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TERRRMRTRIX INC. ID~1-970-879-9048 <br />Cyprus Twentymile Coal Mine <br />Yeua °e" <br />Principal Spillway "As•built" Evaluation <br />MRR 29'96 11 27 No .004 P.02 <br />The objective of this study was to determine if Pond E maintains the necessary minimum detention <br />time of 24 hours for runoff from the 10-year, 24•hour design storm event following the installation <br />of the principal spillway decant orifices. <br />Epp & Associated performed a pond survey of Pond E, dated 11/01/95, which was used to develop <br />the stageJstorage curve that TertaMattix used in November 1995 for the initial design evaluation. <br />The initial pond evaluation, fillet!, "Asbrrilf"Design Envluorion acrd F:nrrrgency S'piJ/way Drcign ", <br />was dated 11/15/95, and utilized the SEDCAD' (5EDCAD) Hydrologic Model (Schwab, 1992). <br />5EDCAD was also used in this evaluation. <br />Cypnu Yampa Cual Company (CYCC) modified Pond E subsequent to the NovG»bcr 1995 <br />evaluation and TerraMatrix reevaluated the storage capacity and spillway specifications to determine <br />if sufficient storage capacity and detemion time were maimained for runoff from the 10-year, 24-hour <br />storm event. CYCC provided the cutrertt stagdstorage curve and construction specifications for both <br />the principal and emergency spillways. The stage/storage curve is provided as an attachment to this <br />evaluation. The principal spillway cotutmGion specifications included the number of decant orifices, <br />the orifice dimensions, and elevations. The principal spillway decant orifices were constructed to the <br />following specifications and dimensions: <br />• • Two 3 inch by 3.5 oblong orifices at 6,840.6 fcet above mean sea level (MSi.) <br />• Four 2.5 inch diameter orifices at 6,641.4 feet above MSL <br />Tltc emergency spillway was constructed with a 17 foot bottom width, sideslopes of lH:I.SV and <br />1H:S.SV for the right and left sideslopes, respectively, and a SS foot bottom length. <br />The Novanber 199.5 5EDCAD evaluation was used as the basis for the Principal Spillway "As-built" <br />Evaluation. Subwatershed areas, the design storm precipitation amount, routing parameters, and <br />sedimemology model inputs remained the same from the November design evaluation. Input changes <br />included the stage/storage curve and the spillway specifications. <br />Due to model input limitations, three decant orifice scenarios were evaluated. 5EDCAD dots nol <br />allow the user to input an unequal number of orifices at different elevations, consequently, it should <br />be noted that the exact number of orifices could not be evaluated. Additionally, 5EDCAD assmnes <br />that an orifice is circular. A sensitivity analysis was conducted to determine a minimum, a maximum, <br />and the calculated nnroff retention times for the design storm. <br />The minimum retention time was determined by inputting four orifices per elevation versus the "as- <br />buih" which specified two orifices at 6,840.6 feet above MSL and four at 6,841.4 feet above MSI.. <br />In addition, the orifices at 6,840.4 feet were input ea a 3.3 inch diameter opening, which is a slightly <br />• Pocc~70~wk~pwq_c~p~ncIPJ.N r/rV% <br />