2012-02-28_REVISION - M1980244 (73)

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selected two channel designs to carry all flows. As depicted on the "Channel Cross Section" figure, cut -slope Channel cross sections are to have a 48" minimum bottom width and a 48" depth, with 3H:1V slopes, or a'design of appropriate capacity for the contributing area. Road channel cross section show the 12" wide by 38" deep ditch that is incorporated in the road design. The capacity of these channels is depicted as a function of slope and depth of flow in the two graphs provided. Each of the computed channel velocities for the peak flows from the 100 -year, 24 -hour precipitation event, for each subbasin, was compared with the design capacities of the channels. In all cases, the design capacities are at least capable of carrying the flows. Riprap computations are provided in the attached summary report. Riprap in the area of the Arequa Gulch overburden storage area will be the oxide rock comprising the overburden. While CC &V will examine areas as they are constructed and operated to assess the need for riprap, most channel areas will already be covered by sufficiently coarse rock to control excessive erosion, as we noted above. Computations based on the projected channel flow velocities and reported in the summary report show the largest D that may be needed is 0.24 feet, or about 3 inches. The oxide overburden material being placed will normally meet or exceed this size distribution. Because all of these channels,-with the exception of channel G -H, are temporary and lead to sedimentation ponds, control of channel erosion with riprap is only required to prevent significant silting of drainage structures, that is, siltation that jeopardizes short -term retention capacities. Ponds that are developed to contain the runoff from the 10 -year, 24 -hour precipitation event, or which serve as detention basins along upstream flow paths, will assist in controlling sediment. CONCLUSION This Plan documents the basis for designs of storm -water runoff controls for the Arequa overburden storage area. The designs are based on safely passing the flows resulting from the 100 -year, 24 -hour precipitation event and containing the runoff from the 10 -year, 24 -hour precipitation event when that runoff is from disturbed areas that have not been reclaimed. The methodology documented in TR 55 has been used to compute flows and volumes. The requisite impoundments that serve to contain and detain runoff exist and are currently used for storm -water runoff control. Thus, the principal construction requirements remaining for the evolution of the storage area will be sloping of surfaces and development of the drainage ditches leading to impoundments, followed by soil placement and revegetation.. Attachments: FILE: AREQUA.DNG (1) Summary of Storm -Water Runoff Computations (2) Plan Views of Arequa Gulch Overburden Storage Area (5 plans) — (3) Plan View of Detention Structures Serving Arequa Gulch Overburden Storage Area. 6