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Cyprus Twentymile Coal Mine ~~/e~N Pond "E^ ~Xiyl~ / Principal Spillway "As-built^ Evaluation ~'~ ~~, ,~ /~,~=~rG•l/S The objective of this study was to determine if Pond E ma'urtains the nece: time of 24 hours for runoff from the 10-year, 24•hour design storm event 1..4vwmg the installation of the principal spillway decant orifices. Epp & Associated performed a pond survey of Pond E, dated 11/01/95, which was used to develop the atagelstorage curve that TerraMatrix used in November 1995 for the initial design evaluation. The initial pond evaluation, titled, "As-built"Design Eunluotion and Emergency Spillway Design ", was dated 11/15195, and utilized the SEDCAD' (SEDCAD) Hydrologic Model (Schwab, 1992). SEDCAD was also used in this evaluation. Cyprus Yampa Coal Company (CYCC) modified Pond E subsequent to the November 1995 evaluation and TuraMatrix rawaluated the storage capacity and spillway specifications to determine if sufficient storage capacity and detention time were maintained for tvnoff from the 10-year, 24-hour storm event. CYCC provided the cutrratt stagdstorage curve and construction specifications for both the principal and emugency spillways. The stage/storage curve is provided as an attachment to this evaluation. The principal spipway Corrsituction specifications included the number of decant orifices, the orifice dimensions, and elevations. The principal spillway decant orifices were constructed to the following specifications and dimensions: • Two 3 inch by 3.5 oblong orifices at 6,840.6 feet above mean sea level (MSi.) • Four Z.S inch diameter orifices at 6,841.4 feet above MSL Tlic emergency spillway was constructed with a 17 foot bottom width, sideslopes of 1H:1.SV and 1H:S.SV for the right and left sideslopea, respectively, and a 55 foot bottottT length. The Novembu ]99.5 SEDCAD evaluation was used as the basis for the Principal Spillway "As-built" Evaluation. Subwatershed areas, the dcaign storm precipitation amount, routing parameters, and sedimemology model inputs remained the same from the Novembu design evaluation. Input changes included the stage/storage curve and the spillway specifications. Due to model input limitations, three decant orifice scenarios were evaluated. SEDCAD flocs not allow the user to input an unequal number of orificca at diffuetrt elevations, conscqucntly, it should be noted that the exact number of orifices could not bo evaluated, Additionally, SEDCAD assumes that an orifice is circular. A sensitivity analysis was conducted to determine a minimum, a maximum, and the calculated ninoff retention times for the design storm. The minimum retention lima was determined by inputting four orifices per elevation versus the "as- buih" which specified two orifices at 6,840:6 feet above MSL and four at 6,841.4 feet above MST.. In addition, the orifices at 6,840.4 feet were input as a 3.5 inch diameter opening, which is a slightly Fikc VUOik~i`~_a~pincipJ.dJ~7Ii9/%