2021-01-04_GENERAL DOCUMENTS - C1981019

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DocuSign Envelope ID: EBE28081-13782-41342-BAD13-D8C9313687131B December 22, 2020 Angela Aalbers Page 10 of 12 Reference: East Taylor Seep Investigation Table 2 Taylor Creek Flow Survey Station Flow (Jpm) TC-01 37.2 TC-02 9.31 TC-03 4.75 Taylor Creek Flow Diversion Options Stantec evaluated options for eliminating or reducing seepage of Taylor Creek stream flow through the highwall miner holes. There are options for temporary and permanent diversions. Temporary diversions could be used to confirm connectivity between Taylor Creek and the seep, while a permanent diversion could be used to reduce seep flow rates. Reducing the seep flow rates may decrease the size of the seep treatment system, or diverting the flow may reduce the variability of flow rates from the seep. Eliminating the connectivity between Taylor Creek and the seep is not expected to eliminate seep flows considering the seep flow measurements indicate a connection to the greater hydrogeologic system. To better understand the diversion options and feasibility, Stantec used StreamStats to calculate peak flows in Taylor Creek upstream from Outfall 010. StreamStats is a flow calculation program prepared by USGS that uses regression equations and the watershed area and location to calculate peak flows (USGS 2020). The peak flows for various flood events are listed below in Table 3. Pipes could be used to convey stream flows past the area with highwall miner holes, and Stantec used culvert -sizing methods to perform these calculations. The culverts in Table 3 were sized using the channel slope, measured at 7% and for a headwater to diameter ratio of approximately 1. The required pipe sizes range from 24-inch for the 2-year flood to 48-inch for the 100-year flood. These flow values do not include the flow through the alluvium that should also be captured, since this water would also be captured by the highwall miner holes. A smaller pipe than indicated in Table 3 could be used to only convey low -flows, but higher flows would likely still be intercepted by the highwall miner holes. The pipe would have to be approximately 1500 feet long to bypass both highwall miner hole areas. A temporary piping diversion could be used to confirm the connection between the stream and seep. Permanent diversions through a pipe are typically not allowed in mine reclamation; therefore, this is not likely to be a long-term solution. None of the pipe sizes are extremely large, and costs for pipe plus installation likely range from $50 per foot to $150 per foot. Costs associated permitting and with intercepting the alluvium and creating a subsurface barrier are not included in these costs. Design with community in mind pk c:\users\pkos\documents\_projects\colowyo\easttaylorseepwaterbalance_22dec2020_final.docx