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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 the <br /> vicinity of the drain is at approximately 5040.4 or greater. <br /> It is recommended to collect topographic survey data of the Orr Property to resolve the elevation issue.There are <br /> a couple of potential explanations for the differences in elevation. The differences in elevation may be because of <br /> the difference between the NGVD 29 and NAVD 88 vertical data, which is approximately 2.9 feet.The <br /> differences in elevation may also be attributable to interpolation between contours on a map. <br /> The upstream invert of the permanent drain is approximately at elevation 5036.5. The downstream invert of the <br /> drainage channel is approximately at elevation 5036.12. Features of the permanent drain will include a headwall <br /> near the Bull Seep cut no steeper than 3 (horizontal)to 1 (vertical) (3:1). This headwall will be reinforced with <br /> Type M riprap founded on 12 inches of Type 2 riprap bedding. The riprap will not be grouted. The riprap-lined <br /> headwall will convey flows to a concrete structure with an 18-inch diameter opening. <br /> It is anticipated that some over-excavation of the slurry wall will be required to construct the channel from the <br /> headwall to the reservoir. When this occurs, the slurry wall will be repaired with a compacted clay liner to <br /> maintain the integrity of the low permeability liner. Turf Reinforcement Mat(TRM)will be placed on top of the <br /> clay liner to form a stable subgrade for the concrete structure. <br /> The flows through the concrete structure will be regulated with a sluice gate and stop logs. The stop logs will <br /> allow for"fine-tuning"of the water levels and flow rates to the drain, allowing for maintenance of the <br /> compensatory wetlands near the Orr Property. The sluice gate will be closed when the Fulton Ditch releases <br /> flows to the Bull Seep. This will allow the historic flow pattern to return to the Bull Seep and prevent water from <br /> entering the City of Thornton Reservoir. When releases from the Fulton Ditch to the Bull Seep end, the sluice <br /> gate will be opened and the lingering Bull Seep water that contributes to the rise in groundwater levels will be <br /> drained back to the Bull Seep via the City of Thornton reservoir. As mentioned above, the stop logs will then be <br /> used as necessary to maintain the compensatory wetlands. <br /> Downstream of the concrete structure flow will occur in open channel conditions to reduce pipe obstructions <br /> observed in the previous temporary drains. The channel geometries will vary. Immediately downstream of the <br /> concrete structure, it will be necessary to provide a crossing for a perimeter mine/reservoir road. This road <br /> crossing will have slopes no steeper than 10:1 horizontal to vertical and will be 10 feet wide to allow vehicular <br /> passage. A 6-inch thick concrete slab will be constructed at the road crossing. <br /> Downstream of the road crossing, the channel side slopes will transition to slopes of 3:1 horizontal to vertical <br /> which will lead to a 24-inch Parshall flume. The entrance to the Parshall flume will have formed concrete wing <br /> walls and a concrete cutoff wall extending below the flume. The Parshall flume will be equipped with a wet well <br /> into which a transducer may be placed. The transducer may be equipped with a data logger in order to track <br /> flows into the mine/reservoir as required by the Division of Water Resources (DWR). <br /> The operating range for a 24-inch Parshall flume is 0.423 cfs to 33.1 cfs. The depth of water at the measuring <br /> point for this operating range is 0.15 feet to 2.50 feet. To reduce the frequency of tailwater submergence, which <br /> impacts reading accuracy, the flume is set to discharge directly into the steep rundown channel. However, when <br /> the reservoir is above the crest elevation of the between the steep and mild slope channels, impacts to reading <br /> accuracy are expected. No design range of flows was stipulated for this design. The intent of this installation is <br /> to reuse an existing 24-inch flume already on site. <br /> Beneath the flume, flow will be conveyed onto a riprap lined trapezoidal rundown channel constructed on the <br />