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JD -6 Mine — Drainage Design Plan <br />m <br />d <br />0 <br />q <br />0 <br />W <br />0 <br />Storage (cubic -feet) <br />Figure 13. Stage- Storage Curve for Planned SWRP <br />Surface <br />Storage <br />28 <br />The planned SWRP spillway will be lined with riprap as shown in Plate 6. The size and thickness of the <br />riprap were determined based on overflow predicted by the HydroCAD model for the 100 -yr 24 -hr storm <br />event (Section 5.4). Using the USBR (1998) method with a peak spillway discharge of 0.19 cfs and a <br />factor of safety of 3x, the spillway requires a Type II granular bedding with D50 = 0.75 inches. The <br />minimum thickness of protective riprap layers generally is specified as a minimum of twice the D50 or <br />equal to the D100 size rock in the layer, or a minimum of 2 inches thick. A filter fabric or 2 inches of 0.25 - <br />inch Type I bedding will be installed at the crest of the spillway and draped below the rock apron in the <br />spillway exit section. <br />4.5.3 Upstream Storage Ponds (USPs) <br />"Upstream storage" refers to the storage of storm runoff close to the points of rainfall occurrence, including <br />road embankments, borrow pits, parking lots, property line swales, parks, and on -site basins and ponds <br />(UDFCD DP -17, 2001). Several upstream storage ponds or depressions exist at the JD -6 Mine, one of <br />which is identified on Figure 2 as USP -2. Additional upstream storage occurs in the perimeter ditch around <br />the mine facilities area (Figure 4) and in small ponds or pools on site. Although this upstream storage is <br />beneficial to reducing stormwater runon to the site, the runoff modeling conservatively does not take credit <br />for this storage. <br />Although the Drainage Criteria Manual published by the Urban Drainage and Flood Control District <br />(UDFCD, 2001) is not directly applicable to the JD -6 site, the manual identifies the benefits of upstream <br />storage and the difficulty in quantifying those benefits: <br />4148A.131028 Whetstone Associates <br />