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<br />,. <br /> <br />Between locations, where the discharges were specified, the <br />discharge values used in computations were gradually adjusted by <br />proration based on the distance between the points of known discharge. <br /> <br />Hydraulic Analyses <br />Water surface elevations were computed through the use of the <br />September 1990 release of the Corps HEC-2 standard step-backwater <br />computer program for the personal computer (Reference 3). Head <br />losses due to bridges were computed using the Special Bridge <br />routine of the HEC-2 program. All bridges were considered to be <br />unobstructed. Flooding due to ice conditions was not included in <br />this study. <br /> <br />The original HEC-2 hydraulic models used in the Corps 1989 SFH <br />study were modified from models created by a conSUlting firm (using <br />the 1973 discharges). Channel cross sections for these models were <br />field surveyed in 1987. The flood plain overbank elevations in the <br />original models were taken from topograhic mapping. The values for <br />the discharges in these models were changed to the updated discharge <br />values and the water surface profiles recomputed. Other changes made <br />in the original hydraulic models for the 1989 SFH study are described <br />in more detail in the report for that study. <br /> <br />The flood flow patterns of the Cache La Poudre River in the study <br />area were complicated because some of the flow of the river separated <br />from the main flow path during certain flood events. In some areas <br />large excavations from gravel mining had considerable effect on flood <br />flows. Flows not following the main channel were considered to be of <br />two different types - divided flow and spills. <br /> <br />4 <br /> <br />. <br />