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V. Hydrologic Balance <br />1. COMMENT <br />The Division has identified a number of problems with the design calculations <br />for the Pit 4 sediment pond. The applicant used incorrect 10-year, 24-hour <br />rainfall for the site (1.8" vs. 1.6") and did not account for the watershed <br />shape factor as found in Appendix E of "Peak Flows in Colorado". This resulted <br />in an overestimation of peak inflow during the design event. Therefore, the <br />hole size needed to dewater the pond in 24 hours was overestimated. The <br />applicant's calculations yielded a 5" radius or 10" diameter hole. As a result <br />of the incorrect assumptions used, necessary storage volume is also incorrect <br />and the primary spillway elevation should be recalculated. The following <br />aspects of the sediment pond design must be recalculated: <br />a. The storage volume needed for a 10-year, 24-hour event. <br />b. Principal spillway elevation. <br />c. Dewatering aperture diameter needed to dewater the 10-year event inflow <br />in 24 hours. <br />d. Outflow velocities resulting from dewatering. <br />e. Design of an adequate non-clogging dewatering device over the dewatering <br />aperture. <br />1. RESPONSE <br />The original rainfall value of 1.8 inches was taken from a larger scale NOAA <br />atlas for Colorado rather than the smaller scale map shown in "Procedures for <br />Determining Peak Flows in Colorado". The watershed shape factor was not used in <br />the original calculations since the watershed will deviate from its "natural <br />condition" throughout the course of Pit ~4 mining activities. However, to <br />remain in compliance with the Division, the calculations have been revised to <br />reflect the 1.6 inch rainfall value and watershed shape factor. The calcula- <br />tions are attached and specific points are addressed below. <br />a. Maximum storage volume required = 6.4 acre-feet. Available storage <br />volume from top of 1 year sediment accumulation to top of riser = 10 <br />acre-feet. <br />b. Principal spillway crest elevation will remain the same as previously <br />identified; 7523.5. <br />c. Dewatering aperture diameter = 5 inches. This will provide 24-hour <br />detention time. <br />d. Outflow velocities during dewatering (8-inch diameter pipe) range from <br />maximum of 11.3 fps to minimum of 0 fps. Riprap will be provided at <br />discharge point to Grassy Creek. <br />e. A baffle type device will be placed around the dewatering aperture. See <br />attached sketch. <br />