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5.0 <br />.1 <br />4.2 Initial Assessment of Adequacy of Facility for Flood Events .. .............................26 <br />4.2.1 Flood Events Evaluated ................................................ .............................26 <br />4.2.2 South Diversion Ditch ................................................... .............................26 <br />4.2.3 Drop Structure Inlet Acceleration Chute ...................... .............................27 <br />4.2.4 Flip Bucket and Plunge Basin ....................................... .............................27 <br />4.3 Results of New Evaluations Performed as part of Detailed Dam Safety <br />Evaluation Process .......................................................... ............................... <br />4.3.1 Hydrologic Check Analysis ................................ ............................... <br />4.3.2 HEC -RAS Modeling of South Diversion Ditch and Drop Structure. <br />POTENTIAL FAILURE MODES .............................................. ............................... <br />5.1 General Description Dam and Loading Conditions ........ ............................... <br />5.1.1 Dam Data (elevations in NAVD 88) ................... ............................... <br />5.1.2 Hydrology Data ................................................... ............................... <br />5.1.3 Seismic Data ....................................................... ............................... <br />5.2 PFMA Process Summary ................................................ ............................... <br />5.3 Potential Failure Modes .................................................. ............................... <br />5.3.1 PFM -1 Overtopping of Drop Structure at Left Dam Abutment <br />during Extreme Flood Event leading to Scour Erosion and Breach <br />ofDam ................................................................ ............................... <br />5.3.2 PFM -2 Internal Erosion or Piping of Embankment Materials <br />Exiting on Downstream Face of the Dam near the Under -drain <br />.27 <br />.27 <br />........29 <br />........34 <br />........34 <br />........34 <br />........35 <br />........36 <br />........36 <br />........41 <br />.41 <br />PipeOutlets ................................................................... .............................45 <br />5.4 Potential Failure Modes Ruled Out (Category IV) ................... .............................48 <br />5.5 Summary of Risk Estimates ...................................................... .............................51 <br />CONCLUSIONS AND RECOMMENDATIONS ............................... .............................53 <br />6.1 Overall Evaluation .................................................................... .............................53 <br />6.2 Recommended Maintenance, Repairs, and Upgrades to Address current <br />conditions.................................................................................. .............................54 <br />6.2.1 Maintenance and Improvements to South Diversion Ditch .......................54 <br />6.2.2 Drainage Outlet Cleanout ............................................. .............................57 <br />6.2.3 Erosion Protection of Right Groin Area ....................... .............................58 <br />San Luis Project Miller Geotechnical Consultants <br />Tailing Dam Data Report iii February 2014 <br />