13000000095 Continental Dam Inundation Report

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CONTINENTAL DAM - 4 - July 11, 2013 INUNDATION MAPPING REPORT Table 3. Continental Reservoir Dam Failure Summary of HEC-HMS Results Downstream of Continental Dam Location Estimated Time of Arrival from Start of Breach Development Estimated Time to Peak from Start of Breach Development Estimated Peak Flow Rate (cfs) Continental Dam 0 hr 0 min 1 hr 5 min 283,400 Confluence w/ Rio Grande 0 hr 58 min 1 hr 36 min 269,500 Creede 3 hr 59 min 4 hr 15 min 146,300 South Fork 6 hr 57 min 7 hr 13 min 92,600 Del Norte 11 hr 44 min 12 hr 8 min 54,400 Monte Vista 17 hr 41 min 21 hr 16 min 34,800 Alamosa 1 d 21 hr 25 min 2 d 1 hr 50 min 10,200 Conejos/Costilla County Line 2 d 23 hr 33 min 3 d 11 hr 17 min 4,100 Lasauses 3 d 8 hr 42 min 4 d 0 hr 30 min 3,200 The estimated peak flow rate at Lasauses due to a clear-day breach of Continental Dam (3,200 cfs) is less than the estimated 100-year flood (8,000 cfs). The 100-year flood peak was estimated using a Log-Pearson Type III distribution analysis because there was no flood study available on the Rio Grande near Lasauses. Maximum monthly stream flow data for the USGS gaging station “Rio Grande River above Trinchera Creek Mouth near Lasauses” from 1936 to 2011 was downloaded from the DWR’s CDSS website1. Using this data to approximate maximum annual stream flows, a Log-Pearson Type III analysis estimated the 100-year flood to be 8,000 cfs. The Log-Pearson spreadsheet is provided in Appendix E. 2.3 Hydraulic Modeling in HEC-RAS The USACE’s water surface profile modeling computer program HEC-RAS was used to create a steady-state hydraulic model of the area downstream of Continental Dam using the flow rates and locations shown in Table 3. Cross-sections of the flow path were developed from the best available USGS 10-meter DEM using the HEC-GeoRAS extension in ArcGIS. The average cross section spacing over the modeled reach was roughly 0.5 miles, with critical areas such as towns having a closer spacing, and less-critical uninhabited areas having a farther spacing. Cross sections were cut perpendicular to the direction of flow in the channel and in the left and right overbanks. Manning’s n-values were estimated from aerial photography and adjusted according to the depth of overbank flow. No bridges in the study reach were modeled and all were assumed to be overtopped. A summary of the HEC-RAS results at critical sections is shown in Table 4, and a summary of the full HEC-RAS model results is attached in Appendix F. 1 The maximum monthly stream flow data is the maximum daily average stream flow during each given month.