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2 <br />Without the construction of the dam, the capacity of the downstream <br />channel will be insufficient to meet large flows and the channel along the <br />road embankment will have to be protected somehow. Either the dam must be <br />built or the downstream channel protected. The decision to build the dam <br />wi a so eterm ne when several areas (area 9--Davis Gulch Dam and area <br />15--Southdraw Dam material stockpile) will be reclaimed. If the decision <br />is deferred too long, considerable erosion and sedimentation can occur in <br />these areas. <br />'~5) There is no evidence that either the Davis Gulch Dam or Middle <br />Fork Dam are necessary for sediment control. The original plan left open <br />the possibility that the dams be dismantled upon completion of the project. <br />Neither is vital to the permanent reclamation of the areas as long as the <br />owns ream c anne s are protecte suc that t e can carr their natural <br />ows. There appears to be a very substantial margin of safety for even the <br />samller Middle Fork Dam under consideration, as its outflow is limited to <br />70 cfs whereas the downstream channel capacity is nearly 900 efs. Design <br />considerations for the dam, however, should be further explained. <br />~6) If the Davis Gulch Dam is not constructed, the interim facili- <br />ties (cofferdam, etc.) will need to undergo thorough analysis to determine <br />their suitability for long term use. These structures were not intended for <br />long term use and may be subject to untimely failure. <br />7O The narrative provides inadequate discussion as to how the pre- <br />cipitation falling on the relatively large, flat surface (roads, mine bench <br />surface, La Sal Laydown site and Plateau site) will be controlled. Will it <br />be detained or carried off site? <br />8) Several areas with substantial slopes are expected to be <br />stabilized simply by application of dust suppressants (Coarse Ore Gulch and <br />Plateau Site). Dust suppressants are not necessarily designed to prevent <br />water erosion and their application for this purpose is suspect. Dust <br />suppressants are warranted for flat areas that are not likely to be seriously <br />affected by water erosion <br />In general,~my impression of the reclamation efforts of Exxon on <br />their Colony Project are that they are fairly reasonable and done essentially <br />in good faith. Some problems exist particularly in the Coarse Ore Gulch <br />and Plant Site areas. IC is imperative that the decision on the Davis Gulch <br />Dam be made as soon as possible such that the downstream channel can be <br />properly protected if the dam is not build and so the dam site can be <br />reclaimed. Indefinite continuation of the disturbance at the dam site is <br />unacceptable. <br />If you have any questions regarding these comments or any other <br />matters, please do not hesitate to contact me. <br />Sin erel/y, <br />Pete Gowen <br />Consulting Hydrologist <br />