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~ ,..' \'i Colorado Department of Corrections April 10, 2003 Page 3 A seepage analysis was initially perfoTmed to evaluate whether a significant portion of the seepage was OccuTTing through the constructed embankment dam, which compTises up to about 50% of the TeservoiT slope area. The analysis was perfoTmed using the finite element progTam SEEP2D (GMS v3.0, BYU Environmental Research LibTaTY, 2000). A simplified, 2-dimensional model was developed using design information discussed above, and a permeability of 1.0 x 1 0~7 em/see for the 12-inch thick liner and the embankment dam material. Due to unsaturated; non- steady state flow conditions occurring through the lineT system, it was not possible to accurately estimate seepage quantities and corresponding material permeability values using the finite-element model. However, the seepage flow net produced by the analysis generally indicated that most of the seepage is occuTring through the lineT, and a comparatively small portion is occurTing through the embankment. This finding generally agrees with our undeTstanding that no flows have been observed exiting the existing downstream toe drain, which suggests that the seepage is not intersecting the downstTeam blanket drain. Subsequent seepage analyses were performed using hand and spreadsheet calculations based on infiltration flow rates through a 12-inch lineT of varying permeability at different Teservoir levels. Stage-volume Telationships fOT the Teservoir developed by the dam and reservoir civil designer, GMS, Inc. (March 9, 2000), were used to evaluate the surface area of the Teservoir liner at different wateT storage elevations. A water storage height of 15 feet, cOTTesponding to the approximate normal high Teservoir level at Elevation 6094 feet was used to estimate seepage rates for the original and bentonite-amended liners. A uniform permeability of 1.6 x 10-7 em/see was used for the original liner system, and a peTmeability of 1.0 x 10-7 em/see was assumed fOT the bentonite-amended liner system based on the estimate by GTG discussed above. The analyses also assumed no seepage !brough the constructed embankment dam. Based on the analysis results, seepage rates of 0.014- acre-feet/day and 0.009 acre-feet/day weTe calculated for the original, natural lineT and bentonite-amended liner, respectively. Analyses weTe then performed to estimate the approximate range of permeability for the current, bentonite-amended Ii neT system. The analyses were perfoTmed using estimated average daily seepage rates at diffeTent TeseTvoir levels compiled by Bishop Brogden Associates and based on inflow/outflow measurements for the years 2000 and 2001. To bTacket the impact of tt.le embankment dam on controlling seepage through the liner, analyses were performed fOT two conditions: assuming no seepage through the portion of the reservoir including the embankment dam, and assuming that the seepage Tate through the embankment dam was similar to that fOT the 12-inch liner. The analysis results aTe summarized in Tables 1 and 2 below. C:\My Documents\MJ\leners\03-'.210.gjm.doc Kumar & Associates, Inc.