2006-06-12_REVISION - M1982015 (23)

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CONDITIONAL LETTER OF MAP REVISION {CLOMR) FOR • ST. VRAIN LAKES Page 10 100-year Floodwav/ Encroachment Model - A floodway analysis was performed on the St. Vrain River to determine the required floodway width needed such that the 100-year water surface elevation is not increased by more than 1.0 foot. This was done by method 4, which determines encroachment stations, so that an equal loss of conveyance occurs on each overbank if possible. When the main river channel is encroached upon, the water surface is raised and therefore more flow overtops the lateral weirs and flows into the "Existing Lakes" reach. This increases the flow rate in the "Existing Lakes" reach and therefore raises the water surface level in this reach before any encroachment has been applied. A balance of the encroachments on both sides of the river has been attained. This creates a condition where both the north and south side of the river share the encroachment and maintain both sides to be developable. In the vicinity of cross-sections 60842.5, 58930 and 58830 a vertical increase of 1.0 feet is not obtainable due to the oxbow configuration of the river. The bounding cross sections at WCR 13 were not encroached until a vertical increase of 1-foot was obtained because of the overtopping flow on the north • side of the bridge at WCR 13. A path for this overtopping flow needs to be maintained and therefore the encroachment limit is outside this area. In several locations, a vertical increase of 1.0 foot on one cross-section would produce a condition with a higher than 1.0 foot vertical increase at the upstream cross- section, and therefore not all cross-sections are increased by 1.0 foot. The floodway boundary is plotted on the work maps and the results from this analysis are included in Appendix C. Below is a table of the cross-sections with the encroachment stations and the vertical increase this encroachment would produce. Reach River Sta Profile Delta (ft) Enc Sta L (ft) Enc Sta R (ft) -25 to Junction 71770 0.1 1000.0 1277.0 -25 to Junction 71765 0.0 1976.0 2271.0 I-25 to Junction 71735 -0.3* 1991.0 2211.0 I-25 to Junction 71701 0.1 1879.0 2206.0 I-25 to Junction 71634 0.2 1859.0 2200.0 -25 to Junction 71608 0.2 1858.0 2201.0 I-25 to Junction 71168 0.3 1933.5 2510.0 I-25 to Junction 70728 0.3 2129.0 2780.0 I-25 to Junction 70235 0.4 2069.0 2950.0 I-25 to Junction 69741 0.6 2009.0 3130.0 I-25 to Junction 69293 0.4 2030.5 3074.0 I-25 to Junction 68844 0.1 2052.0 3000.0