2021-07-22_REVISION - M2010071 (6)

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Cell 2A As specified in Exhibits E and F, Cell 2A will construct a compacted clay embankment around the perimeter of the reservoir. For the purpose of mitigating potential groundwater mounding up-gradient of the reservoir. The Cell 2A underdrain was designed as follows: Since the only historical onsite water table data in the vicinity of Cell 2A includes elevation data form piezometers P4 and P5, regional water table elevation contours published by the USGS Front Range Infrastructure Resources Project (FRIRP) Fact Sheet 113-98, were used to determine baseline upgradient and westerly water table information. This was done by developing a 3 dimensional surface of the USGS data, then lowering it by a uniform value to correspond with the Goose Haven site datum. The 3d surface water table surface from onsite data was then merged with the USGS data to yield an approximate overall water table contour map which is shown in Exhibit G Sheet 1. An analytical solution to groundwater flow based on Darcy's law was then used to determine the volumetric flow requirement of the drain. Given a conservative assumption that that groundwater mounding could saturate the full vertical extent of the soil profile above the historical water table, the calculation is as follows; Darcy's Law ; Q=(K*dh/dl)*A where; Q= volumetric flow K= saturated hydraulic conductivity = 28 ft/day (from NRCS web soil survey) dh/dl = hydraulic gradient = .02 (from Exhibit G — Sheet-1) A= Cross sectional area = 2300 ft x 4ft= 9,200 ft2 Therefore Q= 28 May x .02 x 9,200 ft' = 5,152 ft3 /day = 0.06 c.f.s. The underdrain alignment extends for 30+95.34 ft of which 2,300 ft are perforated pipe and Exhibit G, Attachment A gives the modeled flow capacity for 2300 ft of perforated section and 795.34 ft of solid pipe. Both sections are calculated to exceed the design flow of 0.06 c.f.s. The end of the underdrain alignment ties into an existing 8" pipe which daylights into the lower Boulder Ditch. This existing pipeline to Lower Boulder Ditch was historically used to drain the agricultural fields occupied now by Cell 2A. Underdrain Construction The underdrain was constructed by excavating a trench to the elevations shown in Exhibit G Cell 2A Underdrain Plan & Profile Sheets 2-7. For the first 2,300 ft, 6" SDR-22 PVC pipe was perforated with 2 tows of 3/8" holes spaced 1 ft apart. The pipe was backfilled with 6" of d50 = 1" rock surrounded with mirifi geotextile fabric surrounded by 6" of sand to create a filter pack. The remaining trench solid 795.34 ft section was bedded with 6" of sand. The entire trench was then backfilled with native silty sand (ML).