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constructed from the native gravel that has a high silt content ( -30 %). The berms will be <br />compacted during construction to minimize piping and potential for erosion failure. The <br />collection ditches will have to be able to handle the 100 -yr 24 -hr event stormwater channel flow <br />that enters them, which is the flow from the Small Stormwater Channel. See the Stormwater <br />Collection Ditch Cross Section at the end of this exhibit for design details. <br />A set of 24" corrugated metal pipe (CMP) culverts will be installed on the south end of the east <br />sediment pond at the termination of collection ditches B and C. The culvert designs are shown at <br />the end of this exhibit, and the location can be seen on Map G. <br />The drainages will have stormwater channels installed in them in order to control flow during <br />and after mining. Two designs have been assembled, a large stormwater channel for SWB 6 and <br />7 and a small stormwater channel design for SWBs 2, 3, 4, and 5. SWB 1 will flow directly into <br />a sediment pond. The design details and requirements for these stormwater channels can be <br />found at the end of this exhibit (Large Stormwater Channel Cross Section and Small Stormwater <br />Channel Cross Section) <br />Spillways will be constructed at each sediment pond to facilitate any overflow flowing into the <br />natural drainages on the north side of the site. These spillways will be trapezoidal with equal <br />2H:1V sideslopes, be eight feet wide on the bottom, and be 1.5 feet deep. The spillways will be <br />armored with rock with a D50 size of six inches. Each spillway will be long enough to daylight <br />into the nearest natural drainage north of the respective sediment pond. The invert of the spillway <br />is the top of the pond for the purpose of volume calculations (ex: the Western pond's final depth <br />from original ground will be at least 6.5 feet: 1.5 ft spillway + 5 ft pond storage depth). <br />The transitions between the natural grade to the south and the mined out pit will be on 3H:1 V <br />slopes. However, since these slopes will be steeper than the drainage channels typically are, they <br />will be armored to prevent erosion of the slopes. The armoring of this transition will consist of 4- <br />6" rock grouted on the bottom to a total depth of 9 ". The design of the armoring can be seen on <br />the attached detail: Stormwater Transition Detail. <br />Craig Ranch Pit, March 2011 G -3 <br />