13000000095 Santa Maria Dam Inundation Report

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SANTA MARIA DAM - 3 - July 11, 2013 INUNDATION MAPPING REPORT The MLM-WA method resulted in a predicted bottom breach width of 245 feet, an average breach width of 425 feet, and a formation time of 3.53 hours. The “Results Check” section of the MLM- WA spreadsheet also predicted that the erosion rate calculated by the equations was slower that what can be reasonably expected. For this reason, the breach parameters predicted by the MLM- WA method were not considered for use in the breach simulation of Santa Maria Dam. 2.2 Dam Breach Simulation in HEC-HMS The U.S. Army Corps of Engineers’ (USACE) hydrologic modeling computer program HEC-HMS was used to simulate a clear-day piping failure of Santa Maria Dam using the breach parameters estimated by the Froehlich empirical equations. For the HEC-HMS simulation the water surface was set at the emergency spillway crest (gage height 93 feet) at the onset of the breach. The assumption was made that the breach would extend from the dam crest (gage height 101 feet) to the natural ground elevation (gage height 11 feet), and all water stored above the breach invert was assumed to drain completely. The (constrained) breach bottom width was entered as 175 feet and the breach side slopes were set to 1.316:1 (horizontal:vertical) in order to model the average breach width of 293 feet. These breach dimensions are illustrated on the Typical Sections sheet in Appendix A. As recommended in the Guidelines for Dam Breach Analysis the piping failure was set to start at the midpoint of the final breach height, and both linear and sine wave breach progressions were simulated. The SEO elevation-capacity table was used to model the reservoir storage. The dam breach parameters and resulting peak outflows are summarized in Table 2. Table 2. Santa Maria Reservoir Dam Failure Comparison of Dam Breach Parameters and Peak Outflows Linear Progression Sine Wave Progression Water Height* (ft) Dam Height* (ft) Vol. of Water* (ac-ft) Ave. Breach Width (ft) Breach Development Time Time to Peak Outflow Peak (cfs) Time to Peak Outflow Peak (cfs) Froehlich 82 90 40,210 293 1 hr 26 min 1 hr 26 min 298,400 1 hr 5 min 276,900 *Height of water, height of dam, and volume of water are measured relative to the breach invert elevation. The linear breach progression resulted in the greatest predicted peak outflow of 298,400 cfs with a time to peak of 86 minutes. This hydrograph was routed downstream through representative reaches using the Muskingum-Cunge method in HEC-HMS. A trapezoidal channel with 1:1 side slopes was assumed. The reach lengths, Manning’s n-values, and bottom width input parameters for the Muskingum-Cunge method were estimated in ArcMap using aerial photos, and the elevations used to compute the routing reach slopes were taken from the USGS 10-meter digital elevation model (DEM). The Manning’s n-values and bottom widths were calibrated and fine- tuned through several iterations between this HEC-HMS model and a steady-state HEC-RAS model of the routing reaches. The reach descriptions, routing times, and predicted flow rates are summarized in Table 3. The HEC-HMS output is attached in Appendix D, and the full HEC-HMS model is included on the Project CD.