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<br /> <br />Wright Water Engineers, Inc., 818 Colorado Ave., Ste. 307, Glenwood Springs, CO 81601 <br />Tel. (970) 945-7755; Fax. (970) 945-9210, e-mail: jkelly@wrightwater.com <br /> <br /> MEMORANDUM <br />To: Jessica Wilczek, P.E. <br /> Mountain Coal Company <br /> Via Email <br />From: Wright Water Engineers, Inc. <br /> Jonathan Kelly, P.E. <br />Date: November 3, 2023 <br />Re: Hydrologic Consequences of Pillaring in the Sunset Mains South <br />Wright Water Engineers, Inc. (WWE) has evaluated the hydrologic consequences of pillaring in the <br />Sunset Mains South. WWE had previously analyzed the subsidence associated with the longwall <br />mining in the adjacent panels and found that the mining would not have adverse effects on the <br />hydrology of the overlying stream channels. Our findings were documented in Exhibits 55B and 60E <br />of the mine permit. <br />Agapito Associates modeled the projected subsidence resulting from the pillaring in the mains. Their <br />results indicate that the maximum subsidence would be up to 3 feet, and this would be limited in areal <br />extent. WWE imported the Agapito modeling results to analyze the potential impact on the stream <br />channels in the vicinity of the pillaring activity. <br />Lick Creek and tributaries of South Prong are within the influence of the projected subsidence <br />resulting from the pillaring activity. The maximum subsidence underlying the stream channels is <br />limited to 1 foot. WWE evaluated the potential for the projected subsidence to impact the stream <br />channels within these drainages and found that there should not be hydrologic impacts. These streams <br />have previously been undermined by longwall mining that resulted in significantly larger subsidence <br />without experiencing any change in flow characteristics. The steepness of the channel grades and the <br />ability of the alluvial and colluvial materials to heal and/or seal following differential subsidence has <br />resulted in no observable impacts to these streams. The small amount of projected subsidence <br />resulting from the pillaring in the Sunset Mains South is not expected to adversely affect the <br />hydrology of Lick Creek and South Prong drainages. <br /> <br />G:\WWE\831-032\Sunset Pillaring.docx