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<br />100-year frequency flood will peak between 12 and 14 hours during a 24 <br /> <br />INTERPRETATION AND USE OF REPORT DATA <br /> <br />hour storm. The timing of peak flows on the San Juan River at Pagosa <br /> <br />Flood hazard exhibits for the San Juan River, McCabe Creek and its <br /> <br />Springs during flood periods is highly variable. <br /> <br />tributaries, are in the Appendix following the text of this report. <br /> <br />Included in the Appendix are:_ Flood hazard area map - index, flood hazard <br /> <br />Duration of flooding will vary, depending on the season in which flood- <br /> <br />area maps, flood profiles, typical valley cross-sections, and tabula- <br /> <br />Ing occurs. In the spring when the streams are high with runoff from <br /> <br />tions for the flood frequency-elevation and discharge data. The <br /> <br />snowmelt, coupled with spring rains, high water could persist for <br /> <br />following are descriptions and suggested uses of the exhibits. <br /> <br />several days. During the late summer months when the streams are <br /> <br />normally low, runoff from thunderstorms will recede in a few hours. <br /> <br />Maps <br /> <br />The flood hazard area on all reaches subject to inundation by the 100- <br /> <br />For planning and flood zone regulation purposes, the 100-year flood is <br /> <br />year flood under present conditions is about 524 acres. The area increases <br /> <br />used locally as the base flood. Larger floods, such as the500-year <br /> <br />to about 680 acres for the 500-year flood. The following tabulation for <br /> <br />areas flooded is by study reach. Location of the study reaches ar~ <br /> <br />flood are also considered in the planning and management of flood-prone <br /> <br />areas. The lOa-year and 500-year events are outlined on the Flood' <br /> <br />shown on the Flood Hazard Study Area Map, sheets 1 and 2. <br /> <br />Hazard Area Maps. In many instances the outerlimlts of the 100- and <br /> <br />500-year floods will appear as one line on the maps. This is due to the <br /> <br /> Flood Hazard Area <br /> (excluding channels) <br /> Study Reach lOa-Year Flood 500-Year Flood <br />Stream No. Acres Acres <br />San Juan River 1 87.4 119.2 <br /> 2 381.3 488.6 <br /> 6 24.4 38.0 <br /> Subtotal 493.1 645.8 <br />McCabe Creek 3 27.2 30.2 <br />West Fork Trib. 4 3.1 3.4 <br />North Fork Trib. 5 0.7 0.8 <br /> Subtotal 31.0 34.4 <br /> Study Area Total 524.1 680.2 <br /> <br />topographic relief and slight difference in elevation between the two <br /> <br />flood events. With the exception of the study area in reach No.6, the <br /> <br />maps, drawings, and other technical data labeled Existing Conditions are <br /> <br />valid as of October 1975.!! The study area in reach No.6 was added in <br /> <br />February 1977 to cover an area of projected development which had not <br /> <br />been anticipated in the original Plan of Study. <br /> <br />The Flood Hazard Area Map Index is useful in respect to location of the <br /> <br />various flood hazard exhibits in the study area. Technical studies were <br /> <br />conducted on 6 reach lengths. The locations of the study reaches and <br /> <br />1/ Date of aerial photography for photogrammetric mapping. <br /> <br />- 15 - <br /> <br />- 16 - <br />