12000000065 Fuchs Dam Inundation Report

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Table 4. Fuchs Reservoir Dam Failure Summary of HEC-RAS Results at Critical Sections HEC-RAS WSEL(ft) Ave Vel Chan Vel Top Width Critical Section Station NAVD 88 Q(cfs) (ftls) (ft/s) (ft) Fuchs Dam 150378 10772.1 6,600 11.0 16.2 100 Burro Creek confluence 123000 9078.9 6,400 9.3 8.1 200 W.Fk.Pinos Creek confluence 113745 8786.2 6,300 8.8 15.1 200 Bennett Creek confluence 96346 8487.6 5,400 8.2 9.2 500 Pinos Creek Rd. 71817 8137.3 3,800 4.3 6.1 1,100 U.S. Hwy 160 57913 7960.3 3,100 7.8 7.8 1,900 Rio Grande confluence 37860 7890.2 1,700 4.1 5.3 900 Oak St./Hwy 112 33070 7874.1 1,600 2.6 2.6 200 Rio Grande split 30926 7867.4 1,500 3.7 4.0 1,100 W.County Rd.5n 2033 7800.0 1,100 0.8 0.8 2,400 3.0 INUNDATION EXTENTS 3.1 Detail of Mapping The water surface profile from the HEC-RAS hydraulic modeling was exported to ArcGIS, and the inundation extent was delineated using the 2012 USDA 10-meter DEM. All bridge crossings in the study reach were assumed to be significantly overtopped. For this reason, ground surveys were not necessary at bridge crossings or roadway embankments. The inundation maps associated with this report are provided in Appendix E. 3.2 Description of Inundation Area The flood from a clear-day piping failure of Fuchs Dam will reach a peak flow rate of 6,600 cfs at the breach, and will flow through a steep and narrow canyon along the East Fork of Pinos Creek for approximately 7 miles to the confluence with the West Fork. This reach contains very limited development; however, a few structures as well as Pinos Creek Road near the West Fork confluence are expected to be inundated. Channel velocities calculated in this reach average 16 feet per second, and maximum depths range from 6 to 14 feet. Below the confluence of the East and West Forks, the Pinos Creek Canyon very gradually begins to widen,but the steep grade will cause the flood wave to continue down at a high velocity. Before the flood reaches the wider and flatter farmland near the Pinos Creek Road crossing, flow velocities will average 10 feet per second and maximum depths will range from 2 to 8 feet. Several structures in this reach are expected to be inundated. The flood will begin to expand laterally as it approaches the Pinos Creek Road crossing because of the flatter terrain. Pinos Creek Road is expected to be overtopped by the flood wave. Downstream of the crossing, the flow will likely split and follow multiple drainage paths until rejoining the main stem of Pinos Creek downstream of U.S. Highway 160, which is also expected to be overtopped. FUCHS DAM -4 - June 15,2012 INUNDATION MAPPING REPORT