13000000095 Santa Maria Dam Inundation Report

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SANTA MARIA DAM - 7 - July 11, 2013 INUNDATION MAPPING REPORT The flood peak will have attenuated to a discharge of 48,900 cfs when it reaches the City of Monte Vista. Most of the flooding will occur through the wide open, sparsely populated pastureland located to the north of the city. Several structures located in this pastureland will be inundated, and the U.S. 285 bridge over the Rio Grande is expected to be overtopped. A few structures on the northern end of the main part of town will be inundated as well. A maximum overbank depth of 4 feet and maximum overbank velocities of 4 to 5 feet per second are expected through the pasture north of Monte Vista. After leaving the Monte Vista area the flood will follow the Rio Grande southeast through a very wide floodplain for approximately 25 miles to the City of Alamosa. There are a small number of buildings located in this floodplain that are expected to be inundated by the flood peak, and all bridges in this reach are expected to be overtopped. Most of the overbank flooding between Monte Vista and Alamosa will be 3 feet deep or less, and the peak overbank velocities will be 3 to 4 feet per second. Though most of the flood volume will be contained within the main Rio Grande floodplain, some shallow flooding is expected in a secondary floodplain which branches from the main floodplain and heads due east. This will occur in the vicinity of Alamosa County Road 106. This shallow flooding was not modeled in HEC-RAS, but it is included in the inundation mapping. The peak flow rate at the northwest end of Alamosa will be approximately 19,900 cfs. The Rio Grande in Alamosa is bounded by levees on its left and right banks. However, because the flow on the overbanks will be so far out of the main channel as it approaches Alamosa, the levees are not expected to fully contain the flood. Flow on the north overbank as it approaches Alamosa will be mostly 3 feet deep or less, and it will travel southeast until it reaches Highway 17. The Highway 17 embankment is expected to redirect the flow south toward the Rio Grande main channel. After crossing Broadway Avenue most of the north overbank flow will be constrained by the embankment of U.S. Highway 160 until the Rio Grande diverges from the highway at the southeast end of the city. A maximum flow depth of 4 feet and a maximum velocity of 2 feet per second is expected through Alamosa in the north overbank region described above. Some shallow ponding is also expected to occur north of U.S. Highway 160 and east of Highway 17 in Alamosa. The inundation extents on the south overbank through Alamosa will be very wide – about 1 mile outward from the Rio Grande at the northern end of the city, and nearly 3 miles outward from the Rio Grande at the southern end of the city. Maximum overbank depths will range from 1 to 4 feet, and the peak overbank velocity is not expected to exceed 1 foot per second. The peak depth between the levees will reach up to 12 feet, and the average channel velocity is expected to be roughly 2 to 3 feet per second. All bridges in Alamosa are expected to be overtopped.