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<br />001799 <br /> <br />Flood Prone Areas <br /> <br />The topography of the Ri 0 Grande Ri ver fl oodpl ai n in the San Lui s <br />Valley is extremely flat. The 100-year floodplain boundaries in the <br />Al amosa area are shown in Fi gure 1. Due to the fl at nature of the <br />floodplain, the existing levee system would be outflanked by the river <br />upstream of the City during high, infrequent flood events. <br /> <br />Natural obstructions to floodflows along the Rio Grande include trees, <br />brush and other vegetation growing in and near the stream channel and <br />between the levees. The meanders of the stream further reduce the <br />flow effi ci ency. Manmade obst ruct ions i ncl ude 1 andfi 11 s along the <br />levee, two road bridges and a railroad bridge. Additionally, the West <br />Side Ditch diversion structures are obstructions to flood flows. <br /> <br />Local citizens constructed the levee system along the Rio Grande River <br />to protect the City of Alamosa in the 1940's. The levee system begins <br />north of the Ci ty and continues to the southeast of town near the <br />sewage lagoons. Major flood events of approximately 25-year or <br />greater magnitude, wi 11 outfl ank the upper end of the levees and <br />become tributary to the areas behind the levees. <br /> <br />Areas of fl oodi ng withi n the study 1 imit duri ng the 100-year runoff <br />event include most of the City of Alamosa, East Alamosa and some areas <br />to the southwest of the City of Alamosa, as shown in Figure 1. <br /> <br />During the June, 1985, flood, the levees contained the floodwater, <br />however, a number of critical parts of the levee system were eroded <br />and leakage occurred at other locations. The leakage possibly <br />occurred due to dense tree population on and adjacent to the levees. <br />Also, flood waters will seep beneath the existing levees. The levee <br />system through the City of Alamosa withstood the June flood with 2-3 <br />feet of freeboard with a number of locations maintaining only a 1-foot <br />freeboard. Had the levee failed, many areas not normally flood prone <br />would have been inundated. <br /> <br />The June, 1985, flood flows were roughly equivalent to a 10-year <br />runoff event. Concern for the adequacy of the exi st i ng 1 evee system <br />is high, should runoff events of greater magnitude occur on the Rio <br />Grande. <br /> <br />-7- <br />