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© Mr. Tom Kaldenbach <br />October J0, 1997 <br />Page 2 <br />The original design for the Rail Loop Pond, originally designated as Pond B, was completed <br />in 1980 as pan of the H-G Coal Company permit, Permit No. C-80-003. The original <br />design criteria are presented in Appendix 12-1 of the existing, valid HGTI Permit. <br />Calculations presented in Appendix 12-1 indicate that the required storage capacity for the <br />Rail Loop Pond is 1.89 acre-feet (ac-k). As shown on Exhibit 12-4, Rail Loop Pond2 <br />Existing Conditions, 1.84 ac-k of storage capacity is available. <br />In discussions between HGTI and the Colorado Division of Minerals and Geology <br />(Division), it was agreed that the correct course of action to bring the pond capacity into <br />compliance was to re-analyze the storage capacity requirements for the Rail Loop Pond. <br />2.0 STORAGE CAPACITY DEMONSTRATION <br />Storage capacity requirement calculations utilized SEDCAD+ Version 3 (SEDCAD) <br />(Schwab, 1992). SEDCAD modeling assumptions and results are included as Attachment A. <br />As discussed in the accompanying Rail Loop Pond Technical Revision Table, which <br />summarizes changes to the existing permit as a result of this TR, Attachment A needs to be <br />inserted into the existing HGTI Permit as Appendix 12-1a. <br />Exhibit 12-2 of the existing Permit indicates that the contributing drainage area to the Rail <br />Loop Pond is approximately 13.8 acres. Figure 1, Sediment Pond Drainages, which is <br />presently found in Appendix 12-1 of the existing Permit, and which was prepared during the <br />original pond design, indicates that the contributing drainage area (in 1980) for which the <br />Rail Loop Pond (Pond B in Figure 1) was originally designed, was approximately 30.2 acres. <br />As part of this evaluation, hydrologic and vegetative conditions were calculated to reflect <br />current conditions. In the original design, it was assumed that the land use for all the <br />contributing watershed was disturbed roadway, with barren ground cover. Since 1980, <br />portions of the contributing watershed have been revegetated, i.e., subcatchment 2-2, with a <br />rangeland land use, with fair ground cover. <br />The SEDCAD results presented in Attachment A include 10 years of sedimen[ <br />accumulation. The estimated volume of sediment over ten years is 0.07 ac-k with a <br />corresponding sediment elevation of 6,379.52 feet above mean sea level. As discussed above, <br />1.84 ac-k of total storage capacity is available (from Exhibit 12-4) in the pond and the 10- <br />year, 24-hour storm event would generate 0.68 ac-k of runoff volume. In addition, the <br />sedimentology analysis calculated that outflow settleable concentrations (reported as <br />milliliter per liter [ml/lll would be zero, indicating that pond outflon~ will meet effluent <br />litnitations for settleable solids (0.5 ml/1) prior to discharging into receiving waters. <br />3.0 EMBANKMENT MITIGATION <br />Not only did the review of Exhibit 12-4 suggest that the Rail Loop Pond existing capacity <br />was slightly less than the design criteria, but it also indicates that the pond embankment does <br />not have sufficient freeboard between [he design storm event discharge water surface and the <br />top of embankment. Rule 4.05..6(11)(e) states that the minimum elevation of the top oI the <br />