1998-01-10_REVISION - M1988112 - Laserfiche WebLink

Home Browse Search

1 L 1 1 1 BATTLE MOUNTAIN RESOURCES, SAN LUIS PROJECT CMLRD ADEQUACY LETTER RESPONSES Page 23 of 28 41. In order to encourage drainage from the reclaimed surface of the facility, a grade in excess of 0.5% should be created. We recommend that a I.5% grade be established. This grade would facilitate the removal of water from the surface of the facility in a non-erosive manner, while minimizing the potential for ponding and infiltration to occur. RESPONSE: Under the current design, discharge velocities resulting from the 100-yr, 24-hr storm range from three to four fps across the tailings surface. Under these conditions, the flows are subcritical in nature and stabilization by revegetation is anticipated to be sufficient minimize erosion of the tailings surface. Increasing the surface gradients to 1.5 percent would create supercritical discharge velocities approaching 10 fps during the 100- yr event. Under these conditions, erosion of the tailings surface would be accelerated. During the flow events erosion will be controlled by armared low flow trickle channels. The channels are described in Item 42. 42. No post-mine channels have been proposed for the top of the reclaimed tailings other than those resulting from final deposition as shown on Figure F-3. Flow paths will approximate the pre-minieg condition as noted. A significant reduction in channel slope will occur between pre- and post-mine conditions. Although the reduction in slope will tend to reduce the potential for erosion, it will not ai~leviate the potential as stated on page E-6. The broad flow zones (channels) created on the tailings surface and shallow slopes will result in low velocities during flood conditions. However, during smaller runoff events, channelized flow from upstream areas may tend to cut a channel through the topsoil which has been respread on the tailings. It may be advisable to direct this flow in a constructed low-flow trickle channel rather than allow the topsoil to be eroded away in finding its preferred path. please address that stability of these channels and any channel bottom treatment required for 1ow- flow conditions. RESPONSE: In order to reduce erosion of the tailings surface during low flow events and to prevent stream channel meandering, low flow channels will be created on the surface of the tailings impoundment. An analysis of runoff resulting from the 2-yr, 24-hr storm, which produces 1.4 inches of rainfall, was performed using the computer program WASHED. The analysis indicates that inflows from areas peripheral to the tailings impoundment are negligible during this event. However, direct precipitation produces a peak discharge of 53 cfs at the reclamation spillway.