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<br />~ <br /> <br />2.3 Principal Flood Problems <br /> <br />The Rio Grande Basin in Colorado, because of its relatively high <br />elevation and sheltered position, is less subject to destructive <br />floods than any other major basin in Colorado. The earliest flood, <br />that of 1869, is mentioned in newspaper accounts of considerably <br />later date and little is known about it except that some damage did <br />occur in Alamosa. A flood in 1884 was caused by the melting of a <br />very heavy snow cover, augmented by heavy rains. This flood lasted <br />longer than any other known flood (Reference I}. The Rio Grande was <br />out of banks at Del Norte from about May 24 to June 20. The flood <br />peaked on June 14 and 15, and was probably the greatest known flood <br />in the San Luis Valley. The flood of 1905 was the only destructive <br />flood in which rainfall was not a factor. This flood was caused by <br />snowmelt. The flood of 1911 was the only one recorded in the upper <br />Rio Grande Basin that was caused wholly by heavy rains. The flood of <br />1927 was the highest since stream gage records have been kept. This <br />flood was caused by melting snow and increased by precipitation. <br /> <br />Scenes of the 1911 flood at Alamosa are shown in Figures 2 through 5. <br /> <br />Natural obstructions to floodflows along the Rio Grande include <br />trees, brush, and other vegetation growing in and near the stream <br />channel and between the levees. The meanders of the stream further <br />reduce the flow efficiency. Manmade obstructions include landfills <br />along the levee, two road bridges, and a railroad bridge. Additionally, <br />two diversion channels and their associated embankments are obstruc- <br />tive to floodflows. <br /> <br />Flooding is also caused in some areas by street and runway drainage, <br />an increased water table, direct rainfall, and drainage from other <br />zones via rivulets. <br /> <br />Floods of approximately the 25-year or greater magnitudes will <br />outflank the upper end of a levee system built along the Rio Grande <br />in the area of the city, and become entrapped in the overbank areas. <br /> <br />2.4 Flood Protection Measures <br /> <br />No flood control structures exist on the Rio Grande above Alamosa to <br />protect the study area. Local interests have constructed a levee <br />system along the Rio Grande to protect Alamosa. The levee system <br />begins just north of Alamosa and continues to the south of the <br />community. Floods of about 25-year or greater magnitude will out- <br />flank the upper end of the levees and become entrapped in the overbank <br />areas. <br /> <br />5 <br />