13000000095 Santa Maria Dam Inundation Report

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SANTA MARIA DAM - 4 - July 11, 2013 INUNDATION MAPPING REPORT Table 3. Santa Maria Reservoir Dam Failure Summary of HEC-HMS Results Downstream of Santa Maria 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) Santa Maria Dam 0 hr 0 min 1 hr 26 min 298,400 Confluence w/ Rio Grande 0 hr 21 min 1 hr 37 min 293,000 Creede 3 hr 25 min 3 hr 39 min 209,400 South Fork 6 hr 34 min 6 hr 54 min 119,800 Del Norte 11 hr 7 min 11 hr 28 min 75,200 Monte Vista 17 hr 0 min 19 hr 58 min 48,900 Alamosa 1 d 18 hr 55 min 2 d 0 hr 19 min 19,900 Conejos/Costilla County Line 2 d 22 hr 28 min 3 d 14 hr 59 min 9,300 Lasauses 3 d 6 hr 34 min 4 d 3 hr 54 min 7,400 The estimated peak flow rate at Lasauses due to a clear-day breach of Santa Maria Dam (7,400 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 website2. Using this data to approximate maximum annual stream flows, a Log-Pearson Type III analysis estimated 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 Santa Maria 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. 2 The maximum monthly stream flow data is the maximum daily average stream flow during each given month.