1988-09-26_ENFORCEMENT - M1977378 (2)

Home Browse Search

� i i specifications for the diversion ditches. To achieve near-optimum ditch I � I efficiency and to maintain side slope stability, trapezoidal ditches with side slopes of 1 .5 horizontal to 1 vertical are recommended. Separate diversion systems will be required for each tailing pond. One diversion ditch will be required around tailings ponds No. 1 and No. 2. It is recommended that two separate ditches sloping in opposite directions be constructed around tailings ponds No. 3 and No. 4 (two each). Construction ! of two separate ditches around each of these ponds will effectively reduce i the size, cost, and difficulty of constructing one ditch designed to divert total peak runoff. Refer to sheet S-3, Diversion Ditch Location Plan, for the recommended locations of diversion ditches. The majority of the diversion ditches will be constructed in well graded glacial soils. To preclude undue erosion and scour of these soils, the ditches should be excavated entirely in natural soils and a riprap blanket should be +' k ' placed in the invert and along the side slopes of all ditch sections which exceed a gradient of . 01 ft/ft. In addition, the invert for ditches aligned parallel to hillsides should be sloped slightly into the hillside. This will aid in confining normal seasonal runoff and any ensuing erosion to the upslope side of these ditches. i Riprap should be comprised of a well graded mixture of angular, durable i i gravel , cobbles and boulders, ranging from 1 .5 inches in diameter to 18 inches in diameter. Local talus could be used as riprap material if other sources f are unavailable. However, care should be taken in the selection of any talus borrow site to avoid hazardous talus slope failures. ;I jl Flood velocities in excess of 13 fps will displace riprap 18 inches in 1 ' I diamater. Since the design storm water depth in most ditch sections wi.11 be � l 21 i .