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Page 2 Mr. Carl Mount March 9, 2007 ~~ CIVIL RES's~URCES,LLC calculated aquifer flow currently being conveyed through the Heit Pit above and below each run of underdrain pipe. Due to the pit configuration and existing topographic features, the West and South underdrain systems are designed to gravity drain to Little Dry Creek. Both underdrains daylight at the same point, roughly 120 feet west of the southwest corner of the slurry wall, onto a riprap pad and surface flow though an existing drainage swale to Dry Creek. South Side Underdrain The south side of the proposed slurry wall is within the modeled ground water "shadow" of the Koenig Pit immediately south. Based on WWE's groundwater model Figure 2 (see Attachments) this shadow decreases the average groundwater gradient to a localized gradient of 0.0022 feeUfoot paralleling WCR 20. The shadow has also caused. the phreatic surface elevation drop on the upslope side of the Heit Pit by roughly 1.3 feet. The shadow impacted groundwater surface elevations are considered the historic, pre-Heft slurry wall, elevations, as modeled by WWE. The South Side Underdrain collects water in a 6 inch diameter perforated ADS pipe covered with a filter sleeve. The pipe run is 800 feet in length at a flat grade day lighting at elevation 4849.0 feet into an existing drainage swale. Refer to construction drawings Sheet 2. This pipe is sized to capture potential groundwater flows above the pipe invert and thereby limit groundwater mounding. Groundwater flows to this pipe were calculated based on Darcy's Equation using the phreatic surface gradient of 0.0022 feet/foot and a hydraulic conductivity (K) of 450 feet/day, from WWE. The total bedrock to phreatic surface potential groundwater flow is 159 gallons per minute (gpm) (0.35 cubic feet per second (cfs)). At the maximum allowable mounding elevation, the average head above the underdrain pipe is 1.5 feet which matches the head necessary for the pipe to carry the'0.35 cfs of water using the Hazen- Williams friction coefficient. The perforated pipe will accept all the groundwater inflow as the pipe will inflow a calculated 100 gpm per linear foot of pipe with 1.5 feet of head, per ADS Technical Note 2.105. This flow will exit the pipe at 1.8 ft/s onto riprap and surface flow to Little Dry Creek. Total bedrock to phreatic surface groundwater flow 159 gpm Inflow rate of pipe ea r foot of pipe ai 1.5 feet of head 100 gpm Pipe flow capacity at 1.5 feet of head 159 gpm West Side Underdrain The west side of the proposed slurry wall is impacted slightly by the modeled groundwater °shadow" of the Koenig Pit and more importantly by the water surface elevation defined by the proximity of Little Dry Creek to the west. The groundwater flows that could mound on the west side of the constructed Heit slurry wall are controlled by Little Dry Creek, the South Side Underdrain, and the proposed west side underdrain pipe. Little Dry Creek is a natural drainage control feature with a flowline elevation five to seven feet below the ground surface elevation at the proposed slurry wall. Normal groundwater levels are likely at or near the invert of Little Dry Creek as evidenced by the established wetlands in the channel bottom. Therefore, groundwater that begins to mound will naturally release to Little Dry Creek. Groundwater flowing north along the west side of the Koenig Pit will be intercepted by the proposed south side underdrain pipe and Little Dry Creek, however, a short length of underdrain pipe has been designed for the west side of the pit to further reduce the mounding potential.