2015-02-26_REVISION - C2010089 - Laserfiche WebLink

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COLORADO Division of Reclamation, Mining and Safety Department of Natural Resources 1313 Sherman Street, Room 215 Denver, CO 80203 February 26, 2015 To: Brock Bowles From: Jason Musick RE: New Horizon North Mine, C- 2010 -089 TR -9, Adequacy Review Memo Per your request, I have reviewed the proposed Pond NHN -002 designs and NHN -002 North Ditch designs associated with the above referenced revision. According to the revision WFC is proposing to relocate the inlet of the NHN -002 North Ditch from the south end of Pond NHN -002 to approximately 250' north on the east side of the pond. The Division reviewed the proposed activities to determine if the relocation of the North Ditch inlet would compromise the integrity of the pond as it pertains to Rules 4.05.2(2) and 4.05.9. With the relocation of the ditch, the pond must be sized to have adequate detention time to function as required. Based on the proposal, approximately half of the pond will be utilized for retention for the run- off and sediment load received from the North Ditch, rather than the entire pond. Based on the proposed revision, the Division has found that the detention time of 15.92 hrs and the trap efficiency of 93.65% identified in the SEDCAD 4 design associated with Pond NHN -002 decreases by approximately 10 hrs and 15% respectively. Based on the modeling the installation of a baffle north of the inlet or locating the inlet further south on the pond would aid to increase detention time. Although the design and modeling of the pond appears to be adequate, the pond effectiveness may be reduced if the detention time is reduced or limited based on the location of the North Ditch inlet. 1313 Sherman Street, Room 215, Denver, CO 80203 P 303.866.3567 F 303.832.8106 http: / /mining.state.co.us John W. Hickenlooper, Governor I Mike King, Executive Director I Virginia Brannon, Director OF'c \876.