2005-03-23_REVISION - M1977424 - Laserfiche WebLink

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DS9: The DS9 channel is located on the west side of the Lower Bench. The channel will control runoff from the bench area west of DS8. The estimated geometry of channel DS9 is a 2-ft base, a 0.3 ft depth, and side slopes of 3H:1 V. The channel will be constructed within the bench materials of silt to cobble-size shale fragments and will be seeded to establish native vegetation. 4.0 OUTLET DESIGN The stormwater control plan at Logan Wash Mine focuses on the collection and diversion of storm runoff in channels that convey the water to several main discharge points or outlets. These dischazge points include DP1 and DP2 which dischazge runoff collected from upper subbasins off the lower high wall into a lower subbasin (A7), and dischazge points DP3, DP4, and DPS which dischazge runoff into natural ephemeral drainages. Outlet design calculations were conducted for channel DSS and outlet channels (discharge points) DP3, DP4, and DPS. Calculations for DSS were conducted because of the potential high flow velocity in this channel, construction in unconsolidated waste rock and fill, and its location immediately upstream of discharge point DP3, where a sharp increase in slope will occur. Outlet design focused on riprap sizing needed to armor these high velocity channels/outlets. Two equations were used to approximate riprap size. The "Abt equation" (Abt and Johnson 1991) was used because of the steep conditions of the outlet channels. For comparison purposes, the Army Corps of Engineers {ALOE 1994) riprap equation was also used. Tables 5.0 and 5.1 show the parameter input values and results of the calculations. As anticipated, the calculations show that large riprap sizes are ' required in the steep outlet channels. Side slope riprap sizes aze slightly less (see Table 5.0). The design criteria for riprap are summazized as follows: • The design DSo size for outlet channels DP3 and DP4 is 1.8 and 2.0 ft, respectively. The embankment side slope Dso riprap size is approximately 1.9 fr. • The design D5o size for bottom channel and side slope DSS and DPS is approximately 0.7 ft. • Specific gravity of riprap material shall be no less than 2.3. • If available and reasonably cost-effective, riprap will be angular and not rounded. • Riprap thickness will be twice the DSo size, plus or minus 15 percent. ' • A geotextile fabric will be installed after grading and compaction of each outlet channel surface, riprap will be installed directly on the geotextile fabric. This specification may be removed if the fabric prevents effective anchoring of the riprap on the steeply sloped channels. • To optimize placement and stability, riprap will be installed both mechanically and manually, where feasible. Western Water & Land Inc. 14