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i I i ,- "'....."i.~.~ .t<~.... ". . ..~..... -. "" "r The U.S.G.S. Bench Mark "E 357" was used by one survey crew as the datum for a level circuit that established a temporary bench mark (TBM) on the headwall of a concrete box culvert for Boxelder Creek under Prospect Street. A brass cap bench mark (#25) set for Larimer County using U.S.G.S. datum for control of development adjacent to the Cache La Poudre River Floodplain was used by a different survey crew as the datum for a second level circuit to check the elevation of the TBM on the box culvert under Prospect Street. The independent surveys, both in excess of 1.5 miles in length, established the TBM elevation within 0.05 feet of each other. The elevation of 4900.15 feet above mean sea level established by the level circuit from the U.S.G.S. BM "E 357" was used in all topographic infor- mation gathered for Prospect Meadows and for this investi- gation of Boxelder Creek. The topographic information gathered included a grid-type pattern of elevations for all of Prospect Meadows; cross-sections at key locations through- out the reach of Boxelder Creek; and detailed information on Prospect Street, the concrete box culvert under Prospect Street and the flume-type crossing of the Cache La poudre Reservoir Inlet Canal over Boxelder Creek. The U.S. Corps of Engineers' H.E.C. 2 computer program "Water Surface Profiles" was used to make the hydraulic investigations. Manning's Coefficients, "n" values, of 0.05 for overbank flow and 0.03 for channel flow,were used to be conservative in the analysis (the majority of over- bank flow is over short grass and weed cover with little variation in the shape or direction of the floodplain in this reach). The obstruction of Prospect Street with the undersized culvert for Boxelder Creek was investigated using the "normal bridge routine" option of the H.E.C. 2 program, while the canal flume with pier supports over ) }