1990-01-09_REVISION - M1988112 (6)

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' 90 01109 18:42 ~ $ 303 985 9947 ]RK DE N!~E~• BATTLE MOUNTAIN RE50URCES, SAN LUIS PROJECT CMLRp ADEQUACY LETTER RESPONSES Page 23 of 28 41. In order to encourage drafnege from the reclaimed surface of the facility, a grade in excess of D.SX should be created. Me recommend that a 1.5% 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 armored low flow trickle channels. The channels are described in Item 42. 42. Na post-mine channels have been proposed for the top of the reciaimed tailings other than those resulting from final deposition as shown on Figure F-3. flow paths will approximate the pre-mining condition as noted. A significant reduction in channei slope will occur between pre- and post-mine conditions. A)though the reduction in slope will tend to reduce the potential for erosion, it will not alleviate the potential as stated on page E-6. The broad flow zenes {channels) created an the tailings surface and shallow stapes will result in low velocities during flood conditions. However, during smaller runoff events, channe]ized fiow from upstream areas may tend to cut a channei through the topsoil which has been respread on the tailings. It may be advisabie to direct this flow in a constructed low-flow trickle channei rather than ailow the topsoil to be eroded away in finding its preferred path. Please address that stability of these channe]s and any channe] bottom treatment required for 1ow- flow conditions. RESPDNSE: In order to reduce erosion of the tailings surface during low flow events and to prevent stream channel meandering, low flaw channels will be created on the surface of the tailings impoundment. An analysis of runoff resulting from the 2-yr, 24-hr produces 1.4 inches of rainfall, was performed using program WASHED. The analysis indicates that Inflows peripheral to the tailings impoundment are negligible event. However, direct precipitation produces a peak 53 cfs at the reclamation spillway. storm, which the computer from areas during this discharge of 13