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TETRA TECH <br />Page 2 of 3 <br />Mr. Peter S. Hays <br />July 8, 2011 <br />4. The Applicant states the spillways are in accordance with the best practices of the Urban Drainage <br />and Flood Control District guidelines. Please provide additional information pertaining to the design <br />and construction of the inflow and outflow structures. At minimum, please include the following <br />information: <br />a. Justification for the spillway locations <br />b. Spillway invert elevations <br />c. Cross - section details (both lateral and longitudinal) <br />d. Design parameters <br />The design parameters should include the expected flood flow elevations and peak flows, frequency <br />of the expected flood flows, the expected water elevation in the lake if the spillways are designed to <br />control the lake surface elevation. Additionally, a copy of the Weld County Drainage Report (draft or <br />final version) would be helpful in the Division's evaluation of the spillway design. <br />The spillway locations are placed in such a way that would allow river water currents to enter and <br />leave the pond in a manner that follows the natural flow of the river. <br />The Army Corps of Engineers conducted a general investigation study of the Cache La Poudre River <br />at Greeley, CO. According to the hydrologic analysis prepared for this study, a flow greater than 1600 <br />cfs would overtop the Cache La Poudre River bank and flow onto the Iverson Mine property. 1600 cfs <br />is slightly greater than the 2 -year peak flow of the river. The spillways for the reclamation pond are <br />designed to convey one third of the stream flow of the river, or 533 cfs. <br />UDFCD guidelines do not set any flow rate requirements for spillways. The length criteria set forth in <br />UDFCD guidelines require very large spillways relative to the final storage capacity of the reclaimed <br />mine. Calculations for the spillways show that the inflow fills the reclaimed mine in approximately 1.2 <br />hours, reducing the pressure on the riverside banks due to varying water surface elevations on either <br />side of the bank. <br />Because the pond will be unlined, the water surface elevation of the pond will change seasonally with <br />the groundwater elevation. However, the water surface elevation shall not exceed the crest elevation <br />of the outflow spillway of 4679.1 ft. <br />Cross - sections and plans are presented in Attachment 3. These plans are conceptual at this time <br />and may change in the future due to regulatory requirements and County review. <br />As requested, a copy of the most current version of the Drainage Report is provided as Attachment 4. <br />6.5 Geotechnical Stability Exhibit <br />5. The Applicant's revised slope stability analysis with stockpile and tower surcharge loads of 1,100 and <br />3,000 pounds per square foot respectively was duplicated for verification by the Division using <br />Galena. No potential failure surfaces with lower safety factors than the Applicant's identified critical <br />slop circle were located. The safety factor produced by Galena is slightly higher than that produced <br />by the Applicant's model. The accuracy of the Applicant's analysis is confirmed. A copy of the <br />Galena cross - section is attached for reference. <br />Noted; thank you for a copy of the Galena cross - section. <br />We understand that the current Decision Date for this application is July 25, 2011. Please notify us as <br />soon as possible or no later than July 18, 2011 if you need more time to complete your review of this <br />application and determine that the items have all been satisfactorily addressed so that you can issue the <br />permit. If you need more time, we will request a Decision Date extension. <br />