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HomeMy WebLinkAboutPARAGON POINT PUD PHASE TWO PRELIMINARY - 48 91C - CORRESPONDENCE - DRAINAGE RELATED DOCUMENTL&TONE & ANDERSON, I4 �P Water Resources and Environmental Consultants / 736 Whalers Way, Suite F-200 Fort Collins, Colorado 80525 (303) 226-0120 Final Report August 10, 1992 Date Mr. Glen Schlueter City of Fort Collins Storm Water Utility 235 Matthews Fort Collins, CO 80524 Re: Evaluation of Trilby Road Culvert on Fossil Creek Dear Glen: This letter summarizes the results of the hydraulic analysis of the Trilby Road crossing of Fossil Creek at the south end of the Paragon Point Development. The analysis supplements the work previously performed for both the existing and fully developed conditions documented in the Preliminary Drainage Plan for the Overall Development Plan of the Paragon P.U.D. and submitted by Lidstone & Anderson, Inc. (LA) on November 27, 1991. The Fossil Creek Drainage Basin, Master Drainageway Planning Study, completed August 1982 was also used in the analysis. The existing culvert on Fossil Creek at Trilby Road is an 8-foot diameter corrugated metal pipe (CMP). The minimum roadway elevation is 4892.5 ft at the low point of the crossing. The 100-year, existing condition discharge at Trilby Road is 2,720 cfs which produces a 100-year ponded water elevation upstream of Trilby Road of 4895.1 ft. This translates to 2.6 ft of overtopping depth over Trilby Road for the existing condition. In the Preliminary Drainage Plan, a 24'W x 6'H three -sided reinforced concrete box (RCB) was evaluated with the roadway embankment raised from elevation 4892.5 ft to 4896.0 ft. To account for the existing detention storage upstream of Road, the developed condition SWMM model included the routing of -inflows through the detention area. The 100-year, fully developed condition discharge upstream of Trilby Road was found to be 3,841 cfs while the discharge would be 1,798 cfs downstream of Trilby Road due to the detention routing. This results in a 100-year ponded water surface elevation upstream of Trilby Road of 4896.2 ft and an overtopping depth of 0.2 ft over Trilby Road. An additional analysis was performed for the 100-year, fully developed condition peak discharge with the existing 8-foot diameter CMP and with the roadway embankment raised 2 feet from elevation 4892.5 ft to 4894.5 ft. A SWMM model similar to that used in the analysis of the 24'W x 6'H RCB was used to model the detention ponding upstream of Trilby Road. The results -of the analysis identified a 100-year ponded water surface elevation upstream of Trilby Road of 4895.5 ft along with an overtopping depth of 1.O ft over Trilby Road. �}89J S/ACaZes' /.O�l ©UPI. /�Y so �( /ev. _ �89f(-,J Branch Office: Box 27, Savery, Wyoming 82332 Mr. Glen Schlueter August 10, 1992 Page Two Based on the results presented in the preceding paragraphs, the following conclusions are provided: 1. The existing 100-year water surface elevation upstream of Trilby Road is 4895.1 ft msl. This elevation identifies the lower limit of the floodplain easement recently purchased by the City of Fort Collins. 2. The 100-year water surface elevation associated with fully developed conditions and the proposed culvert (241W x 6' H RCB) is 4896.2 ft msl. This elevation identifies the upper limit of the floodplain easement recently purchased by the City of Fort Collins. 3. The water surface elevation associated with the 100-year, fully developed condition peak discharge and the existing Trilby roadway embankment raised 2 feet is 4895.5 ft msl. This means the flooding limit for this condition falls within the floodplain easement recently purchased by the City.of Fort Collins. If you need additional information related to this analysis, do not hesitate to contact me, or in my absence, Chris Doherty. Sincerely, &t,&AdXJAw%oo Bradley A.'Anderson, P.E. Principal Engineer BAA/tlt