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Flood Insurance Study Report Data Flood Insurance Study - Cooper Slough, Larimer County, Colorado Page 8 of Fort Collins and Larimer County (Reference 3). These maps were compiled by pl10togrammetric methods from 6" flight line vertical aerial plhotography exposed between 1984 and 198EI. The stream crossing structums were surveyed by the Larimer County Surveyol"s office to obtain elevation data and structural geometry. All elevations are refermced to the National GI30detic Vertical Datum of 1929. Elevation reference marks used in this study are srlown on the maps. Based on field observations, debris blockages were determined for culverts and the Mannings "n" roughness coefficients used in the '1987 SLA sltudy were verified or changed. The followin9 debris blockage percentages were determined for each culvert: 100% for the 36" CMP at Vine Drive due to size; 100% for the 48" CMP at the Colorado & Southern Railroad due to the amount of sedimEmt currently in the culvert; 25% for the double 48" CMPs at State Highway 14; and 100% for the box. culvert opening along Lake Caral at Sherry Drive, (because LakEl Canal is assumed to be full during a storm event:,. The Manning's "n" roughness coefficient used for the Cooper Slough channel and overbanks is .045. The Manning's "n" used for the Overflow south of State Highway 14 ranges from .025 to .045; and the "n" value used for the flow along Sherry Drive ranges from .045 to 0.1. Tile starting water surfac3 elevations for the CoopElr Slough HEC-2 model upstream of State Highway 14 were determined by HEC..2 modellin!;1 assuming that the centerline of the westbound lane is at critical depth and acts as a weir (See Appendix. C). Flow through the two 48" CMP's were determined by hand calculations (see Appendix C). Because Sherry Drive and the Overflow both tie into the Boxelder Creek floodplain, the starting water surface elevations for theSES models were taken from the 1981 SLA Boxe der Creek Master Drainaqeway Planninq Study (Reference 10) . Initial HEC-2 analyses on Cooper Slough between Vine Drive and State Highway 14 indicated that a high ground island north of the cas Railroad is shallowly flooded in the 100- and 500-year storms. At the request of City and County officials, a separate HEC-2 analysis was dev1sloped for the 100-year event that diviided flow around the island. This analysis wa:, comprised of two models - one for Bach channel around the island. Floodplains and floodways depicted on the maps accompanying this report were developed from the divided island flow analysis. Appendix 0 presents the divided island flow rrodels. Initial HEC-2 analyses ah.o indicated that Cooper Slough overtops its eastern bank between sections E and G creating a split in flow. Bank overtoppings at sections E through G travel due so Lith and rejoin the 100-yeal" water surface of Cooper Slough at section C. Boxelder Creek overflow discharges were added to the HEC-;~ model