2020-11-17_REPORT - M1988112 - Laserfiche WebLink

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INSTRUMENTATION Piezometers: Piezometer readings from July 2018 through July 2020 are presented graphically on Figure 2. The piezometers all indicate dry conditions or minor amounts of water present at the bottom of the casings due to moisture condensation. This behavior is consistent with historic ' observations and indicates that the sub-drains are functioning. Low phreatic conditions are maintained within the dam embankment. No abnormalities are indicated in the measurements. Underdrains: Table 1 summarizes estimated discharge rates from the underdrain system, measured from a collection pipe where it discharges to the seepage collection pond. Data are presented from June 2019 through July 2020 in Table 1. Figure 3 shows a graph of the discharge rates for the 7-year period between May 2014 and July 2020. Prior to June 2018, flow rates ' generally ranged from 37.5 to 42.5 gpm,with an average of approximately 39 gpm. The recorded flow rates for this past two years from June 2018 through July 2020, ranged from 28.5 to 36.5 gpm,with an average of approximately 34 gpm,or about 5 gpm lower than historic averages. The cause for this reduction in underdrain flow rates is unknown, and may be attributable to sustained drought conditions in the region since 2018. It is also possible that drain flows may be partially restricted due to increased clogging by sediment or precipitates. It is recommended that the pipes be cleaned and inspected again to confirm that the reduction in flows is due to drought conditions and is not due to pipe clogging. This recommendation is carried forward from the 2019 annual report. Table 1. San Luis TSF Underdrain Flow Measurements: July 2019—July 2020 ' Date Flow m 7/30/2019 35.5 8/27/2019 35 9/30/2019 35 10/31/2019 34.5 11/27/2019 34 12/31/2019 33 1/30/2020 32.5 ' 2/27/2020 32.5 4/2/2020 33 4/30/2020 33.5 5/28/2020 36 6/30/2020 36.5 i 3 2020 Dam Safety Inspection Report San Luis Tailings Dam