13000000095 Santa Maria Dam Inundation Report

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SANTA MARIA DAM - 2 - July 11, 2013 INUNDATION MAPPING REPORT close to the normal storage listed in the state’s records. The datums were matched up using the spillway crest elevation as a reference. A summary of the elevation data is shown in Table 1. Table 1. Santa Maria Reservoir Dam Failure Summary of Elevation Data Description 1911 & 1990 Plans (feet, unknown datum) Capacity Table (feet, gage height) Dam Crest 9490 101 Emergency Spillway Crest 9482 93 Approx. Natural Ground under Embankment 9400 11 The elevations cited in the remainder of this memo are in reference to the gage height. The approximate surface area of Santa Maria Reservoir was estimated for each gage height from the capacity table. At the spillway crest elevation (gage height 93) the surface area was estimated as 621 acres. This was very close to the reservoir pool surface area which was measured from a USGS quad map in ArcMap (617 acres), therefore validating the assumption that gage height 93 was a reasonable estimate of the spillway crest elevation. An elevation-area-capacity curve based on the capacity table is provided in Appendix B.1 The modeled breach location for a clear day failure was assumed to be a piping failure of the main dam down to natural ground below the embankment (gage height 11 feet). A clear day failure of the outlet tunnel below the auxiliary dam would not cause as large of a peak outflow because the majority of the tunnel is excavated in rock and the natural ground interface below the centerline of this embankment is approximately gage height 81 feet. Two empirical methods for predicting the average breach width and failure time were compared. The methods, which are included as spreadsheets in Appendix C are the Froehlich (2008) equations and the MacDonald & Langridge – Monopolis/Washington State (MLM-WA, 2007) equations (collectively referred to as the “empirical equations”). The empirical equations predict the average breach width and failure time as a function of depth of water, dam height, and volume of water stored. The MLM-WA method also takes into consideration the estimated volume of embankment eroded. The Froehlich method resulted in predicted bottom and average breach widths of 230 and 293 feet, respectively, and a breach formation time of 1.44 hours. However, based on the Typical Sections sheet in the 1911 plan set, the canyon will constrain the breach to a bottom width of approximately 175 feet. The breach formation time of 1.44 hours remains unchanged because it is independent from the breach width in the Froehlich method. 1 Discontinuities were identified in the elevation-area curve by D&A which showed drops in reservoir pool surface areas corresponding to increases in gage height. D&A determined that this was likely due to error in the handwritten capacity table, so D&A adjusted the storage values as shown in the capacity table in Appendix A.