2020-03-31_PERMIT FILE - C1981019A (13)

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EXHIBIT 7, ITEM 20, PART PP—PROSPECT SEDIMENTATION POND The progression of mining in the East Pit was generally from east to west, climbing upward in topography and moving away from the Prospect Pond. As a result, the watershed tributary to the pond in the early years was intercepted by the inactive East Pit, and the watershed increased as backfilling and grading advance westerly. Currently, the entirey of the East Pit is reclaimed except a small portion at the headwaters of the watershed where a long term haul road exists. Ditch Design, overview As the reclamation in the East Pit advanced to the west, three permanent ditches were constructed to convey runoff form the disturbed and reclaimed lands southeasterly toward the Prospect sedimentation pond. From north to south these ditches are Prospect Ditch, North Tributary East Pit Ditch, and Final East Pit Ditch. Although the total drainage area tributary to Prospect Pond is 711.1 acres, the largest watershed for any single ditch is 153.3 acres, that being Prospect Ditch just before it releases into undisturbed lands. The ditches are therefore all designed for the 10 year-24 hour storm in accordance with Rule 4.05.3. The two southern ditches are rip rap lined for their entire length, while the re-established Prospect Ditch is rip-rap lined only in its lowermost reach. Minimum stone size (D50) for each ditch was determined in SEDCAD using the Simons/OSM model. The largest riprap size determined in SEDCAD for any ditch and design flow is D50 equal to or greater than 6 inches. All ditches are nevertheless designed with a minimum D50 of 12 inches. For channels with a vegetative erosion control lining (upper reaches of Prospect Ditch, uppermost reach of North Tributary East Pit Ditch) the maximum peak flow velocity for the design storm is 1.98 fps (Prospect Ditch). This velocity is well below the maximum allowable velocity of 5 fps for the vegetative cover lining. Stock Ponds In order to assure compliance with effluent requirements at the outlet of Prospect Pond, five stock tanks have be constructed near the re-established main drainage channels of Prospect Ditch, North Tributary East Pit Ditch, and Final East Pit Ditch. These stock ponds include PD1, PD2,NTEP1, EP1, and EP2 respectively. The stock ponds were designed and constructed in accordance with Fig. 2.05-6 "Stock Tank Plan/Profile — Typical Design". In many cases these stock tanks completely intercept the runoff from their tributary watersheds in the 10 year 24 hour design storm. Inlet and outlet details relating to tie-in to their respective ditches are presented on Figure Exh. 7-15R-8. The SEDCAD modeling shows the runoff volume from the 10 year, 24 hour storm to be 6.98 acre feet, and the modeled outflow from Prospect Pond is well Exh. 7-PP-1 Revision Date: 1/6/20 Revision No. TR-136