2020-11-17_REPORT - M1988112 - Laserfiche WebLink

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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 was completed in the fall of 2019. Improvements were designed to minimize risk to the dam during major flood events. The completed project substantially increased the hydraulic capacity of the South Diversion Ditch and included 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. Photos 9-15 of the completed structure as observed during the 2020 annual inspection are attached with this report. • Maintenance Issues: Three maintenance items are summarized as follows: (1) Surface erosion on downstream slope and potential for erosion around Drop Structure: Surface erosion gullying that was noted in the 2019 annual report on the downstream slope Iof the dam in the vicinity of the lower left (south) lined drainage ditch appeared to have been repaired. The right(north)downstream slope area that was repaired in 2013 and again in 2014 to correct significant erosion damage that was impacting the ditch liner on the north end of the downstream slope is performing well. Vegetation in that area continues to become established and mature (see Photos 2 and 4), and the upgraded erosion-control systems (cross-berms and riprap-lined down-drain channels) appear to be generally effective. Similar measures may be needed on freshly disturbed areas on both sides of the new South Diversion Drop Structure. The drop structure disturbed areas were seeded following construction in late 2019,but due to sustained drought conditions the vegetation has become only sparsely established on the flatter slopes(see Photo 9).Monitoring of this area is recommended, and if surface erosion starts to develop, it may be necessary to re- seed and install erosion control measures similar to those used on the other side until vegetation is well-established. (2) Maintenance in downstream toe drain area: There continues to be sediment buildup in the downstream toe area due to surface runoff that appears to originate from the downstream right groin area. Minor ponding in the area was observed during this 2020 inspection due to leakage from the outfall pipe downstream from the toe drain manifold (see Photos 4 and 5). Evidence of cattle in the area (probably attracted by the ponded water)was seen and the ground around the pipes was disturbed although the pipes appeared undamaged other than the small leak. A continuing recommendation is that the drainage coming off the south groin be graded to direct storm-water runoff away from the toe area. The pipe leak should be repaired to reduce ponding. s 2020 Dam Safety inspection Report San Luis Tailings Dam