12000000065 Fuchs Dam Inundation Report

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Several more structures in this reach will likely be flooded. Channel velocities can range from 2 to 8 feet per second, and depths may exceed 5 feet. The Montoya-Mexican Ditch, which diverts water from an eastern branch of Pinos Creek about a half mile upstream of U.S. Highway 160, is expected to convey some of the flood to the east toward Del Norte. The ditch can possibly overflow and spill into the fields it irrigates to the north, which may also result in some shallow flooding across the western side of Del Norte. The shallow flooding is expected to re-converge with the main flood wave in Pinos Creek and in the Rio Grande upstream of the Oak Street Bridge. Though this shallow flooding was not modeled in HEC-RAS, it was included in the inundation mapping. The flood wave will follow Pinos Creek to the confluence with the Rio Grande at the western end of Del Norte. The Rio Grande is expected to convey the flood through a majority of the town without significantly overtopping its banks. However, the Oak Street Bridge may be overtopped, and the banks downstream of the bridge may also overtop which will cause several structures to be inundated. The channel velocities calculated in this reach range from 1 to 8 feet per second, and maximum depths can exceed 9 feet in some places. Downstream of Del Norte the Rio Grande splits into two channels, and the flood wave may follow one or both of the channels. Most of the flood will likely be contained within either channel,but overtopping of the banks is possible. If overtopping occurs,multiple structures on the outer fringe of either channel, as well as some structures between the two channels are in danger of being flooded. Channel velocities calculated in this reach range from 1 to 4 feet per second, and depths range from 1 to 4 feet. As reported in the effective FEMA FIS,the 100-year and 10-year floods at Del Norte are 10,734 cfs and 7,701 cfs,respectively—both significantly greater than the 1,100 cfs estimated 3.5 miles downstream of Del Norte due to a clear-day breach of Fuchs Dam. For this reason,the inundation mapping extent was terminated downstream of Del Norte near West County Road 5n, which is approximately 3.5 miles southeast of Del Norte. FUCHS DAM - 5 - June 15,2012 INUNDATION MAPPING REPORT