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4'3-2 Stormwater Runoff within Disturbance Areas <br /> Stonnvvater runoff within the Upper pd and East Pit would be stored within the pits. Tierra Group <br /> developed SEDCAD models to determine the volume of sediment and water reporting to each pit <br /> and determined e rnininnunn area and depth required to store the water and sediment from the <br /> 10-yeor. 24-hour event within each pit. The pits would be developed in a manner that creates a <br /> sloped surfoce, grading to drain to o |ovv area within the pit. The location of the low area within <br /> each pit would change as the pit is developed and maintained so that water does not accumulate <br /> against the pit vve|!e, Be,nna would be constructed at the pit perimeter as an additional measure <br /> to prevent runoff from exiting the pit. <br /> Stornnvvater runoff from the new 3Cc8aS roads would report to existing sediment control ponds <br /> N1501 and K80{]1. Tierra Group developed 8E[)[}AD noDde|o for the existing sediment ponds, <br /> accounting for the additional sediment from the new disturbance areas. Tierra Group determined <br /> the existing S8dinnnnL ponds can aoCOmonnOd8t* the additional sediment load. Table 4.2 <br /> summarizes required C8paCiU8S for the 10'year. 24-hour event by tgCi!ity, including existing <br /> sediment pond M501 and K8801 surveyed capacities(with 1 foot of freeboard). <br /> TABLE4'2: REQUIRED STORMVVATER RUNOFF CAPACITIES <br /> Upper Pit in-pit <br /> Upper Pit 53.65 4.9 N/A <br /> storage <br /> Varies I'] East Pit in-pi <br /> East Pit Varies t storage N/A <br /> Notes: t1l Watershed basin varies from 54.0 to 82.6 acres throughout pit development. Required capacity varies <br /> hnm5.OLo77oova�eoL depending on"he size of the active miniogo:daoe. Capacities mberovioiVsd <br /> periodically throughout operations. <br /> p1 Actual capacity ofM5O1 used site survey data provided by American Gypsum (Gombu. 2018). <br /> w <br /> Actual capacity ofMOO1 used site Survey data provided by American Gypsum(8!ag|e. 201S). <br /> The surveyed capacity of pond M0[)1 does not meet the required capacity of 0.57acre-f8et� <br /> hovvever, additional sediment accumulation was present as of the 2019 survey and the pond <br /> would be cleaned out to maintain the 0.57-aore-foot required capacity before the East Pit area is <br /> disturbed. The existing gtormnvvater management plan would also be modified and submitted to <br /> the appropriate regulatory agencies for approval before new areas are disturbed. Drawing 14 <br /> ehovvG drainage basin delineations and locations for existing sediment ponds. <br /> 4_3_3 Culverts <br /> Culverts would be constructed during operations to convey upland flows under new construction: <br /> one is anticipated vvmat of the Upper Pit (under the road extension and growth medium stockpile) <br /> and a second is anticipated west of the East Pit(under the access road)as shown on Drawings 05 <br /> and 07. Culverts were sized to accommodate flows from the 100-yeer. 24-hour event using the <br /> upland drainage basin area and the United States Army Corps of Engineers (USACE) Hydrologic <br /> Modeling 8yatenn (HEC-HMS) (2010). The west culvert would be on extension of the Upper Pit <br /> 0900Acceaa Road ou|ved (BLyW. 2019: CO [)R|K8{S. 2013o). which was modified toa 30'inoh <br /> 14-gage corrugated steel pipe to match the existing upstream culvert.The east culvert would be <br /> 24-inch 15-gage corrugated steel pipe. as determined by the Federal Highway Administration <br /> 41 <br />