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Model Peak Q Volume <br />Design 7.55 cfs 3.2 ac-ft <br />Current 58 cfs 8.2 ac-ft <br />Differences are apparent and in some cases significant. For example, the design model <br />assumes more than 700 acres draining to Prospect pond but the current model assumes only <br />533 acres contributory to Prospect pond. The design model never uses a curve number <br />higher than 68 (for 265 acres) yet the current conditions model uses curve numbers of 86 and <br />91 on a total area of 140 acres. Weighted curve numbers, while somewhat complicated, <br />differ by nearly 10, a weighted CN of 58 for the design model and a weighted CN of 67.5 for <br />the current conditions model. <br />While the Division acknowledges that specific comparison of curve numbers or other <br />individual model input parameters does not tell the entire story, what is clear is that the <br />current, on-the-ground conditions in the Prospect watershed exceed the worst case <br />assumption used to design and construct Prospect pond. The model based on current <br />conditions estimates a peak discharge of about 7 times higher (58 cfs vs. 7.55 cfs) than the <br />currently approved Prospect pond design predicted. Total runoff volume estimated for <br />current conditions is about 2.5 times higher (8.2 ac-ft vs. 3.2 ac-ft). The dramatic and <br />substantial difference in these two important SEDCAD model runs clearly demonstrates what <br />the Division's position has been for the last two years, clearly stated in a Division memo from <br />February 2008 "The main issue in the Prospect drainage is that the SEDCAD modeling does <br />not and will not reflect the on-the-ground conditions in the East Pit. There is a very strong <br />possibility that the Prospect Pond is undersized, which could lead to a catastrophic failure." <br />Surface roughening techniques have been encouraged and recommended by the Division in <br />the Prospect watershed. If surface depressions of only three inches in depth existed on 33% <br />of the disturbed surface area, about 100 disturbed acres would easily retain the 8.2 acre-ft of <br />storm runoff on those areas prone to producing high runoff (i.e. high curve numbers). This <br />reclamation technique has shown to be effective in Colorado at other large surface <br />operations and the practice can even be considered in storm runoff modeling and pond <br />design. Retaining storm runoff on the disturbed areas and the subsequent reduction in peak <br />flow to the pond is critical to the future success of the Prospect pond, not only from a safety <br />standpoint on Highway 13 but a water quality position as well. <br />In summary, although the actual storm that occurred appears to have been more intense than <br />what SEDCAD modeling assumes, the 10-year, 24-hour storm was not exceeded based on <br />data provided. Modeling does show that on-the-ground conditions are in a worse case <br />hydrologically than what currently approved Prospect pond modeling assumed. On-the- <br />ground measures should be taken as soon as possible to reduce run-off from rainfall and <br />snow melt events, reduce peak flows, retain sediment on disturbed areas, reduce channel <br />erosion, and prevent large volumes of storm runoff from entering Prospect pond. It is critical <br />to public health and safety that the Division and Colowyo do everything necessary to prevent <br />disturbed area runoff and sediment from impacting Highway 13.