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From available information reported in Technical Revision 2, the lowest elevation on the Orr property was <br />believed to be approximately 5038.5; however, surveyed information shows that the property elevation in <br />the vicinity of the drain are at approximately 5040.4 or greater. <br />It is recommended to collect topographic survey data of the Orr property to resolve the elevation issue. <br />There are a couple of potential explanations for the differences in elevation. The differences in elevation <br />may be because of the difference between the NGVD 29 and NAVD 88 vertical data, which is <br />approximately 2.9 feet. The differences in elevation may also be attributable to interpolation between <br />contours on a map. <br />The invert of the temporary pipes are approximately at elevation 5038.8. Two pipes were installed to <br />temporarily drain the On- property. Based on field observation, this system appears to have lowered the <br />surface water elevation by at least 2 feet. The relation of surface water to groundwater on the Orr <br />property is unknown. The impact on the Orr property groundwater surface elevations is unknown and <br />should be evaluated. <br />Al has directed Tetra Tech to reuse the existing 18 inch ADS pipes. The proposed permanent system will <br />have pipe inverts set at elevation 5038.8, will cross the slurry wall and discharge into the pit. A concrete <br />headwall will be constructed on the upstream end. Notches for stoplogs will be cast in place in the <br />wingwalls. The stoplogs are intended to allow fine tuning of the water surface elevation in the Bull Seep <br />and associated wetlands. The pipes will discharge into a riprap-lined rundown channel to the mine floor. <br />The maximum capacity of the pipes is estimated to be 31.5 cfs. The capacity assumes that approximately <br />4.2 feet of surcharge (up to elevation 5043.0) may be applied to the pipes during high water condition. <br />Elevation 5043.0 is the lowest approximate existing grade elevation in the vicinity of the pipe. That <br />elevation was considered the maximum water surface elevation possible for this analysis. The actual <br />maximum water surface elevation may vary. The capacity of the pipe was calculated using the <br />CulvertMaster computer program. Calculations for the culvert capacity are attached. <br />The rundown channel has a trapezoidal cross-section. The proposed channel will have a 4' bottom width <br />and 3H:1V side slopes. The longitudinal slope is approximately 3H:1V, matching the finished reclamation <br />slopes. The velocity of the flow in the channel is approximately 10.5 fps. Using the UDFCD riprap sizing <br />equation (Eq. MD -13), the sizing factor exceeded the maximum value allowed for plain riprap. Therefore, <br />the rundown channel will be lined with grouted riprap. At least one foot of freeboard is provided within the <br />channel. Calculations for the channel velocity are attached. <br />A series of monitoring points will be used to monitor both ground and surface water levels and their <br />responses to the various stop log levels. These monitoring points will consist of a series of staff gauges <br />and monitoring wells. These points will be monitored by Aggregate Industries. The data collected will be <br />forwarded to the DRMS. <br />2 Tetra. Tech <br />