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<br />7 <br /> <br />III. HYDRAULIC ANALYSIS <br />Hydraul ic analysi s was performed to determi ne water surface profil es for <br />the 10-, 50-, 100- and 500-year floods.. The hydraulic analysis util ized the <br />U.S. Army Corps of Engineers HEC-2 computer model (Reference 3). A general <br />description of the procedures used in the hydraulic analysis follows. <br /> <br />Mapping and Surveying <br />Aerial photography of the Town of Telluride \'Ias taken in June, 1985. <br />Topographi c maps were developed from the aeri al photographs (Reference 4). <br />The map scale was one-inch equals OnE! hundred feet with two foot contour <br />interval. Ground control for the mapping \'Ias performed by a profe!;sional land <br />surveyor and utilized the U.S. Geological Survey benchmark located adjacent to <br />the school. The ground control consisted of spot elevations surveyed at each <br />street intersection throughout the Town. The benchmark used for vertical <br />control was also utilized in the 1977 study mapping. Additional ground <br />control surveys were performed to identify the channel bottom and top of road <br />elevations of bridges. Several recent surveys had previously been completed <br />for private property owners in the Town of Telluride. Surveys us'ing the same <br />benchmark and performed by 1 i censed surveyors were util i zed to suppl ement the <br />mappi ng for channel bottom el evati ons of the San Mi gue 1 River. A channel <br />baseline was drawn to provide a reference point along the channel. <br /> <br />Selection of Parameters for Hydraul ic t~odel <br />Input data necessary for the HEC-2 model include ground elevations, <br />flows, starting water surface elevation, roughness coefficients, and bridge <br />data. The starting water surface elevation for the model was computed using <br />the slope area method because the profi "I es contai ned in the 1978 Flood <br />Insurance Study (FrS) appeared to be in error. The channel bottom profi le of <br />the 1978 FrS was several feet above the topographic map elevation shown on the <br />mapping prepared for the 1977 study (the majority of the 1978 FIS profile and <br />the 1977 study maps are in agreement for channel bottolfi elevation).. <br />Ground elevations were interpolated from the topographic maps and supple- <br />mented with field survey information for thl~ under water portion of the cross <br />sections. Bridges were field inspected and measured. Loss coefficients used <br />