2020-08-07_PERMIT FILE - C1981010 (116)

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East Pyeatt Ponds The East Pyeatt Pond system consists of three sediment ponds in series at the locations shown on Figure 1, "East Pyeatt Area As-Built Ponds." The ponds are designed to operate in series to treat the runoff from the 10 year/24 hour storm to meet the 0.5 ml/I settleable solids standard, and to safely pass the 25 year/24 hour storm runoff through each embankment's spillway. The SEDCAD computer program was used to model the performance in both storm events. The 10 year/24 hour and 25 year/24 hour storms (hereafter 10/24 and 25/24) are 1.40" and 1.80" respectively. The combined watershed of all three ponds is 1,227.43 acres, 1,157.23 of which report to the uppermost pond, with an additional 40.50 acres reporting to the middle pond and an additional 29.71 acres reporting directly to the lowermost pond. The effluent quality exiting the lowermost pond is the parameter of interest in the 10124 storm, while spillway adequacy for each of the three ponds is the parameter checked for the 25/24 storm. Currently the entire 1,227.4 acre watershed is either undisturbed native land (376.2 acres), reclaimed mined land (758.5 acres), or currently active disturbed land (93.0 acres). This last category consists of a haul road crossing northwest to southeast as shown. In the years 2018 and 2019, additional areas of acres of reclaimed land will be re-disturbed, as shown on Figure 2 "East Pyeatt Area As-Built Ponds, Year 2019." By 2021 the pit will intercept most of the increasingly larger portions of the watershed, so Year 2019 is therefore considered the worst case condition from a hydrology and sedimentology perspective. All 139.33 acres to be re-disturbed exist south of and tributary to a current haulroad crossing. That crossing is modeled as a "null" in the SEDCAD model, that is, simply a collection point. Its 1,110.18 acre watershed includes 223.67 acres of undisturbed land, 795.53 acres of currently reclaimed land, and 90.98 acres of active disturbed land, namely the haul road. By 2019, 139.33 acres of the currently reclaimed 795.53 acres will be redisturbed, as described above. The sediment capture modeling using the SEDCAD program has therefore been updated and re-run incorporating this change in disturbed area. Runoff is calculated using the SCS Curve Number method which assigns a cure number based on land use and condition. As shown on Figure 2, " East Pyeatt Area As-Built Ponds, Year 2019", most of the watershed (669.37 acres) will remain reclaimed land which has been assigned a Runoff Curve Number of 67 in accordance with the SCS Cure Number procedure. The next largest land use category is 327.76 acres of undisturbed land, assigned a runoff Cure Number of 49. As discussed above, by 2019, there will be 139.33 acres of re-disturbed land above the haulroad crossing, assigned a Runoff Cure number of 75 in recognition that re-disturbed previously reclaimed land will have a very deep and relatively loose soil structure making it capable of higher infiltration (less runoff) than other active disturbed lands such as the haulroad. The existing 90.98 acres of the haul road will remain, with its cure number of 87. The sediment yield from the entire watershed is determined by the SEDCAD program using the MUSLE (Modified Universal Soil Loss Equation). Input parameters used in this part of the analysis are presented on the results page (page 18) of the model output for the 10/24 storm. The three sediment ponds have trapezoidal earthen channel spillways, with control section elevation, bottom width and side slopes as described in the SEDCAD output. Two of them, (the uppermost and lowermost) also have a v-notch weir establishing the normal pool 2 feet below the elevation of the emergency spillway, while the middle pond has a narrow 3-foot wide earthen channel spillway 2 feet below the emergency spillway elevation. These v-notch weirs Revision: -- 1 Approved: . —_-— -:A-gust, 2018