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Subdrain flows were consistently lower over the past year,in the range of approximately <br /> 31 to 35 gpm(Table 1),compared to the previous 5 years where flows ranged from 37.5 <br /> to 42.5 gpm. There was an historic drought in the region in 2018 and the dry conditions <br /> may explain the reduction in underdrain flows. At the time of this 2019 inspection, the <br /> slope above the pipes remains dry, and all piezometers remain dry, and it is considered <br /> unlikely that the observed reduction in underdrain flows is attributable to significant <br /> pipe clogging. <br /> In order to confirm that the pipes are not becoming clogged it is recommended that <br /> jetting and another camera inspection of the pipes be performed. The camera <br /> inspection completed in May 2018 showed some accumulation of sediment or <br /> precipitates was occurring. Damage and partial collapse of the pipes that has been <br /> documented by previous camera inspections (2018, 2014) has not been considered to <br /> be a dam safety concern as long as the pipes are flowing. It is prudent to continue <br /> maintaining the downstream ends of the pipes to ensure that they are clean of sediment <br /> and precipitates that could clog the flows from the sub-drain system, and to avoid the <br /> risk of pore pressure buildup at the toe of the dam. <br /> In summary,maintenance-related recommendations include: <br /> 1. Continue to maintain the surface erosion control measures (cross berms and rock-lined down <br /> drains) in the repaired area at the right downstream slope of the dam. <br /> 2. Monitor erosion gullies that are forming on left downstream slope area next to the lined ditch. <br /> Perform maintenance grading as needed to prevent gullies from undermining the ditch liner. <br /> 3. Clean the underdrain pipes by jetting to remove accumulated sediment and precipitate buildup, <br /> and conduct a camera inspection to evaluate the condition of accessible lengths of the pipes. <br /> 4. Modify the grading in the lower right groin area to direct surface runoff away from the <br /> downstream toe and to reduce erosion, sediment transport and accumulation of sediment at the <br /> toe of the dam which builds up around the under-drain pipe outlets. <br /> The overall condition of the dam is "Conditionally Satisfactory", pending completion of <br /> construction of the South Diversion Ditch drop structure. Please feel free to contact me if you <br /> have any questions regarding this report. <br /> Regards, <br /> Debora J. Miller, Ph.D., P.E. <br /> MILLER GEOTECHNICAL CONSULTANTS, INC. <br /> 7 <br /> 2019 Dam Safety Inspection Report San Luis Taiiings Dam <br />