the north, Weld County Road 13 on the east, the St. Vrain River to the south, on the southwest by Interstate 25 (I-25), and on the west by Weld County Roads 11 and 9'/2. The site is located in the segment of the St. Vrain River ranging from approximately 56,414 feet to 70,610 feet from the point of confluence with the South Platte River. • II. BACKGROUND INFORMATION The St. Vrain River has a tributary area of approximately 500 square miles. Its headwaters originate from the Continental Divide in the Rocky Mountains down to the North and South St. Vrain River. The St. Vrain River basin has elevations, which range from 4,900 feet to over 14,000 feet at Long's Peak. The St. Vrain River is a part of the South Platte River Basin. The study reach extends from approximately 3,800 feet downstream of the Weld County Road 13 bridge to downstream edge of the bridge at the I-25 frontage road. Native grasses make up the existing vegetation. Gravel mining has occurred in the study reach of the St. Vrain River creating existing lakes adjacent to the river. Previous Studies. The Colorado Water Conservancy Board, in conjunction with the Army Corps of Engineers (ACOE), prepared a detailed study on this stretch of the St. Vrain River in 1977. The input HEC-2 model from this study was obtained and used as the base model. However, the output data from this study was not published and was not available in any form. Information from FEMA was requested and FEMA has indicated that there are no detailed studies available for this stretch of the St. Vrain River. A previously submitted CLOMR for this same location of the St. Vrain River was • approved in 1999. The request was based on a different proposed design concept and was not constructed. The data used to prepare the above-