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INSPECTION OBSERVATIONS AND MAINTENANCE RECOMMENDATIONS <br /> Specific dam-safety related observations are summarized as follows: <br /> • South Diversion Ditch Improvements: The Detailed Inspection Report (MGC 2014) <br /> included recommendations for design and construction of improvements to the South <br /> Diversion Ditch to minimize risk to the dam during major flood events. In 2015 to early <br /> 2016, MGC, in collaboration with Engineering Analytics (EA) and DOWL-HKM <br /> engineering firms, completed hydrologic and geotechnical engineering studies and <br /> prepared final engineering design,construction drawings and technical specifications for <br /> a project that will substantially increase the hydraulic capacity of the South Diversion <br /> Ditch. The project includes improvements to the diversion channel that collects and <br /> conveys storm water runoff from the major contributing watershed areas south of the <br /> tailing facility, and replacement of the hydraulically deficient conduit drop structure at <br /> the left dam abutment with a more robust reinforced concrete chute structure. Channel <br /> improvements were completed by BMRI in 2016. The drop structure replacement project <br /> construction was in progress on the date of this current inspection (June 20, 2018) as <br /> shown on Photos 9-11. Construction phase engineering services are being provided by <br /> Engineering Analytics(EA)with consultation by MGC. <br /> • Maintenance Issues: Three on-going maintenance issues have been identified: <br /> (1) Surface erosion on downstream slope: Minor surface erosion(rilling and gullying) <br /> has occurred in an area on the downstream right(north)groin of the dam embankment <br /> slope. This area was repaired in 2013 and again in 2014 to correct significant erosion <br /> damage that was impacting the liner. Repairs consisted of surface re-grading and <br /> seeding efforts. Additional erosion protection measures (cross-berms) were <br /> recommended and implemented after re-grading in 2014 to try to prevent ongoing <br /> gullying and erosion in this area. The erosion-control measures were enhanced <br /> further in 2015 by adding riprap-lined down-drain channels, reshaping the cross <br /> berms and re-seeding. The upgraded erosion-control systems appear to be generally <br /> effective as vegetation continues to get established and mature (see photo 20). <br /> Ongoing maintenance is recommended to correct for minor erosion where flow from <br /> the rock-lined down-drains is jumping over the contour berms at the bottom of the <br /> rock channels, and the rock in the steep down-drain channels is slipping off of the <br /> underlying geotextile in some places(see Photo 19). Vegetation appears to be getting <br /> established and the erosional stability in this area is anticipated to continue to <br /> improve over time as vegetation matures. <br /> An erosion gully was observed on the upper left (south) downstream slope adjacent <br /> to the access road (Photo 8). Final grading associated with the drop structure <br /> 5 <br /> 2018 Dam Safety Inspection Report San Luis Tailings Dam <br />