13000000095 Continental Dam Inundation Report

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CONTINENTAL DAM - 7 - July 11, 2013 INUNDATION MAPPING REPORT The flood peak will have attenuated to a discharge of 34,800 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 3 feet and a maximum overbank velocity of 4 feet per second is 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 2 feet deep or less, and overbank velocities are not expected to exceed 3 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 10,200 cfs. The Rio Grande in Alamosa is bounded by levees on its left and right banks. However, because the flow on the left (north) overbank will be so far out of the main channel as it approaches Alamosa, the north levee is not expected to fully contain the flood. Flow on the north overbank as it approaches Alamosa will be mostly 2 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 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. Conversely, the right (south) levee is expected to contain all or most of the flood peak through Alamosa. The south levee is not expected to overtop, but there will be some shallow flooding through the city which would occur due to a small amount of water flowing around the upstream extent of the levee. This shallow flooding was not modeled in HEC-RAS, but it is included in the inundation mapping. The assumption that the south levee will contain the Continental Dam breach flow is reasonable because the attenuated flow rate at the northwest end of Alamosa (10,200 cfs) is comparable to the 100-year flow rate at Alamosa reported in the effective April 21, 1999 FEMA Flood Insurance Study (approximately 11,000 cfs). On the effective FEMA Flood Insurance Rate Map the south levee was shown to contain this 100-year flood. The peak depth between the levees will reach up to 11 feet, and the average channel velocity is expected to be roughly 2 feet per second. All bridges in Alamosa are expected to be overtopped. The flood will spread out over a very wide floodplain after leaving Alamosa. Overbank flood depths from Alamosa to the small community of Lasauses about 22 miles downstream are expected to be 1 to 2 feet deep or less. The average overbank velocity in this reach will be less than 1 foot per second, and the peak overbank velocity is not expected to exceed 2 feet per second. The Alamosa County Road S 13.7 bridge and the Conejos/Costilla County Road Z bridge will be