2001-05-16_REVISION - M2000158 (2)

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Appendix D - Porenria! Hydrologic Srrucrurer ro Minimise /mpaar and ,4firtlrtR Methods Drop Structures and Weirs A variety of drop structures and weirs can be placed in the channel to prevent head cutting upstream and downstream of a mining operation. The invert elevation should be no lower than the pre-mining channel invett elevation. Type VH rock is trenched 5 feet deep into the channel for at least 20 feet from the bottom of a 10:1 (H: V) side slope (Figure D-14.) This structure is anchored with a double row of sheet piling spaced 5 feet apart and driven to refusal. The wingwalls should extend across the width of the floodplain. Photographs of drop structures and vortex and W-shaped weirs are provided in Figttre D-15. Several of the photographs are from ! non-muting applications. These structures must also be anchored into the stream bed to prevent ~; undercutting and erosion. '~:~ Berm Stabilization Spillways minimize the potential for failure of the berms during flood events. Spillways should l be installed on berms protecting the active pit in split channel mining or along lateral berms running between a stream and a floodplain pit. The invert elevation of the spillway crest should be one-foot above the 2-year 24-hour flow elevation. The side slopes should be 10:1 (H: V) and the channel should be stable during a 25-year 24-hour flow event. In the event that an in-stream mining operation includes a pit within the adjacent floodplain, berms between the river and the pit should be at least 100 feet wide, and should have reinforced side slopes between the stream and the pit. Rip-rap, concrete or soil cement can be used for stabilization. General specifications aze shown in Figure D-16. The minimum top width of the berm increases to 400 feet if no armoring is used. SUMMARY There are a number of techniques that may be used by themselves or in combination to stabilize a channel utilized for an in-stream aggregate operation. These include armoring with rock or vegetation, installation of jetties, and placement of grade control structures. A hypothetical plan view of a river that combines the use of these structures may be reviewed in Figure D-17. Pre- . project planning may also identify the benefits of designation of an undisturbed vegetated buffer zone between the stream and the operations azea. Buffer zones are also identified in Figure D-17.