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<br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br />I <br /> <br />We evaluated the effect of a partial cutoff below the dam on the quantity of seepage. The <br />evaluation was made for feasibUity level studies. A detailed seepage evaluation was beyond the <br />scope of our services. Our analysis indicates that a partial cutoff penetrating about half the <br />depth of the foundation soils will reduce seepage losses from the reservoir by about 50 percent. <br />This would involve construction of a continuous cutoff about 100 feet deep beneath the <br />embankment and the north side of the reservoir. A partial cutoff penetrating 90 percent of the <br />foundation soils will reduce the seepage by about 70 percent. We estimate that a full depth <br />cutoff would reduce the seepage by about 90 percent. <br /> <br />Our experience indicates actual seepage may vary from the values estimated by a multiple of <br />10 to 100. Decisions based on seepage estimates should consider this potential variability. <br /> <br />5.3.6 SUMMARY OF EXISTING CONDITIONS AND RECOMMENDED INTERIM OPERATION <br />PLAN <br /> <br />The results of the analyses indicate that the existing embankment has an adequate factor of <br />safety with regard to slope stability failure. However, phreatic levels in the embankment have <br />been fairly high during periods of reservoir operation. and the subsurface investigations indicate <br />that zones in the foundation materials are highly pervious. Continued, uncontrolled seepage <br />could result in a piping failure of the embankment and/or foundation materials. Particle <br />migration has not been observed in the seepage from recent reservoir operations; however, the <br />operations have been at restricted levels. so the effect of operation at full reservoir levels in <br />unknown. <br /> <br />We recommend that the current reservoir operational guidelines be continued until embankment <br />modifications can be made. Daily monitoring of the reservoir and embankment should be <br />continued during periods of reservoir operation including a visual inspection of the embankment, <br />measurement of seepage in the flow weirs, and measurement of water levels in the downstream <br />piezometers. In addition. water levels should be monitored and evaluated in the fIVe <br />piezometers Installed for this investigation on a weekly basis during periods of reservoir <br />operation, and on a monthly basis at other times during the year. We would be glad to assist <br />you in evaluating piezometer measurements. <br /> <br />5-13 <br />