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• • III IIIIIIIIIIIII III <br />;. ~ sss <br />~' <br />Memo <br />To: Erica Crosby <br />From:Allen Sorenson n / <br />CC: Carl Mount, Tom Schreiner <br />Date: 04/04/01 <br />Re: Adequacy Responses, S&H Mine, Platte Valley Sand & Gravel, File No. M-2000-158 <br />I have reviewed responses numbered 25-27 and 38 in the Applicant's letter and <br />attachments dated March 26, 2001. The Applicant's response number 38 satisfies the <br />Division's concern with mining setbacks to structures including gas lines and wells. <br />Response number 27, relating to the alignment of temporary overburden and topsoil <br />stockpiles, is also satisfactory. Additional clarrfication or commitments related to responses <br />numbered 25 and 26 will be required for the Division to recommend approval of the <br />application. <br />Response no. 25: The Division was not concerned that the operation of the pit will increase <br />floodwater surface elevations in the floodway and floodplain, and agrees with the Applicant's <br />analysis that potential floodwater elevations will actually decrease as the pit is mined out. <br />The Division's concern is with the potential for the high velocity flows of the main stem of the <br />river to be diverted through the pits because of floodwaters down cutting the land separating <br />the pits from the river and land separating the pits from each other. Land that separates the <br />pits from the river, whether that land is built up through placement of fill or is simply native, in <br />situ ground that is left intact, is referred to as a "riverside berm:' Riverside berms are further <br />discussed below. Ground that is left intact or built up using fill between pits, and generally <br />athwart of the anticipated direction of potential flood flows, are called "lateral berms." The <br />Applicant states that there will be no placement of fill in the construction of lateral berms. <br />However, the berms comprised of native material left in place between pits are similarly <br />subject to differential head and down cutting during a flood. This can lead to a number of <br />potential problems including sequential, progressive failure of lateral berms in a downstream <br />direction, temporary or permanent diversion of the main stem through down cut channels in <br />failed riverside and lateral berms and through the pits so linked, and damage to the pipelines <br />that are carried in the berms. If any lateral berms are to be left within 400 feet of the top of <br />the bank of the river, the Applicant must install structures to equalize hydraulic head on <br />• Page 1 <br />