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I I I I I I I I I I I I I I I I I I I I. INTRODUCTION Physical changes to floodplains have taken place at three locations along the San Miguel River in San Miguel County, Colorado. The County contracted with Simons, Li & Associates, Inc. (SLA) to perform hydraulic analysis at the three changed locations. A Flood Insurance Study (FIS) was prepared and adopted in 1978. The 1978 FIS was used as a basis for performing the hydraulic analysis. II. SURVEYING AND MAPPING Aerial photography was taken in 1974 and topographic maps were prepared for use in the 1978 stuqy. The areas where changes have occurred were field surveyed and cross sections measured. The field survey utilized a U.S. Geo1 ogi ca1 Survey (USGS) benchmark used for the previ ous mappi ng. Cross sec- tion plots are contained in Appendix A. III. HYDRAULIC ANALYSIS The U.S. Army Corps of Engineers (COE) HEC-2 water surface profiles com- puter model was used to perform hydraulic analysis. Input data was prepared by using the field surveyed cross sections. Values for hydraulic parameters such as roughness coefficients and bridge loss coefficients were selected to be compatible with those used in the 1978 FIS. Starting water surface eleva- tions were taken from the 1978 FIS profiles. Appendix B contains complete input and output listings of the HEC-2 model for the 10-, 50-, 100- and 500-year floods and the 100-year f100dway. The peak flows were taken from the 1978 FIS. Profiles for the 10-, 50-, 100- and 500-year flood were plotted and are contained in the map pocket. IV. FLOODPLAINS Floodplains were delineated using the water surface profiles generated by the HEC-2 model. The f100dway was plotted using the known width at each cross secti on and i nterpo1 ated between cross secti ons. F1 oodp1 ai n maps are con- tained in the map pocket. "i