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ryj!l Page 9 of 11 <br />Mr. Jared Ebert <br />January 20, 2010 <br />structures please submit the results of the HECRAS model as well as the inputs used <br />for the model with an explanation how the inputs were derived. <br />Sizing of the in, flow/outflow structures is in accordance with methodologies presented <br />in the UDFCD guidelines. As previously discussed, this methodology is expected to <br />be conservative for the Parsons Mine. Therefore, detailed verification of the <br />adequacy of the design is not necessary. A summary of the design parameters for <br />each of the inflow/outflow structures is provided on the attached table titled "Design <br />Parameters for Inflow/Outflow Structures. " <br />3. According to the plan, the inflow/outflow channel's will be excavated to a depth of <br />two feet. Please clarify where the two foot excavation will be keyed off? For <br />example, the channel should be excavated two feet below the top of the pit bank and <br />then excavated at the same elevation from the top of the pit bank to the river. It <br />should not be excavated just two feet uniformly across the land surface between the <br />pits and the river. <br />Inflow/outflow channels will be constructed at a horizontal longitudinal with an <br />invert elevation as specified on the attached table titled "Design Parameters for <br />Inflow/Outflow Structures. " <br />4. For each of the inflow/outflow structure for each mining cell using the unit <br />hydrograph for a 100-year flood event, please submit tables that compare the river <br />water elevation and the pit water elevation. Please time the tables so the water level <br />for the river and the pit are displayed for every 15 minutes during the flood event. If <br />this exercise does not demonstrate that the cells are filled with flood water prior to <br />overtopping of the riverbank, the inflow/outflow structures must be re-designed to <br />achieve that goal. <br />As previously discussed with DRMS Staff, detailed verification of the adequacy of the <br />design is not required if the inflow/outflow structures are designed in accordance <br />with UDFCD guidelines. <br />5. Please commit to grading and armoring the pit-side embankment at the inflow point <br />of the mining cells as soon as possible after mining is complete. <br />A grouted riprap rundown will be constructed along the pit side of the reclaimed <br />embankment at the inflow point of each of the inflow/outflow structures. These <br />rundowns will be constructed as soon as possible after mining is completed. <br />6. Please provide the seed mix that will be planted in the inflow/outflow channel. This <br />should include the names of the native species and the pounds of live seed per acre <br />that will be used. Also, this channel will be prone to erosion until native grass has <br />begun to establish. Please commit to planting a fast growing cover crop in the <br />channel and specify the species of grass that will be utilized. <br />A seed mix for the inflow/outflow spill structures has been added to Exhibits E and F.