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Mineral Joe Mine - Drainage Design Plan 26
<br />4.5.2 Planned Stormwater Retention Pond Expansion
<br />The SWRP will be expanded to the north and a spillway will be constructed across the existing dirt road
<br />leading to the lysimeter (Figure 12, Plate 6). The pond is oriented north -south and will curve slightly
<br />eastward, to fit the existing topography and facilities. The maximum dimensions will be 50 ft by 250 ft,
<br />with a maximum depth of 8 ft (Plate 6). A 10 -ft wide rock -lined spillway will release water to the north
<br />into the existing drainage channel. The spillway will be located at an elevation of 5,872.5 ft, which is 8.0
<br />feet above the floor of the excavation, rendering this a non jurisdictional dam3. The side slopes of the
<br />unlined pond will be constructed to match the current condition of approximately 2.5:1, while the spillway
<br />approach slope is designed at 5:1. The spillway exit slope has been designed to match the existing
<br />topography, and is approximately 10:1. The planned SWRP has a retention storage volume of 30,300 ft3
<br />(0.70 acre -ft) and a maximum surface area of 7,300 ft2 (0.168 acres) (Table 15). Temporary storage above
<br />the spillway would accommodate an additional 0.18 acre -ft. The stage- storage curve for the planned
<br />SWRP is provided in Figure 13.
<br />Table 15. Stage- Storage Data for the Planned Stormwater Retention Pond
<br />Elevation
<br />Depth
<br />Area
<br />Area
<br />Volume
<br />(ft amsl)
<br />(ft)
<br />(ft)
<br />(acres)
<br />(ft)
<br />5,864.5
<br />8.0
<br />328.78
<br />0.008
<br />0
<br />5,865.0
<br />7.5
<br />837.03
<br />0.019
<br />291
<br />5,865.5
<br />7.0
<br />1,221.52
<br />0.028
<br />806
<br />5,866.0
<br />6.5
<br />1,651.52
<br />0.038
<br />1,524
<br />5,866.5
<br />6.0
<br />2,241.76
<br />0.051
<br />2,498
<br />5,867.0
<br />5.5
<br />2,585.17
<br />0.059
<br />3,704
<br />5,867.5
<br />5.0
<br />2,952.61
<br />0.068
<br />5,089
<br />5,868.0
<br />4.5
<br />3,310.10
<br />0.076
<br />6,655
<br />5,868.5
<br />4.0
<br />3,663.13
<br />0.084
<br />8,398
<br />5,869.0
<br />3.5
<br />4,218.41
<br />0.097
<br />10,368
<br />5,869.5
<br />3.0
<br />4,634.97
<br />0.106
<br />12,582
<br />5,870.0
<br />2.5
<br />5,049.37
<br />0.116
<br />15,003
<br />5,870.5
<br />2.0
<br />5,467.77
<br />0.126
<br />17,632
<br />5,871.0
<br />1.5
<br />5,889.15
<br />0.135
<br />20,471
<br />5,871.5
<br />1.0
<br />6,318.40
<br />0.145
<br />23,523
<br />5,872.0
<br />0.5
<br />6,756.47
<br />0.155
<br />26,792
<br />5,872.5
<br />0.0
<br />7,303.79
<br />0.168
<br />30,307
<br />5,873.0
<br />-0.5
<br />7,726.62
<br />0.177
<br />34,064
<br />5,873.5
<br />-1.0
<br />8,194.22
<br />0.188
<br />38,045
<br />Notes: Elevations greater than 5,872.5 are above the spillway and represent temporary storage volumes
<br />3 A "jurisdictional dam" has a reservoir capacity of greater than 100 acre-feet or reservoir surface area greater than 20 acres at the high -water line,
<br />or one that exceeds 10 feet in height from the bottom of the excavation (or lowest point of the natural surface of the ground, if not excavated) to the
<br />crest of the emergency spillway. A "non jurisdictional dam" is smaller than these dimensions and is regulated under sections 37 -87 -102 and 37 -87-
<br />105 of the Colorado Revised Statutes (C.R.S.).
<br />4148B.131029 Whetstone Associates
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