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Oxhow d9inine 1.L.C - Terhniml Rcvition'fR43 11 Wett Coa! Rcfnn F'adlily Tr<bnirat!Idegnary Comment/Rerponru SePtemher 5, 2003 Page 3 11) Because the height of the Pond E dam, ar measured from the bottom of the channel up to the emetgenry rpillway invert, it 12.5 feet, Rule 4.05.9(3) requires that the pond design be in compliance with C.RS. 37-87-105. Please provide proper documentation that the designs for Pand E have been filed at and approved by the Office of the State Engineer. Response: C.R.S. 37-87-105 establishes requirements for design criteria Eor certain classes and sizes of dams in Colorado, however, under C.R.S. 37-87-114.5, Applicability of Provisions-Exemptions, Sectioti 37-87-114.5(1)(d) indicates that; (1) The provisions of Sections 37-87-105 to 37-87-114 shall not apply to: (d) Siltation strucrures permitted under azticle 33 of title 34, C.R.S. (Note: The referenced article and title aze the Regulations of the Colorado Mined Land Reclamation Board for Coal Mining) The general design criteria aze outlined in the "Guide to Construction and Administration of Dams in Colorado" (Office of the State Engineer -Division of Water Resources, June 1994). In this publication, under the heading, "Exempt Structures", coal mine siltation structures aze identified as, `...normally exempt from regulation by the Office of the State Engineer." The corresponding and more detailed "Rules and Regulations for Dam Safety and Dam Consuuction" (Office of the State Engineer -Division of Water Resources, 2CCR 402-1, September 1988) also document the exemption for coal mine siltation strucrures under Rule 18D (refer to page 47). Given this exemption, OMLLC believes that the referenced design criteria are not applicable for Sedimentation Pond E. It should be noted that one of the key design criteria for dam embankments over 10 Eeet in height is that the dam design meet a Factor of Safety of 1.5 for geotechnical stability. As documented in Appendix C of the Sedimentation Pond E Embankment Wall Design (MWH, July 2003) report (included in Exhibit 2.05-E4), the embankment wall has been designed under maximum foreseeable loading conditions (i.e.: sediment to maximum design sediment level and impounded water to top of strucrure) to meet a Factor of Safety of 1.5. 12) Please demonstrate that the existing railroad culvert just below the Pand E dam ran pau the design precipitation event outflow from the emergenry rpiUway. Response: The hydrologic criteria used in designing the UPRR drainage ditches and culverts are unknown. It is important to note that under present conditions, any runoff (from any intensity or duration of storm that may occur) would flow in the existing natural drainage {unconrsolled) to the existing UPRR ditch, along the raikoad right-of-way, and would then pass under the roadbed through the existing UPRR culvert(s). The probability of dischazge from the emergency spillway is very low. A 25-year event occurred several years ago at the site and, based on ongoing hydrologic monitoring, there were no discernable flows in most of the small ephemeral drainages in the azea. Significant vegetation, relatively dry soil conditions, and the presence of relatively deep, permeable alluvial/colluvial deposits in most of the small drainages combine to limit runoff flows. The design for the emergency spillway is conservative, in that calculated design flows are based on the assumption that [he entire II West Coal Refuse Facility area would be disturbed at one time and relatively high runoff curve numbers. The design also assumes that the pond would be full up to the level of the principal spillway at the time the 25- year storm hits, which is very unlikely.