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The proposed structure shall be a trapezoidal concrete weir with a six foot bottom width <br />and 1:4 horizontal to vertical sides as shown on Figure 5. <br />C.2.2.2 Spillway <br />The spillway proposed shall carry the attenuated storm water flows away from the <br />tailings impoundment has been designed in 3 parts based on bed slope and armoring. <br />Reach 1 will have a bottom width of 8 feet and side slopes of 2 to 1, horizontal: vertical <br />(h:v) or flatter. The section of Reach 1 upstream of the control weir will be an outlet- <br />controlled backwater, with a maximum depth of 7.6 feet and a maximum velocity of 2.4 <br />feet/sec (fps) at the weir section controlled by the weir geometry. The velocity is less <br />than the maximum permissible velocity of 4 fps for channels in graded silts to cobbles <br />transporting clear water (Chow, 1959) and additional erosion protection is not deemed <br />necessary. The section of Reach 1 below the control weir will also have a bottom width <br />of 8 feet with 2 to 1 (h:v) or flatter side slopes. The downstream section of Reach 1 will <br />have a bed slope of approximately 0.002 ft/ft to maintain velocities below 4 ft/sec and <br />prevent the need for armoring. The downstream section of Reach 1 has an estimated <br />normal flow depth of 5.5 feet and a velocity of 4.0 fps during the peak spillway outflow <br />during the PMP. Reach 2 will have a finished bottom width of 3 feet with side slopes of <br />2:1 or flatter (h:v). The channel bed will parallel the existing ground surface, with a <br />slope of approximately 0.22 ft/ft. The estimated normal flow depth in Reach 2 will be <br />2.8 feet, with a peak velocity of 17.9 fps during the PMP. Due to the high design flow <br />velocity, the Reach 2 channel will be lined with either grouted rip rap or an irregular <br />concrete facing. This armoring will protect the channel from erosion and dissipate <br />hydraulic energy over its length. Reach 3 will have a finished bottom width of 5 feet, <br />side slopes of 2:1 (h:v) and a bed slope equal to the natural grade of the terrain, <br />approximately 0.055 ft/ft. The estimated normal flow depth for Reach 3 will be 3.5 feet, <br />with a velocity of 10.0 fps during the PMP. Due to the relatively high design flow <br />velocity, the Reach 3 channel will require rip rap erosion protection.