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<br />Memo to Wallace Erickson 9 May 3 2001 <br />The DMG has determined through independent ground water modeling and by review of the Applicant's <br />model that pit dewatering will not significantly diminish the yield of the Robinson Well. In addition, the <br />Applicant has agreed to install water table monitoring between the proposed pit and the Robinson Well. <br />Also, the Robinson Well is outside the 600 foot radius that could trigger a well interference hearing <br />before the State Engineer. Based on these decision making factors, the DMG recommends approval of <br />the Line Camp Pit application over the objection that offsite well yields may be damaged. <br />The DMG and nearby residents have expressed the concern that excavation of the proposed Line Camp <br />Pit may create adverse offsite impacts during and following a large flood on the Dolores River. Mapping <br />that illustrates the jurisdictional one-percent annual chance flood (the so-called 100-year flood) has been <br />provided in the reclamation permit application. The discussion in the remainder of this paragraph is <br />provided so that parties interested in the Line Camp Pit application will be aware that designs and <br />mitigation for larger floods are not considered. In order to provide a standard national procedure, the <br />one- percent annual chance flood has been adopted by the Federal Emergency Management Agency as <br />the base flood for purposes of floodplain management. This standard is the basis upon which DMG <br />conducts its flood impact evaluations, which is consistent with nationally recommended and established <br />procedures. Extreme flood events with much greater magnitude than the one-percent annual chance <br />flood can and do occur, but with a lower annualized probability of occurrence. This is a significant <br />complicating factor for flood plain management decisions in that there will always be residual losses from <br />extreme flood events above and beyond those for which mitigation is cost effective. <br />Objectors to the permit application have argued that the excavation of the proposed pits will increase the <br />water surface elevation during a flood. The arguments provided cite the reasoning that the pits <br />following reclamation will be full of water and will not provide storage volume for encroaching <br />floodwater. Also, that the storage for flood water within the pore spaces of the alluvium in the pre-mine <br />setting will be eliminated by excavation of the sand and gravel and subsequently filling the pits with <br />water. It has been DMG's experience and it is DMG's position that mining and reclamation of gravel <br />pits in the floodplain generally has the net impact of reducing flood peaks on downstream properties, <br />even when the reclaimed pits are filled with water. A quantified demonstration in support of this <br />position follows. <br />Information provided in the permit application indicates that the average water table elevation at the Line <br />Camp Pit location is approximately five (5) feet below ground surface. Examination of the pre-mining <br />and reclamation plan maps indicates that the freeboard in the reclaimed ponds will be between zero and <br />three feet. In other words, the pond water surfaces will be from zero to three feet below pre-mining <br />ground surface elevations. The northern pond map indicates approximately three feet freeboard, the <br />middle pond shows zero freeboard, and the southern pond shows 2 feet freeboard. As a practical matter, <br />the pond shores and interconnecting pipelines cannot be installed without at least one foot of freeboard, <br />but for the quantitative analysis the conservative assumption of 6 inches freeboard in the middle pond <br />will be used. <br />