2019-10-22_REVISION - M1988112 (2)

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INSPECTION OBSERVATIONS AND MAINTENANCE RECOMMENDATIONS Specific dam-safety related observations are summarized as follows: • South Diversion Ditch Improvements: The South Diversion Ditch rehabilitation project, which was designed to minimize risk to the dam during major flood events, was nearing completion at the time of the June 4, 2019 inspection. The project substantially increases the hydraulic capacity of the South Diversion Ditch. The project includes grading improvements to the diversion channel that collects and conveys storm water runoff from the major contributing watershed areas south of the tailing facility, and replacement of the drop structure at the left dam abutment with a robust, reinforced-concrete, open-chute spillway structure. Channel improvements were completed by BMRI in 2016. The drop structure replacement project construction was in progress on the date of this current inspection (June 4, 2019) as shown on Photos 8-12 and 16-17. Construction phase engineering services are being provided by Engineering Analytics (EA) with consultation by MGC. • Maintenance Issues: Three on-going maintenance issues have been identified: (1) Surface erosion on downstream slope: Minor surface erosion (rilling and gullying) is occurring on the downstream slope of the dam (see Photo 25 for example), and notably in the vicinity of the lower left(south) lined drainage ditch(see Photo 15). Deeper gullies that could eventually undermine the left (south)-side ditch liner should be maintained. The right downstream slope area that was repaired in 2013 and again in 2014 to correct significant erosion damage which was impacting the ditch liner on the north end of the downstream slope is performing fairly well (see Photos 23 and 24). Erosion control measures in this area include surface re-grading and seeding, and installation of cross-berms and riprap-lined down-drain channels. The upgraded erosion-control systems appear to be generally effective as vegetation continues to become established and mature. Ongoing maintenance of this area is recommended to maintain the erosion control measures until vegetation is well-established. (2) Storm Water Diversion Away from Downstream Toe Area. There continues to be sediment buildup and minor ponding in the area due to surface runoff that appears to originate from the downstream right groin area(Photos 18 and 19). It is recommended that the area be graded to direct storm-water runoff away from the toe area and minimize the maintenance required to keep the pipe discharge area open and dry for inspections. (3) Underdrain collector pipe outlets: The three, 8-inch diameter sub-drain pipe outlets at the downstream toe of the dam were connected to a manifold at the time of the inspection (see Photo 19) so it could not be determined if all three pipes are flowing. 6 2M Dam Safety Inspection Report San Luis Tailings Dam