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JD -7 Mine — Drainage Design Plan 39 <br />Figure I& Photograph of Stormwater Retention Pond 001 B -A South <br />4.5.2.2 Retention Pond Expansion <br />The existing stormwater Catchment OO1B -A North will be expanded because its current capacity is <br />insufficient to contain the runoff from the 10 -yr 24 -hr storm, based on the stormwater model input <br />parameters. The 10 -yr 24 -hr storm can be contained by a 60 ft x 29 ft rectangular lined storage pond with <br />2.5:1 side slopes and a total depth of 4 ft with a freeboard of 1 ft. The expanded stormwater catchment <br />pond would be constructed at the location adjacent to the existing pond 001B -A North (Figure 11, Plate 4). <br />Increasing the footprint of the lined catchment pond from the existing 1,040 ft2 to 1,740 and deepening the <br />pond to a total of 4 feet (3 -ft wetted depth) expands the storage capacity from its current 1,863 ft3 to 2,412 <br />ft3 with 1 ft freeboard or 3,939 ft3 bank full. The stage- storage table for this pond is shown in Table 17 and <br />the runoff hydrographs are shown in Figure 19. The pond would be constructed with a 5 -ft broad - crested <br />spillway to pass runoff from storms greater than the 10 -yr 24 -hr storm, including the 100 -yr storm event. <br />The HydroCAD results indicate a peak velocity of 1.5 ft/s over the five foot spillway of Catchment OO1B -A <br />North. The Thorne and Zevenbergen resistance equation was applied to determine an acceptable D84 rock <br />size of 0.75 inches for the spillway and five inches for the rock apron. These specifications are provided on <br />Plate 4. <br />The existing stormwater Catchment OO1B -A South will be improved to pass the 100 -year 24 -hr storm. <br />Catchment 001 B -A South is constructed with a Hypalon liner and will discharge excess stormwater to the <br />west. A rock -lined spillway will be installed along the western side of the existing structure. The <br />HydroCAD results indicate a peak velocity of 0.64 ft/s over the 24 foot spillway of Catchment 00113-A <br />South. The Thorne and Zevenbergen resistance equation was applied to determine an acceptable D84 rock <br />size of 0.75 inches for the spillway and three inches for the rock apron. <br />4149A.140207 Whetstone Associates <br />